PHYSICS - FORCES - GRAVITY

Forces of Attraction

Gravity or gravitational forces are forces of attraction. We're not talking about finding someone really cute and adorable. It's like the Earth pulling on you and keeping you on the ground. That pull is gravity at work.

Every object in the universe that has mass exerts a gravitational pull, or force, on every other mass. The size of the pull depends on the masses of the objects. You exert a gravitational force on the people around you, but that force isn't very strong, since people aren't very massive. When you look at really large masses, like the Earth and Moon, the gravitational pull becomes very impressive. The gravitational force between the Earth and the molecules of gas in the atmosphere is strong enough to hold the atmosphere close to our surface. Smaller planets, that have less mass, may not be able to hold an atmosphere.

Planetary Gravity

Obviously, gravity is very important on Earth. The Sun's gravitational pull keeps our planet orbiting the Sun. The motion of the Moon is affected by the gravity of the Sun AND the Earth. The Moon's gravity pulls on the Earth and makes the tides rise and fall every day. As the Moon passes over the ocean, there is a swell in the sea level. As the Earth rotates, the Moon passes over new parts of the Earth, causing the swell to move also. The tides are independent of the phase of the moon. The moon has the same amount of pull whether there is a full or new moon. It would still be in the same basic place.

We have to bring up an important idea now. The Earth always produces the same acceleration on every object. If you drop an acorn or a piano, they will gain velocity at the same rate. Although the gravitational force the Earth exerts on the objects is different, their masses are just as different, so the effect we observe (acceleration) is the same for each. The Earth's gravitational force accelerates objects when they fall. It constantly pulls, and the objects constantly speed up.

They Always ask About Feathers

People always say, "What about feathers? They fall so slowly." Obviously, there is air all around us. When a feather falls, it falls slowly because the air is in its way. There is a lot of air resistance and that resistance makes the feather move slower. The forces at work are the same. If you dropped a feather in a container with no air (a vacuum), it would drop as fast as a baseball.

What About the Moon?

But what keeps the Moon from falling down, if all of this gravity is so strong? Well, the answer is that the moon IS falling; all the time, but doesn't get any closer to us! Remember that if there wasn't a force acting, the Moon would be traveling in a straight line. Because there IS a force of attraction toward the Earth, the moon "falls" from a straight line into a curve (orbit) around the Earth and ends up revolving around us. The Earth's gravity holds it in orbit, so it can't just go off in a straight line. Think about holding a ball on a string and spinning it in a circle. If you were to cut that string (no more gravity), the ball would fly off in a straight line in the direction it was going when you cut the string. That direction, by the way, is not directly away from your hand, but tangent to the circle. Tangent is a geometry term used to describe a direction that are related to the slope of a curve. Math stuff. The pull of the string inward (toward your hand) is like the Earth's gravitational pull (inward toward the center of the Earth). 

Force of Gravity

• Mass is the amount of matter in an object. It is measured in kilograms. The mass of an object remains the same anywhere in the universe.
• Weight is the force of gravity on an object. It is measured in newtons. The weight of an object differs depending on its position in the universe (e.g. A person's weight on the moon with less gravity will be less than that on earth).
• Rule for Weight
  
  

Weight (newtons)	= Mass (kilograms) × Gravitational Acceleration (9.8 m/s2)
W	= m × g

SCIENCE NEWS - SPACESUITS #

SPACE SUITS

PHYSICS - NEWTON'S FIRST LAW

According to NEWTON'S FIRST LAW, 

• an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. 
• It is the natural tendency of objects to keep on doing what they're doing. 
• All objects resist changes in their state of motion.
• In the absence of an unbalanced force, an object in motion will maintain its state of motion. This is often called the law of inertia.

The law of inertia is most commonly experienced when riding in cars and trucks. In fact, the tendency of moving objects to continue in motion is a common cause of a variety of transportation injuries - of both small and large magnitudes. 

