Question

(a)? Some people say that the “force of inertia” (or “force of momentum”) throws the passengers forward when a car brakes sharply. What is wrong with this explanation? Which Newton’s law can be used to explain what really happens?

(b)? Can we say that the earth is approximately an inertial reference frame? Explain your reasoning.

(c)? Imagine a coin. If you throw it in the air, it will go straight up and at its highest point, its velocity will be zero. Is the coin in equilibrium at this point? Explain your reasoning.

Answer 1

(a) It is the inertia of motion, which is the inabity of a body to change it's state of motion by itself, that causes the person to throw forward when a speedy vehicle applies brakes suddenly. Newton's first law can be used to explain this phenomenon, which is also called the law of inertia according to which unless the application of a force a body at rest or moving with uniform velocity can not change its state by itself. This property is called inertia. When the speedy vehicle stops suddenly the lower part of the body of a passenger which is in touch with the vehicle comes to stop along with the vehicle but the upper part of the body which is not in contact with the vehicle, due to inertia of motion, will tend to move with the same speed for which the person is thrown forward to compensate for this.

(b) An inertial frame is one which is not undergoing any acceleration. The earth is rotating about its own axis as well as revolving around the sun for which it is under the action of a centripetal force and hence an acceleration always. So strictly speaking earth can not be an inertial frame. But as the speed of earth around sun is of the order of 10⁴ m/s which is very small as compared to speed of light, so for a small time interval effect of rotation and revolution can be ignored and speed of Earth can also be taken constant. Hence approximately Earth can be taken as an inertial frame of reference.

(c) A body is said to be in equilibrium when the net force acting on the body is zero. At the highest point the coin is still acted upon by the force of gravity which tend to pull the coin towards Earth surface. Hence the coin can not be in equillibrium.