CBSE Class 11-science Answered

A non-inertial frame of reference does not have a constant velocity. It is accelerating. There are several ways to imagine this motion:

• The frame could be traveling in a straight line, but be speeding up or slowing down.
• The frame could be traveling along a curved path at a steady speed.
• The frame could be traveling along a curved path and also speeding up or slowing down.

This is really quite easy to understand. If you are in an automobile when the brakes are abruptly applied, then you will feel pushed toward the front of the car. You may actually have to extend you arms to prevent yourself from going forward toward the dashboard. However, there is really no force pushing you forward. The car, since it is slowing down, is an accelerating, or non-inertial, frame of reference, and the law of inertia no longer holds if we use this non-inertial frame to judge your motion.

If all of this is viewed relative to the ground, it becomes clear that no force is pushing you forward when the brakes are applied. The ground is stationary and, therefore, is an inertial frame. Relative to the ground, when the brakes are applied, you continue with your forward motion, just like you should according to Newton's first law of motion. The situation is this: the car is stopping, you are not; so, you head out toward the dashboard. From your point of view in the car it seems like you have spontaneously been pushed forward. Actually, there is no force acting on you. The imagined force toward the front of the car is a fictitious force.

A similar fictitious force can be noticed by a person in a car when it speeds up. Let us say that you are in a car at a stop light. The car is standing still. The light turns green, and the car accelerates forward. While undergoing this acceleration, the car is a non-inertial frame of reference. If the acceleration is large enough, you will feel yourself "pushed" into the seat. Actually, no force is pushing on you. Again, as viewed from the inertial frame of the ground, you are just maintaining your velocity, as you should according to Newton's first law of motion. You were still when the light was red, and you are attempting to remain still when the light turns green. However, the car started to move when the light turned green. The car actually comes up from behind you, and, using the seat, the car pushes you forward. As the seat comes forward and pushes on you, the back seat cushion compresses a bit. You may interpret this feeling as your body being pushed backward into the seat. Really, you are attempting to maintain your velocity of zero, and the seat is coming up from behind to push on you. There is no backward force. The imagined force is a fictitious force. Fictitious forces arise in non-inertial, or accelerating, frames of reference.