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INERTIA

A good party trick is to pull out a cloth from under a tea set without moving the cups
The seat belts in a car will become tight if the car stops quickly
You can tear a piece off the end of a kitchen roll without holding the roll if you give it a sharp pull
If you are sitting in a car when it corners you seem to be thrown sideways into your partners lap
The handles of a loaded plastic shopping bag may break if it is jerked upwards
It is easy to balance on a bicycle when it is moving but almost impossible when it is standing still
Spacemen on the Moon found that once they were moving it was difficult to stop. Although they weighed less they still had the same mass and so the same inertia as on the Earth.

All these things are examples of INERTIA. All objects have inertia. The more massive they are the more inertia they have.

INERTIA is the reluctance of an object to change the way it is moving. If it is still it stays still and if it is moving it carries on moving in the same way unless a force acts on it.

In the example of the cornering car you try to travel along in a straight line but your partner and the car come round to push you out of this line. It only feels as if you are being thrown into their lap. If a car stops really suddenly the people inside it will carry on moving forwards and so be thrown forward.

You can also show the effects of inertia by knocking away the bottom block from a pile of wooden blocks using a sharp blow with a hammer without moving the others. The blocks have inertia and there is not enough force on the rest of the pile to start moving.

Another example is two people on swings, one heavy person and one light one. The heavy one has a lot more inertia and will therefore be much harder to get swinging and then much more difficult to stop.

A special piece of apparatus called an inertia balance (or wig wag) was devised to demonstrate the effects of mass on the inertia of an object. (See Figure 2). When the number of solid metal cylinders in the 'tray' were changed the vibration rate also changed. Objects with more inertia vibrate more slowly.

It also showed that the pull of gravity, or weight, has no effect on the inertia of an object.

This was most important in the movement of massive but weightless objects in spacecraft.

INERTIA EXPERIMENTS
1. You can knock away the bottom block from a pile of wood blocks without the other blocks moving.
2. Put a piece of card on the top of a beaker and then put a penny on top of that. You can now flick away the card. The penny has inertia and will not move but drop straight down into the beaker.
3. A truck on the linear air track will keep moving for a long time. There is not much to stop it.
4. Hang up two pendulums, one heavy one and one light one. Swing both of them by the same amount and now try and stop each one. The more massive one has more inertia and you will find that it is much more difficult to stop.

INERTIA AND A GOLF BALL

The hitting of a golf ball is a good example of inertia. The mass of a golf ball is quite small and so its inertia is also small. However when the golf club strikes the ball the inertia of the ball is enough to prevent it moving instantaneously. It will take a fraction of a second for the ball to reach the speed of the club. During that time the golf ball will distort as shown in Figure 3.

Squash balls are much more elastic that golf balls and so they distort more during the time that they are in contact with the racquet.

INERTIA OF ROTATING OBJECTS

It is quite difficult to show some effects of inertia when objects move in a straight line so we use a rotating table and a large flywheel to help us.

1. Stand on the rotating table and try to turn yourself round without touching anything. You will find that this is very difficult. You cannot overcome your own inertia and so you don't move much.

2. Get someone to push you round slowly with your arms stretched out. NOW bring your arms into your sides. You will spin round more quickly. You may have seen this sort of effect with ice skaters, divers and gymnasts. Holding two books or weights in your hands makes the effect even more impressive.

3. Stand on the rotating table and very carefully throw a weight to someone. As the weight moves off you will find that you spin in the opposite direction.

4. Stand on the ground and hold the spinning flywheel. Try and change the direction of the axle of the flywheel. You will find that it is very difficult. The axle "does not like" to have its direction changed. This is the reason why bikes don't fall over when they are moving along.

5. Now stand on the rotating table and get someone to give you the spinning flywheel with its axle horizontal. Turn the flywheel so that its axle is upright, you will start to spin in the opposite direction to the flywheel. You won't spin as fast as it is spinning because you are much heavier.

PROBLEMS
Where do you think inertia is important in the following:
1. One car is towing another?
2. A supertanker docking
3. An astronaut walking on the Moon (HINT: friction depends on the weight of the astronaut).
4. When a car or plane accelerates quickly the passengers feel as if they are pushed back into their seats.

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