Inertia experiment 1

In this experiment a trolley is allowed to roll down a ramp so that it hits a solid barrier at the bottom. The marble is not pressed in to the plasticine; it rests loose on top. The trolley carries on down the ramp in a straight line. When it hits the barrier, the barrier exerts a force on the trolley to stop the trolley. We would call this an external force, a force external to the system of the trolley and marble. Since the force acts on the trolley and the marble is not physically attached to the marble, the marble is unaffected by the external force and carries on as before in a straight line. The marble is not thrown forward by the collision; it merely continues with what it was already doing, completely unaware that a force has acted on the trolley. This is why you should wear a seat belt. It demonstrates the property of mass called inertia, as expressed in Newton's First Law. Objects with mass carry on in a straight line with a steady speed unless an external force acts on them.