Forces_Concept_1

A force is a push or a Forces are caused by objects acting on other objects, and can be experienced in a variety of ways. They can vary in strength, direction and duration. general terms a force can: start something moving; make something go faster or change direction; make a moving thing slow down or stop; change something's shape (either permanently or temporarily).

The action of a force requires the expenditure of energy (see Energy),

Sir Isaac Newton was the first person to describe force and movement in terms of mathematical relationships and universal 'laws' of motion. Our standard unit of force was named after him: the newton (small n) or N.

Force and movement

Forces and their effects are often misunderstood. This is frequently the result of a misinterpretation of the evidence: the story' has not been taken into consideration, and important elements have been ignored or overlooked. Some of these are discussed under Common misconceptions lower down.

On the basis of superficial evidence, it would appear that to cause constant movement requires a constant force: if you stop pushing the object, it will stop moving. A closer analysis will reveal that frictional forces act against the motion of a moving object, stowing it down (see Concept 4), if you could get rid of the friction, then once you pushed an object to start it moving it would keep moving with a constant speed and direction. This can only happen in outer space. The assumption that an object slows down after the motive force has been removed needs to be challenged: what is slowing it down? The idea that a frictional force can be balanced by a motive force is discussed under Concept 4.

Push a large book across a smooth table, keeping your fingertip in contact with it. When you started to make the book move, you probably felt quite a push against your fingertip; but once it had reached the speed that you wanted it to move at the pressure on your finger lessened. To get it to that speed, you needed to make the book accelerate from a so you needed to push with a greater force, Once it was moving at a suitable speed, you only needed to push with enough force to overcome the friction between the surface of the book and the table. if you were pushing the book over a rougher surface (with more friction between the book and the table), you would have needed to push it harder to keep it moving at that speed.

Newton's First Law of Motion states that an object will stay as is unless a net (unbalanced) force is applied to it. Unless there is more force acting on it in one direction than in another a moving object will keep moving in the same direction at the same speed; and a stiff object will stay put.

Force and direction 1 : getting started

When you kick a football along the ground, what is making it go? Your kick. But as soon as it leaves your foot, it begins to slow down because of friction. Other forces may be involved: the ball may be running uphill or downhill (gravitational force), or be aided or hindered by the wind (air pressure). These forces either help the ball to keep going or slow it down more quickly. So it is possible for forces to combine and either work together or work opposition,

What happens if the wind direction is at right angles to the direction of the kick? The force of the wind changes the direction of movement making it veer off to one side (see Figure l). Note that the ball does not take an L-shaped path: its path curves somewhere in between the directions of the two forces. If the kick were harder; the ball would travel more in that direction; if the wind were stronger; it would travel more in that direction.

Force and direction 2: slowing down and stopping

Sometimes you might apply a force to an object in the opposite direction to its movement For example, somebody throws you a ball and you catch it. To do that, you have to apply a 'stopping force' to the ball and the longer the distance it slows down over, the less it will hurt your hands, if you push a loaded shopping trolley to your car you will probably find yourself desperately pulling back in an attempt to slow It down as you approach the can In both cases, the force is being applied in the direction of the change in the motion.

Again, try pushing a large book across a smooth table. Push the book then release it and watch it slide across the table. As soon as you have let it go, the only horizontal force acting on it is the frictional force slowing it down, working against the direction of movement.

Put your back into it!

Imagine a road race without petrol, which the vehicles have to keep pace with each other. A Mini and a double-decker bus are side by side on the start line. I'll let you push the Mini. You could probably push it on your own, but will need considerable help with the bus: it will require a much greater force (push). First point: the more mass an object has, the greater the force required to accelerate it to a given speed.

Once we are both up to walking pace with our respective burdens while you may have eased up and not have to put much effort into keeping the Mini moving I (and my team) would be best described as 'not struggling quite as much as before'. Second point: the force required to keep each vehicle moving at G constant speed is just sufficient to overcome friction.

At the end both vehicles must stop together on the line, Both you and t go round to the front of our respective vehicles and app!y a force opposite to the direction of movement. Again, you will find that to begin with, it takes a little more effort than expected; but you soon have the Mini slowing gently to a stop. The bus, however. doesn't appear to want to stop. It will take nearly as much effort (friction is on our side this time) to stow it down as it did to get it going. Third point the more mass an object hos, the greater the force required to decelerate it from a given speed The lesson to be learnt here is a more generalised version of 'If you're given the choice of pushing a bus or a car, always choose the car'. Also, note that two people pushing the bus can get it up to speed twice as quickly as one person can, So the moss of the object and the force used are both important when you are trying to make an object accelerate (or decelerate). This leads us to Newton's Second Law of Motion: force = mass ^xacceleration (or f = ma). So increasing the force applied to a given mass will increase the rate of in proportion.

