Answer to Question #144217 in Mechanics | Relativity for Mikkos Joshua Garcia

Question

1. You are driving a car at a red stoplight. A small stuff toys hangs from your rear-view mirror. As you wait inside the car, the stuff toy is at rest relative to you. Gravity pulls the stuff toy down while the string pulls it up. These two forces are equal, resulting to a zero total force. The stoplight signals "Go" and you step on the gas. The stuff toy swings towards the back of the car. Is Newton's First Law applicable on the stuff toy?
2. Let say, an observer is standing on the sidewalk watching your car in number 1. As it sits at the red light the stuff toy is stationary relative to you. When the light turns green and the car begins to accelerate to the right, the stuff toy remains stationary relative to you until the force exerted on them by the string forces them to follow the motion of the car. Is Newton's law of inertia tru to the observer? What about to you who is sitting in the car?

Answer 1

1. No. Newton's First Law is not true: at one moment stuff toys are stationary and at the next moment they begin to swing towards the back of the car although no net forces are acting on them.
2. When the light turns green and the car begins to accelerate to the right, stuff toys remain stationary relative to an observer until the force exerted on them by the string forces them to follow the motion of the car. So Newton's First Law is true for the observer stand on the sidewalk, but is not true when you are sitting in the car.