Inertia experiment 5

 Push in the springy bolt on the front of a trolley.

 Catch the slot under the front lip so that it stays in place.

Put a marble onto one of the drilled holes on top to hold it in place on the still trolley. I like to keep one finger beside it to remember whereabouts above the bench the marble started.

 Press the trigger release. The bolt pushes the trolley backwards away from the wall.

Notice that the marble has stayed roughly above its initial position on the bench. This is because it has inertia. The external force acts on the trolley not the marble. You experience this if you are standing on a bus or train that moves off quickly. You fall backwards not because there is a force throwing you backwards - quite the opposite. No force acts on you - you are merely trying to stay in one place. It is also why cars have headrests to prevent whiplash injuries, but that's another post!