Changing shape

Are you sitting comfortably? If so, can you feel the force that you are exerting on your chair? There are sensors in parts of your body that are currently telling you that you are in contact with something and pressure being applied. Depending on the nature of the material that you are sitting on, one or both of two things will have happened when you sat on the chair: parts of you will have changed shape and/or parts of the chair will have,

Hopefully these changes of shape are only temporary in nature; shortly after getting up you and the chair will revert to your original shapes. It is worth mentioning here that two different types of materials are associated with temporary and permanent changes. Elastic materials return to their original shape once the external force is removed Plastic materials remain their new shapes.

Stretching and releasing

As was noted in Energy, energy is needed for a force to actually do something. Find a thick elastic band and stretch it out as far as you can, This act requires you to use energy As you stretch the elastic, you can feel it pulling back at you. You then have to keep up the force or the band revert to its original shape. if it were a piece of a plastic material such as dough, the initial effort would be alt that was needed. But with an elastic material you need to use your muscles to maintain the tension, When you release the elastic band, you have two options:

Release the band slowly, exerting slightly less force than the band, so that it brings your hands back together slowly. This shows that the band is exerting a force throughout the process.

Just let the band go, and see what happens when that amount of force is applied over a very short period of time,

This situation can also be described in terms of energy You used energy to stretch the bands so energy has been stored in the elastic band If can either be released at once (as in a. catapult) or slowly (as in a balsa wood plane where an elastic band is twisted and then released to turn the properly. You also need to use energy to hold a book in the air; because human muscles do not stay taut without expending energy; but a table does not expend energy in keeping a book off the ground.

Unbalanced forces

Throughout this section, rt has been clear that where an object is changing speed shape or direction, the change is caused by a force acting on the object in a particular direction being bigger than the force opposing it. Whenever forces are unbalanced, something will change.

Why you need to know these facts

Force a phenomenon that we experience constantly — so constantly that we tend to forget about it and ignore it. By observing in greater detail, we can begin to appreciate the effects and characteristics of force. Being aware of farces in everyday life help children to grasp this important aspect of science.

Vocabulary

Force a push or pull.

Accelerate/decelerate — to increase or reduce speed of movement in a particular direction.

Amazing facts

The motor racing driver David Purley crashed in 1 977, going from 173km/h to Okm/h in 66cm a deceleration force of 178.9 times the Earth's gravity and survived

Common Misconceptions

Forces are to do with living things.

'A forceful character: ^IMay the Force be with you.' *The life force.' Many common uses of the word 'force' give children the impression that it is something intrinsic to plants and animals, rather than a concept within physical science. As with all scientific words in common usage that have alternative meanings, the key is to model the correct scientific usage for the children and encourage them to use this.

Constant motion needs a constant force.

This is usually true, but only because of friction. Ask the child why an object slows down: Why doesn't it just keep going when you stop pushing it? Introduce the idea of friction the force that opposes movement. As long as there is friction in the system, you will need to keep up a small push to overcome it. if you can reduce the ffctiom you will need to push less to keep the object moving Pushing a metal block over sandpaper will need more force than pushing it over ice. If you could reduce friction to nothing, you wouldn't need to keep pushing the object to keep it moving.

The same push will always result in the same movement.

Ask the child to push a small box across the carpet then do the same with a larger and much heavier box, Is there the same force [or the same motion? Hopefully, the child will realise that mass is a factor too and make progress towards developing an understanding of Newton's Second Law.

Teaching ideas

Exploring forces (exploring, observing)

The children can take a collection of moving toys and explore what happens when they push or pull them, then what happens when they push or pull harder. What happens when they make them heavier and then try to make them move? They can also explore playground equipment: How do you make the equipment move? What happens when you push harder? What happens when you push in a different direction?

Pushes and pulls (identifying, explaining)

The children can act out pushes and pulls in PE, both individually and in pairs or groups. Ask them to identify things in the classroom that need pushing or puffing (chairs, doors, pencils and so on), and to explain how these things work.

Squashing and stretching (developing vocabulary)

Place a lump of play dough in the middle of a table.The children can take turns to pick it up and change the shape of it in different ways, saying the appropriate verb (such as bend', 'squeeze', 'twist'. 'press') as they do so.

Identifying forces (observing, recording)

Children should be asked to look at and think about a range of different situations in terms of pushes and pulls, Good examples would be:

the p\ayground (swings and slides)

bail games

going from the classroom door to your seat.

The children can then talk through various physical actions, using 'pushes' and 'pulls' as the key verbs.

Report abuse