# B Frame of reference in which Newton's first law is not valid

1. Sep 13, 2016

### parshyaa

Newtons first law of motion depends on frame of reference
• So what are some examples , which shows that in this frame of reference F = 0, but a is not equal to zero or vice versa.

2. Sep 13, 2016

### jbriggs444

Suppose that you are on a rocket out in deep space. The engines are firing so that the craft experiences one gee of acceleration. You hold out your hand and drop a pencil which, from your accelerating point of view, falls and hits the floor. There is no force on the pencil, yet it accelerates. This is a case of an "accelerating frame of reference".

3. Sep 13, 2016

### A.T.

If F includes the inertial forces in an non-inertial frame, then Newtons 1st and 2nd still apply (that's what inertial forces are introduced for). Otherwise any non-inertial frame is an example where they don't apply.

4. Sep 13, 2016

### parshyaa

Thank you so much. I wonder you said "deep space" so that gravitational force is 0.

5. Sep 13, 2016

### jbriggs444

Yes, that is why I put you in a rocket rather than in a chair sitting at your desk.

As A.T. hints, there is a sense in which sitting at your desk and sitting in a rocket are equivalent. Gravity need not be regarded as a force but merely a result of the fact that we choose to consider the surface of the earth to be continuously at rest. But I did not want to complicate a simple question with a subtle answer.

6. Sep 13, 2016

### Staff: Mentor

The other most common non-inertial frame is a rotating frame. An inertial particle will accelerate, leading to the use of the centrifugal force and the Coriolis force to describe the motion.

7. Sep 16, 2016

### zanick

now wait a min. if you are in a rocket and drop that pencil, it doesnt accelerate anymore .. it falls to the rear of the rocket and sticks to the rear wall with 1g of force.

8. Sep 16, 2016

### jbriggs444

It does accelerate -- as measured against the accelerating frame of reference in which the rocket is motionless.
It does not accelerate -- as measured against an inertial frame in which the pencil is motionless.

Both are true.