• Consider for instance the unfortunate collision of a car with a wall. Upon contact with the wall, an unbalanced force acts upon the car to abruptly decelerate it to rest. Any passengers in the car will also be decelerated to rest if they are strapped to the car by seat belts. Being strapped tightly to the car, the passengers share the same state of motion as the car. As the car accelerates, the passengers accelerate with it; as the car decelerates, the passengers decelerate with it; and as the car maintains a constant speed, the passengers maintain a constant speed as well.

• But what would happen if the passengers were not wearing the seat belt? What motion would the passengers undergo if they failed to use their seat belts and the car were brought to a sudden and abrupt halt by a collision with a wall? Were this scenario to occur, the passengers would no longer share the same state of motion as the car. The use of the seat belt assures that the forces necessary for accelerated and decelerated motion exist. Yet, if the seat belt is not used, the passengers are more likely to maintain its state of motion. The animation below depicts this scenario.

If the car were to abruptly stop and the seat belts were not being worn, then the passengers in motion would continue in motion. Assuming a negligible amount of friction between the passengers and the seats, the passengers would likely be propelled from the car and be hurled into the air. Once they leave the car, the passengers becomes projectiles and continue in projectile-like motion.
Now perhaps you will be convince of the need to wear your seat belt. Remember it's the law - the law of inertia.

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http://www.physicsclassroom.com/mmedia/newtlaws/cci.cfm

PHYSICS - REVIEW OF FORCES

Forces


Forces are in play all around us. Things hanging, sitting, balancing, moving and spinning are all using some kind of force. Forces come in different forms and they all result in something.

Let us start the lesson with this short picture — 

" Milly opened the fridge and brought out chilled can of soda. She slammed it, opened the soda and gulped it down. She was upset it was finished too soon and so she crushed the can in her hand and threw the empty can into the bin."

Milly applied force in many of her actions (highlighted actions). Her actions involved the use of force to lift, open, turn, move and even change the shape of something.

Force, together with its various types are applied in almost every single activity in our lives. 

Pushing the shopping cart, pulling the baby stroller, lifting weights at the gym, eating and many other things involve the use of some force.


Can you think of the many ways in which you have applied a force to get results? 

In this lesson, we shall look at Forces in detail and how forces change the shape of objects, get things moving, cause moving objects speed up, slow down or stop and change the way things move.  Weight, pressure and turning moments are all the result of forces too. Ready?

What is a force?

A force can be a push or a pull. It is not something you can see or touch, but can see it in action. Forces can be measured using a device called force meter. The unit of force is called the newton. It is represented by the symbol N. A force of 2N is smaller that 7N.

A force usually results from an interaction. The interaction can be a physical one, or a non-physical one. Forces resulting from physical interaction are called 'Contact Forces' and examples include Frictional, Tension, Air resistance and Spring force.

A force resulting from non-physical interaction is called 
'Action-at-a-distance force' and examples include gravitational, electrical or magnetic force.


Measuring forces





Force meters contain a spring connected to a metal hook. The spring stretches when a force is applied to the hook. The bigger the force applied, the longer the spring stretches and the bigger the reading.

A Force diagram
A force diagram is usually used to show the forces acting on an object. An arrow, with a name, length and direction is used to represent a force.

See this example below:


In a force diagram, the longer the arrow, the bigger the force.

What is Mass

Every object is made up of matter (Matter is anything you can touch physically) The more matter an object has, the bigger it is, and the more mass it has. Mass is measured in kilograms, kg, or grams, g. Things that have a big mass are harder to move, or harder to stop than objects with little mass.

Mass is how heavy something is without gravity.
This means the mass of an object is the same on
earth and in space (or other planets)

A 100gm ball will be 100kg everywhere, even on the moon. This fact is not the same for weight. The weight of an object can change at a different place, such as on the moon.


In the illustration above, notice how the mass of an astronaut remains the same, whiles his weight is smaller in moon as a result of less gravity.

Mass in NOT the same as weight. The difference is that weight is determined by how much something is pulled by gravity. If we compare two different things to each other on Earth, they will both be pulled by the same gravitational force, so the one with more matter will weigh more.

What is Weight?

