Wednesday, November 6, 2013

PHYSICS QUIZZES

quiz - vectors

quiz - acceleration

quiz - forces

quiz - gravity

quiz - speed

Wednesday, October 30, 2013

Pressure is Force per Unit Area

Pressure is the force on an object that is spread over a surface area.
The equation for pressure is the force divided by the area where the force is applied.

Although this measurement is straightforward when a solid is pushing on a solid,
the case of a solid pushing on a liquid or gas requires that the fluid be confined in a container. The force can also be created by the weight of an object.

Questions you may have:

What is the pressure when a solid pushes on another solid?
What happens when a solid pushes on a confined fluid?
What happens when the force comes from gravity?

Pressure of solid on a solid

When you apply a force to a solid object, the pressure is defined as the force applied divided by the area of application. The equation for pressure is:

P = F/A

where

P is the pressure
F is the applied force
A is the surface area where the force is applied
F/A is F divided by A

For example, if you push on an object with your hand with a force of 20 pounds, and the area of your hand is 10 square inches, then the pressure you are exerting is
20 / 10 = 2 pounds per square inch.

Pressure equals Force divided by Area

You can see that for a given force, if the surface area is smaller, the pressure will be greater.
If you use a larger area, you are spreading out the force, and the pressure (or force per unit area) becomes smaller.

Solid pressing on confined fluid

When a liquid or gas is confined in a container or cylinder, you can create a pressure by applying a force with a solid piston. The pressure created in the cylinder equals the force applied divided by the area of the piston: P = F/A.
In a confined fluid—neglecting the effect of gravity on the fluid—the pressure is the same throughout the container, pressing equally on all the walls. In the case of a bicycle pump, the pressure created inside the pump will be transmitted through the hose into the bicycle tire. But the air is still all confined.

Pressure is in all directions in a fluid

Increasing the force will increase the pressure inside the cylinder.

Caused by gravity

Since the weight of an object is a force caused by gravity, we can substitute weight in the pressure equation. Thus the pressure (P) caused by the weight (W) of an object is that weight divided by the area (A) where the weight is applied.

P = W/A

If you place a solid object on the floor, the pressure on the floor over the area of contact is the weight of the object divided by the area on the floor.

Pressure equals Weight divided by Area

Example with shoes

A good example of how a force on small area can result in a very high pressure is seen in women's shoes with high spiked heels. These types of shoes can cause damage to some floors due to the very high pressure on the floor at the heel.
An average shoe distributes the weight of the person over 20 square inches. Thus, a 100-pound person applies 100/20 = 5 pounds per square inch on the floor.
Since a spike-heel is only 0.25 square inches, the 100-pound person would be applying 100/0.25 = 400 pounds per square inch on the floor at the heel! In some cases, that is sufficient to damage the floor.

Fluid weight

If you put a liquid in a container, the weight of that liquid would be pressing on the bottom of the container similar to that of the weight of a solid object. The pressure on the bottom of the container would be the same as if the weight was from a solid:

P = W/A.

The only difference is that pressure in a fluid goes in all directions. So the pressure on the sides at the bottom would be the same.
Gases and liquids exhibit pressure due to their weight at every point in the fluid.

Summary

Pressure is the force on an object that is spread over a surface area. The equation for pressure is P = F/A. Pressure can be measured for a solid is pushing on a solid, but the case of a solid pushing on a liquid or gas requires that the fluid be confined in a container. The force can also be created by the weight of an object.

Put pressure on yourself to excel

Take the Quiz: SUPERHERO SCIENCE

by Ron Kurtus (revised 18 March 2006)

PHYSICS - VELOCITY,SPEED, & MOTION

Velocity, Speed, and Motion... Oh My!

Velocity equals distance divided by time.

Velocity and speed are very similar ideas, but velocity is a vector, and speed is not. Suppose we knew that someone was driving at thirty-five kilometers an hour (35 km/hr), but the direction wasn't given. How would you draw an arrow to represent a vector? You can't know how to draw the vector if you only have one value (either amount or direction). In this example, you were never told about the direction. Physicists would say that the speed is thirty-five kilometers an hour (35 km/hr), but the velocity is unknown. On the other hand, if you're moving at 35 km/hr in a northern direction, then you would have an arrow pointing north with a length of 35. Physicists would say that the velocity is 35 km/hr north.

Velocity is the rate of motion in a specific direction. I'm going that-a-way at 30 kilometers per hour. My velocity is 30 kilometers per hour that-a-way. Average speed is described as a measure of distance divided by time. Velocity can be constant, or it can change (acceleration). Speed with a direction is velocity.

Remember vectors? You will use a lot of vectors when you work with velocity. Our real world example of navigation on the ocean used velocity for every vector. Velocity is a vector measurement because it has an amount and a direction. Speed is only an amount (a scalar). Speed doesn't tell the whole story to a physicist. Think of it another way. If I tell you I'm driving north and ask you how long until we get to the city. You can't know the answer since you don't know my speed. You need both values.

One Moment in Time

There is a special thing called instantaneous velocity. That's the velocity at a split second in time. Above, we were talking about your speed and direction over a long period of time. Why would you need to measure a velocity at one moment? Think about the moment you drove over the manhole. It's important to know if you were going 1 km/hr when you drove over the manhole, or 60 km/hr. It wouldn't help you to know that your average speed was 30 km/hr.

The term "instantaneous" refers to something physicists call a limit. Scientists "limit" the amount of time they do the measurement. When the "limit" moves to zero, that limit is one tiny moment in time. A physicist would measure your velocity as the "limit for a period of time", zero, to get the instantaneous velocity.

Changing Your Velocity

When velocity is changing, the word acceleration is used. Acceleration is also a vector. You speed up if the acceleration and velocity point in the same direction. You slow down (also referred to as decelerating) if the acceleration and velocity point in opposite directions. When you accelerate or decelerate, you change your velocity by a specific amount over a specific amount of time.

Just as with velocity, there is something called instantaneous acceleration. Instantaneous means scientists measure your acceleration for a specific moment of time. That way they can say he was accelerating at exactly this amount at this point during his trip.

Constant Acceleration

There are a few special situations where acceleration may be constant. This type of acceleration happens when there is a constant net force applied. The best example is gravity. Gravity's pull on objects is a constant here on Earth and it always pulls toward the center of the planet (Note: Gravity decreases as you move far away from the surface of the planet.). The gravities of other planets are different from Earth's gravity because they may have different masses and/or sizes. Even though the gravity may be smaller or larger, it will still create a constant acceleration near the surface of each planet.

Tuesday, October 29, 2013

PHYSICS - FORCES - GRAVITY

Forces of Attraction

Gravity of the Earth pulls objects towards the center of the planet.

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

Both the feather and the ball fall at the same speed in a vacuum.

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

Monday, October 28, 2013

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.

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

Monday, October 21, 2013

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.
Pushing a pram

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.
moon and earth

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.
gravity tip

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.

example of pressure

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.

left aligned image

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.

what is a balanced force

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
log in water

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.
example of unbalanced forces

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:

resultant force opposite

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.
snow-sliding and friction

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
uses of moments

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.

Mrs. Remis' Science Blog - 7th grade

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