Question

When you are standing on a train and the train brakes suddenly, you need to stop yourself from falling over. Explain what is happening According to physics, objects cannot apply forces on themselves. So how does a car accelerate? How does a parachute let you fall great heights without serious injuries?

Answer 1

Ans.

1) When the train brakes, you are still moving forward. The force(brakes) acts on the train to stop it (it's not acting on you)> To change your motion there must be net force on you. You provide the force yourself by hanging on to something. So you need to stop yourself from falling over.

2) A car presses down on the road(ground) which in return pushes the car forward. Thus the car accelerates.. So the force acting on the car due to ground accelerates it.

3) An falling abject accelerates 10 m/s every second it falls. Air provides resistance to its downward motion and after sometime the net force on the object becomes zero. After that it moves at constant speed. A parachute provides huge air resistance, thus it takes very small amount to make net force zero on you, so you have very small constant speed(compared to no parachute) at which you fall. Hence a parachute let you fall great heights without serious injuries.