Weight is a force caused by gravity. Because it is a force, it is also measured in Newtons (N). It is the gravitational force between the object and the Earth. An object will have greater weight if it has more mass.

All over the world, people read the weight of objects with kilograms. Thats is not correct. It is done only because it is easy for people to grasp. The proper scientific unit of measurement is Newton, and it is written as N

As mentioned in the previous page, the weight of an object is the same everywhere on earth because the object is under the same pull of gravity. In Space, there is no gravity so the object will not even sit on the scale at all. Is will just stay in suspense. Technically speaking, there is no weight on the Space.

Gravity on the Moon is less and that means an object will weigh less on Moon than on earth.

An object's weight (W) can be determined by the product of its' mass (m) and the magnitude of the local gravitational acceleration (g), thus W = mg.

An object with a mass of 1 kg has a weight of about 10 N, everywhere on earth.

Apparent weight
Sometimes the scale can record the weight of an object and get it wrong. Here is a simple test: The next time you stand on a scale, you will notice that your weight will be slightly more if you try to jump on it. This is because you put more force downwards, in addition to original force of of gravity. This is apparent weight and it is a measure of downwards force, not the weight from gravity.

What is Gravity

All objects have a force that attracts them towards each other. This force is called gravity. Even you, attract other objects to you because of gravity, but you have too little mass for the force
to be very strong.

Gravitational force increases when the masses are bigger and closer. This means that the gravitational force on Moon is less than on earth, because Moon has less mass than Earth.

Good examples of very massive objects that possess gravitational force include the moon and other planets. Consider the earth on which humans live. Everything tends to fall on the ground and stays there. If you jump, you came down again. Throw a ball upwards, and it will surely come down."Down" is towards the centre of the Earth, wherever you are on the planet.

This is a result of gravitational force, which pulls objects towards the center of the earth. 

Pressure

Pressure depends on how much force or weight is exerted, and over the area on which that force is applied: greater force, more pressure.

This is the equation for working out pressure:
pressure = force ÷ area

The unit for pressure is pascal, Pa. Pa is the same as newtons per square metre N/m2 . 1 Pascal = 1 N/m2.

Let us see some classic examples of pressure.

Drawing pins
If you held a drawing pin and pressed the pin the wrong way, what will happen? You surely will hurt yourself.

In the illustration above, there is more pressure at the pointed part of the pin, because that area is tiny and given the same force, the pressure will be more. The pressure at the flat end is less because the area is wider.


High-heel shoes
Take a look at these two shoe types. If a lady wearing the high heel shoe stepped on your feet with her heels, that would almost punch a hole because of the heels little area. It would be less painful if she wore the flat pinky shoe because the sole are is larger and the pressure is less.

Balanced forces

Balance forces are two forces acting in opposite directions on an object, and equal in size. Anytime there is a balanced force on an abject, the object stays still or continues moving continues to move at the same speed and in the same direction. It is important to note that an object can be in motion even if there are no forces acting on it.

Balanced forces can be demonstrated in Hanging, Floating and Standing/sitting objects

Hanging objects
Take a look at this hanging glass bulb shade. The weight of the bulb shade pulls down and the tension in the cable pulls up. The forces pulling down and pulling up can be said to be in balance.











Floating objects

Take a look at this log floating on a pool of water. It is floating because the weight of the log is balanced by the upthrust from the water. If more weight is tied to the log, the force pulling it down may be more and will cause it to sink.


Standing/Sitting on a surface
Consider a metal block resting on a surface of a table. Its' weight is balanced by the reaction force from the surface. The surface pushes up against the metal block, balancing out the weight (force) of the metal block.

Unbalanced forces

Unlike balanced forces, we say unbalanced forces when two forces acting on an object are not equal in size.

Unbalanced forces causes can cause:
a still object to move
a moving object to speed up or slow down
a moving object to stop
a moving object to change direction

Unbalanced forces make the wagon in the diagram speed up.

Notice that because there is a bigger force and a smaller force involved, the direction of the wagon will be determined by the bigger force. The wagon is moving as a result of unbalanced forces.

