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 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.
'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.
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.
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
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.
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.
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.
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.
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.
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 pinsIf 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 shoesTake 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.
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 objectsHanging objectsTake 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 objectsTake 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 surfaceConsider 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.
Unbalanced forces causes can cause:
a still object to movea moving object to speed up or slow downa moving object to stopa moving object to change directionUnbalanced 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.
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:
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.
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.
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).
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 appliedThis 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:
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
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.
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 belowYou will notice that longer spanners undo nuts a lot more easily than shorter spanners. See the illustration belowA 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.