# Inertial adn non-inertial frames

1. Apr 13, 2012

### jd12345

i know these topics are discussed many times and i have read many of them but still have a doubt.

Suppose a car is accelerating and there is an object and a man in the car. Object is ofcourse at rest with respect to man. The man does not know if the car is accelerating or not - he just sees the object at rest. So for him the car is inertial frame and he can apply newtons laws there . How would he know if he is in an non-inertial frame?

2. Apr 13, 2012

### pcm

in car frame frictional force is acting on object ,still the object is at rest.so,it is obviously a non inertial frame.
for example, if he drops a ball it will not fall vertically downwards.

3. Apr 13, 2012

### A.T.

By applying Newtons laws and comparing to reality. If applying Newtons laws (using only interaction forces) doesn't match experiments, then he knows is in an non-inertial frame.

4. Apr 13, 2012

### Creator

Hate to tell you this jd, but even if the car is NOT accelerating, he is still in a 'non-inertial' frame simply because his butt is pressing against the seat due to gravity.

In addition if the car is accelerating horizontally (down the street) as you have said,then he can easily tell simply by hanging the object on a string from the ceiling of the car. If the object does NOT hang vertically, then he and the car are accelerating horizontally.

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