Resultant forces

To understand resultant forces better, let us see these two scenarios:

Any time a stationary object stays still, its' resultant force is zero. As soon as force is applied, acceleration begins. The speed of the acceleration will depend on the force applied and the mass of the object.

In a similar way, each time an object in motion (in constant speed and same direction) stays in motion, its' resultant force is zero too. As soon as a force is applied, it can make it stop, change direction, move slower or move faster. The resulting effect will depend on the force applied and the mass of the object.

It is worth noting that an object may have several different forces acting on it. See example in the illustration below:

All these different forces, F1, F2, F3 can be added up to know the resultant force, F4. The resultant force is the single force that has the same effect on the object as all the individual forces acting together.

If different forces are acting in different directions, a resultant force can be determined as well. See illustration below:

Frictional forces

Friction is a force that stops things from moving easily.

Whenever an object moves or rubs against another object, it feels frictional forces. These forces act in the opposite direction to the movement. Friction makes it harder for things to move.

In the illustration below, the smooth base of the snoblades slides smoothly on the snow. The boy on the grass is having difficulty sliding, because the grass is not smooth and his shoes are getting stuck in the grass. There is more friction between the shoes and the grass than the snow and the snowblades.

Without frictional forces, a moving object may continue moving for a longer period. Frictional forces are usually greater on rough surfaces than on smooth surfaces.

Frictional forces can be good and helpful. For example:

A basketball star can grip a ball and control it better in a dunk because of greater friction.When we walk, we don’t slip easily because of the friction between our shoes and the floor.
Each time you ride your bike, friction between the tires and the road help you not to skid off.

Sometimes frictional forces can be unhelpful.
If you don't lubricate your bike regularly with oil, the friction in the chain and axles increases. Your bike will be noisy and difficult to pedal.

Air resistance

Moving objects like aircrafts, cars and arrows experience air resistance when they are in motion. Frictional forces of the air against the moving object cause this resistance. There is more (bigger) resistance with faster movement, and less resistance with slower resistance.

Cars, aeroplanes and many fast moving objects are usually streamlined to overcome resistance.

Have you seen bike riders in a race? They wear smooth clothing and helmets designed to overcome resistance. This makes them glide through the air with top speed.

Moments

Moments is a scientific name for turning forces around a pivot. Forces can make objects turn if there is a pivot. Take a look at the illustration below. The pivot is the point in the middle of it. This pivot make one end tip up or down depending on the force applied to one end. This means moments can be equal and opposite if the force applied at both ends are equal and the sea-saw is balanced.

To work out a moment, two things are considered:
The distance from the pivot that the force is applied.
The size of the force applied

This is the equation for working out a moment:
moment = force × distance
The unit for moment is Nm (newton metre).

Example
If a force of 10 N acted on a see-saw 2 m from the pivot, moment would be worked as follows:

force × distance = moment
10 × 2 = 20 Nm

Here is an example of balanced moments. 20N at 4m from the pivot is balancing 40N at 2m from the pivot. The objects create moments of 80Nm that are equal and opposite, so the see-saw is balanced.


Equal Moments balances out a see-saw


Moments can be useful in many ways. Here are a few examples

A crowbar uses moments to lift heavy things over a lever. See the diagram below

You will notice that longer spanners undo nuts a lot more easily than shorter spanners. See the illustration below

A see-saw will balance if the moments on each side of the pivot are equal. This is why you might have to adjust your position on a see-saw if you are a different weight from the person on the other end.

