Force
Forces and Motion | Electricity and Magnetism

Do heavier things fall faster?

Teaching Guidance for 5-11 11-14 Supporting Physics Teaching

Falling freely – at the same rate

Wrong Track: It's obvious. If I drop a cannonball and a cricket ball, the cannonball will fall faster.

Right Lines: All objects fall freely at the same rate irrespective of mass (provided the effects of air resistance can be ignored).

Forces, mass and acceleration

Thinking about the learning

This is held as being so obvious that most people wouldn't even bother to check it out. A heavy block of wood, mass 2 kilogram, is clearly being pulled down with a greater force of gravity (about 20 newton) than a lighter piece of wood, mass 1 kilogram (about 10 newton). It seems clear to most that this larger force will make the heavy object fall faster.

Thinking about the teaching

The fact that a larger block of wood is subject to a greater force from gravity is indeed true. However, the greater mass of this wood requires a greater force to maintain its accelerated motion. Overall, the effect of a small force on a small mass is the same as that of a large force on a large mass. The net effect is the same – they fall together. They have the same force to mass ratio. (There is more on this argument in the Gravity and Space episode in the SPT: Earth in space topic.) The most important strategy for teaching is to set up a simple and effective dropping objects practical activity. Pupils should drop the objects themselves and also watch objects dropped by others. It is important to use objects that will not damage the floor or feet.

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