Forces_Concept_3

Subject Facts

The newton

The Sl (International System of Units) unit for the measurement of force is the newton (N). It is defined as the force required to accelerate a 1kg mass by 1ms in one second. The force of gravity, if unopposed would accelerate us towards the centre of the Earth at a rate of 9.8ms-2 — read as '9.8 metres per second, per second' every second it will go 9.8 metres per second faster than the previous second Therefore gravity exerts a force of 9.8N for every kg of mass we possess, By multiplying your body mass in kg by 9.8, you can calculate the force of weight (in newtons) that you exert on the ground.

Rounding 9.8 to 10 makes it much easier to calibrate force measuring machines. For example, a set of bathroom scales can be used as a force meter by multiplying the readings of mass (in kilograms) by 10 to give the force that you exert on the opposite sides. They are only able to measure pulls. For example. you could attach the hook to a door handle to measure the force required to pull the door open; or hang a 1kg mass from the hook to see the force exerted on it by gravity (which should be 9.8N).

Direction

As well as knowing the strength of a force, you may also need to know the direction that it is acting in.

Consider the example of kicking a football. On a windy day the direction of the wind force will dearly be important, The direction of a force is indicated on a force diagram with an arrow; by convention, the larger arrows indicate stronger forces. Balanced forces should be shown as opposite arrows of equal size; where a change of speed shape or direction is occurring, one arrow needs to be bigger than the one opposite to it. On Figure I in concept 1 we could draw arrows to show the forces of the kick, friction and the wind (see Figure 3),

It is easy ta confuse the direction of a force with that of movement. Consider someone throwing a ball up and catching it (see Figure 4), As the ball leaves the hand, the upward force of the throw is greater than the downward force of gravity (ignore air resistance). As soon as the ball leaves the hand the only farce acting on it is downwards, so it immediately begins to slow down. Gravity makes the ball decelerate (slows it down) until it stops moving upwards, then begins to make it accelerate back downwards. When the ball is caught, the hand exerts an upwards force on it greater than that of gravity. When holding the ball, the hand needs to push it up just enough to counteract the force of gravity.

Now consider a hand holding a tumbler The hand has to press against the glass, with the glass pressing back in order to make sure that the friction (see Concept 4) between the hand and the glass is sufficient to prevent the downward pull of gravity slipping it out of the hand's grip.

Teaching ideas

The children can make their own force meter using a long, thin rubber band} as follows (see Figure 5). Attach a cut rubber band to the outside of a cotton reel using tape. Thread a length of dowel through the centre of the cotton reel until it meets the rubber band. Attach a 'hook' (bent paper dip) to the band and down with more tape. The hook can be used for pulls and the other end of the dowel for pushes.

This force meter can be calibrated for measuring as follows. Holding the reel with the paper clip down, attach a 100g hanging mass to the paper dip, The band will stretch and the dowel bob down. Where the dowel enters the cotton reel, make a mark this is how far the band stretches under a force of almost I N. Now try 200g to measure 2N, and so on. (Be careful not to overstretch the band, or it will slip,)

The children can now measure (very roughly) what force is required to make different objects move or accelerate, They could try to give different toy cars 'fair' pushes, or even give a larger mobile toy a constant I N push to see how it accelerates.To do this, they should hold the toy still and push the length of dowel into the back of it until the band has stretched to the 3 N mark Then* holding the force meter by the red and keeping it at the IN mark, they should release the toy and keep pushing it for as long as possible (if it doesn't move, they should use a smaller toy).