PHYSICS - FORCES QUIZ

TAKE THIS QUIZ WITHOUT YOUR NOTES & RECORD YOUR SCORE ON PAPER & TURN IN THE BOX

FORCES

SCIENCE NEWS - WHALE EAR WAX #20

WHALE EAR WAX

Cerumen. It’s a lovely word. Especially considering that it means earwax. And we’re not the only species that produces the stuff. Some whales build up waxes, along with lipids and keratin protein into what’s called earplugs. And researchers now know that examining these plugs tells them about a whale’s lifetime exposure to pollution.
Alternating layers of dark and light in the plugs correlate to seasons of feeding or migration. So the plugs have been used to determine a whale’s age. Think tree rings. 
In the latest study, scientists analyzed an earplug from an endangered blue whale killed by a ship near California. They found that levels of stress hormones doubled over the whale’s life.
They also found evidence that the whale had been exposed to pesticides such as DDT, with the highest levels during the whale’s first six months of life. The whale was likely exposed to the pesticides in its mother’s milk.
They also found a couple of peaks of exposure to mercury. The study is in the Proceedings of the National Academy of Sciences. [Stephen J. Trumble et al., Blue whale earplug reveals lifetime contaminant exposure and hormone profiles]
Future earplugs should offer additional clues about whale lives. So look for researchers to give new attention to leviathan cerumen.
—Cynthia Graber

PHYSICS - MOTION

Energy Around Us

We use the concept of energy to help us describe how and why things behave the way they do. We talk about solar energy, nuclear energy, electrical energy, chemical energy, etc. If you apply a force to an object, you may change its energy. That energy must be used to do work, or accelerate, an object. Energy is called a scalar; there is no direction to energy (as opposed to vectors). We also speak of kinetic energy, potential energy, and energy in springs. Energy is not something you can hold or touch. It is just another means of helping us to understand the world around us. Scientists measure energy in units called joules.

Active Energy vs. Stored Energy

Kinetic and potential energies are found in all objects. If an object is moving, it is said to have kinetic energy (KE). Potential energy (PE) is energy that is "stored" because of the position and/or arrangement of the object. The classic example of potential energy is to pick up a brick. When it's on the ground, the brick had a certain amount of energy. When you pick it up, you apply force and lift the object. You did work. That work added energy to the brick. Once the brick is in a higher/new position, we would say that the increased energy was stored in the brick as PE. Now the brick can do something it couldn't do before; it can fall. And in falling, can exert forces and do work on other objects.

Season of Springs

The study of springs is a whole section of physics. A spring that just sits there doesn't do much. When you push on it, you exert a force and change the arrangement of the coils. That change in the arrangment stores energy in the spring. It now contains energy and can expand and do work on other things. Anything that is elastic (can change its arrangement and then restore itself), such as a rubber band, can store energy in the same way.

A rubber band can be stretched and then it is ready to do something. That stretching involves work and increases the potential energy. You can flatten a solid rubber ball and it will want to bounce back up. You can also pull the drawstring of a bow and the work done stores the energy that can make the arrow go flying. Those are all examples of your putting energy in, and then something happening when the energy comes out.

Gases Storing Energy

Gases? What can they do? Gases are great because they can compress and expand. They act as if they were elastic. If the pressure increases and compresses gas molecules, the amount of stored energy increases. It's similar to a spring, but slightly different. Eventually that energy in the compressed gas can be let out to do something (work).

In your car, there are shock absorbers. Some shocks have compressed gas in the cylinders rather than springs. The energy in those cylinders keeps your car from bouncing too much in potholes. Think about wind. Wind is caused because of pressure differences in the atmosphere. When the wind blows it can do anything - turn windmills, help birds fly, make tornadoes, and do all types of work.

 

Motion

Inertia

• Inertia is the tendency of a stationary object to remain at rest, or the tendency of a moving object to continue at the same speed.
• The heavier the object the greater is the inertia.

          Examples of Inertia

1. When a car brakes suddenly, the driver and passengers tend to keep going in the same direction and at the same speed as before braking. Seat belts are therefore needed to stop them going through the windscreen.

2. It is more difficult to push-start a stationary truck than it is to push a small sedan.
3. It is more difficult to stop a moving truck than it is to stop a small sedan.

PHYSICS - FRICTION

Friction Basics

Friction is a force that holds back the movement of a sliding object. That's it. Friction is just that simple.

You will find friction everywhere that objects come into contact with each other. The force acts in the opposite direction to the way an object wants to slide. If a car needs to stop at a stop sign, it slows because of the friction between the brakes and the wheels. If you run down the sidewalk and stop quickly, you can stop because of the friction between your shoes and the cement.

