The Earth's gravitational pull
Gravitational force can act 'at a distance'; it shows little variation over short distances, but does vary over larger distances.
Apparatus and materials
For each student group
Load with mass up to 1 kg, e.g. bag of sand, brick, heavy book, or 1 kg mass
Loop and hook, to hang load from forcemeter
Small object for studens to drop, e.g. bag of sand
Health & Safety and Technical notes
Use a box or tray lined with bubble wrap (or similar) under heavy objects being lifted. This will prevent toes or fingers from being in the danger zone.
a Stand with your feet apart and hold the load. Feel the force pulling it down. The force is its weight, the force of gravity.
b Imagine that the force is the pull of a long spring stretching down into the ground.
c Lift the load higher. Imagine the spring stretching. Decide whether its force gets bigger.
d Use a forcemeter to check your idea about whether the force got bigger. Find the weight, in newtons, of the load when it is close to the ground, and when it is high up.
e Decide what force is balancing the weight when you hang it from the forcemeter, so that it does not accelerate.
f Drop an object so that it accelerates to the ground. This is to see what happens when the downwards force acts alone, and is not balanced by the force provided by the stretched spring in the forcemeter.
1 Every object attracts every other object with a gravitational force, but the attractions are small unless one of the objects is big. You could compare this with electric (or electrostatic) forces where not only can the force be either attractive or repulsive, but forces between small objects can be large.
2 Some students may apply their knowledge of springs, so that they expect the imaginary spring to exert a larger force as it stretches. If so, you need to explain that this spring goes all the way to the centre of the Earth. It is so long that small stretches would not make a noticeable difference to the force it exerts.
3 Explain that with extremely precise measuring equipment the weight of the load might, in fact, be seen to vary. It would get smaller as it moves away from the centre of the Earth. In the laboratory the change in its position is much too small to be detected by a forcemeter. For spacecraft far from the Earth's centre the effect is significant.
4 In everyday affairs we measure 'weight' in kilograms. While everyday affairs are confined to the Earth, physics is not. We have to recognize that mass and weight are different things. The quantity of material of an object, its mass, stays the same wherever it goes. Its weight depends on the local pull of gravity, and it changes. We measure mass in kilograms and weight in the units of force, newtons.
5 By allowing a load to fall, this activity also demonstrates that unbalanced force produces a change in velocity (acceleration), always and inevitably.
This experiment was safety-checked in October 2004