In these experiments, a tub full of sand and an empty tub are taped to the top of two trolleys. In one experiment you push the trolleys away from you, one with your left hand and one with your right hand. You catch them with elastic bands. The heavier one stretches the elastic band more, showing that a bigger external force is needed to effect the change in motion. You then use the elastic bands to pull them towards you instead and the same thing happens. The bigger the mass, the bigger the external force needed to change the motion. This is INERTIA.