What happens if you run down the sidewalk and you try to stop on a puddle? Friction is still there, but the liquid makes the surfaces smoother and the friction a lot less. Less friction means it is harder to stop. The low friction thing happens to cars when it rains. That's why there are often so many accidents. Even though the friction of the brakes is still there, the brakes may be wet, and the wheels are not in as much contact with the ground. Cars hydroplane when they go too fast on puddles of water.

Friction and Gases

Friction only happens with solid objects, but you do get resistance to motion in both liquids and gases. This doesn't involve sliding surfaces like friction does, but is instead the kind of resistance you get if you try to push your way through a crowd. It's a colliding situation, not a sliding one. If the gas is air, this is referred to as air resistance.

If you were in the space shuttle and re-entering the atmosphere, the bottom of the shuttle would be getting very hot. The collisions that occur between the molecules of the air being compressed by the shuttle, heat up the air AND the shuttle itself. The temperature on the top of the shuttle is also warm, but nowhere near the temperatures found on the bottom.

Friction and Liquids

Although liquids offer resistance to objects moving through them, they also smooth surfaces and reduce friction. Liquids tend to get thinner (less viscous) as they are heated. Yes, that's like the viscosity of the oil you put in your car. Car engines have a lot of moving parts, and they rub on each other. The rubbing produces friction and the result is heat. When oil is added to a car engine, the oil sticks to surfaces, and helps to decrease the amount of friction and wear on the parts of the engine. An engine that runs hotter requires a more viscous oil in order for it to stick to the surfaces properly.

Measuring Friction

Measures of friction are based on the type of materials that are in contact. Concrete on concrete has a very high coefficient of friction. That coefficient is a measure of how easily one object moves in relationship to another. When you have a high coefficient of friction, you have a lot of friction between the materials. Concrete on concrete has a very high coefficient, and Teflon on most things has a very low coefficient. Teflon is used on surfaces where we don't want things to stick; such as pots and pans.

Scientists have discovered that there is even less friction in your joints than in Teflon! It is one more example at how efficient living organisms can be. 

Friction

• Friction is a force opposing the movement of one surface over another.
• The Magnitude of the Force of Friction depends on the following factors:

1. The roughness of the surfaces (e.g. stepping on banana peel compared with carpet)
2. The force pushing the surfaces together (e.g. A heavy truck's tires compared with a bicycle's tires on the road)
3. Whether the surfaces are moving or stopped

• 3 Types of Friction

1. Static Friction - acting between 2 stationary bodies (e.g. holding a person on a chair)
2. Sliding Friction - acting between surfaces where one is moving (e.g. sliding furniture across the floor, writing with a pencil on paper)
3. Rolling Friction - acting between surfaces of objects where one has a rounded shape (e.g. car tyres on the road, ball bearings); less than sliding friction

• 4 Ways to Reduce Friction

1. Reducing the force pushing both surfaces together
2. Using a lubricant such as oil or water between the surfaces
3. Using ball bearings or rollers between both surfaces
4. Polishing both surfaces

SCIENCE NEWS - WHY DOES WATER EXPAND? #19

PHYSICS - FORCES IN LIQUIDS

Forces in Liquids

• Cohesion is the combined effect of the forces of attraction between particles close to each other in the same substance.
• Surface Tension is the cohesion at the surface of liquids. Detergents and liquid temperature affect surface tension.
• Examples of Cohesion and Surface Tension - the rounded shape of a water droplet, clasped pointed hands of divers to reduce surface tension, reduction of surface tension for divers by fountains spraying into the diving pool

• Adhesion is the combined effect of the forces of attraction between the particles in different substances close to each other.
• Capillary Action is the effect that causes liquids to rise or fall in fine tubes. The finer the tube, the higher the liquid will rise up the tube.
• Meniscus - The meniscus is the curved surface of a liquid such as water in a glass or mercury in a thermometer. The curve is caused by the adhesive attraction between the liquid and its container.
• Examples of Adhesion and Capillary Action - Meniscus of water in a glass, the movement of water through the tubes in a tree