1. What is a force?

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       Introduction          Video      


Image:fig a.GIF Modelling the bike as a particle can provide an explanation for the fact that it moves backwards when pulled backwards.

Modelling an object as a particle is an idealisation (or abstraction) which can be useful in many cases.

Image:fig b.GIF Modelling the drum as a particle can explain what happens if the string is horizontal, but not what happens if the string is vertical.

Beware of potential problems when modelling objects as particles.

Image:fig c.GIF Write down a simple definition of a force in your own words.
Image:fig d.GIF Exploring the effects of gravity on objects can help create a definition of a force.
Image:fig e.GIF Gravity can change the motion of a ball in a number of ways.
Image:fig f.GIF A force is something that causes a change to the motion of a body.
Image:fig g.GIF Newton’s first two laws are based on an understanding of this idea of how a force changes motion.
Image:fig h.GIF In the first car the resultant force is in the direction of motion and the car gains speed.

In the second case the resultant force is zero and the car travels at a constant speed.

In the third case the resultant force is backwards and the car slows down.

Image:fig i.GIF In the first case the resultant force is upwards. As the parachutist is also moving downwards, he slows down.

In the second case the resultant force is zero and the parachutist moves with a constant speed.

Image:fig k.GIF The resultant force is always downwards.

If the resultant force were zero at the highest point, the ball would stay there!!

Image:fig m.GIF No, the motion is changing from downwards to upwards.

The resultant force is upwards, for almost all of the time that the ball is in contact with the table.

Image:fig n.GIF The direction of the motion is always changing, so the forces are not in equilibrium.
Image:fig p.GIF
Image:fig q.GIF Image:fig r.GIF
Image:fig s.GIF Image:fig t.GIF
Image:fig u.GIF Image:fig v.GIF


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