Physics Help Forum Force acting in a non inertial frame of reference.

 Nov 5th 2017, 06:49 AM #1 Senior Member     Join Date: Feb 2017 Posts: 205 Force acting in a non inertial frame of reference. A non-inertial frame of reference does not have a constant velocity. It is accelerating. Lets take an example of a non inertial frame of reference or an accelerating frame of reference. Lets say the frame is traveling in a straight line, but speeding up or slowing down. Lets say a ball is thrown horizontally towards a person, now the ball slows down before reaching the other person, could this be a non inertial reference frame? If so how? In a non inertial reference frame newtons law of inertia does not hold which means that the ball is decelerating without a force. This might seem so. But actually the force of friction is causing the ball to slow down. So how can this be a non inertial reference frame? Newtons first law states " An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an external force." So here the speed of the ball slows down but it is due to friction. A force acting on the ball.
Nov 5th 2017, 09:36 AM   #2
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 Lets say a ball is thrown horizontally towards a person, now the ball slows down before reaching the other person, could this be a non inertial reference frame? If so how?

Could what be an inertial frame?

Also remember that all bodies are 'at rest' in a frame of reference co-moving with them.

Nov 5th 2017, 10:04 PM   #3
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 Originally Posted by studiot Could what be an inertial frame? Please be specific. Also remember that all bodies are 'at rest' in a frame of reference co-moving with them.
Assume you are a lilliput and you sit above the ball that is thrown horizontally. The ball is thrown and later its velocity decreases due to friction, so is this a non inertial reference frame since velocity is not constant?

Nov 20th 2017, 03:36 AM   #4
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 Originally Posted by avito009 Assume you are a lilliput and you sit above the ball that is thrown horizontally. The ball is thrown and later its velocity decreases due to friction, so is this a non inertial reference frame since velocity is not constant?
Reference frames are not defined based on the behaviour of the objects inside them. It is totally fine to have an accelerating object within an inertial frame.

In your case above, the situation you describe of a decelerating ball can be described either in the context of an inertial frame or not. If it is described in the context of an inertial frame, you can just use Newton's laws and you'll have no issues describing the motion of the ball. If you use a non-inertial frame (e.g. a linearly accelerating one), you will only get the correct answer if you use additional terms in your equations, terms which depend on the acceleration of your non-inertial frame.

Last edited by benit13; Nov 20th 2017 at 03:41 AM.

 Nov 20th 2017, 05:13 AM #5 Senior Member     Join Date: Jun 2016 Location: England Posts: 1,011 One of the ways Physics works is to try to disentangle the effects of the very many causative actions that apply to any observed situation. By carefully minimising different causative actions in different experiments, the effect of each can be individually assessed and then combined to make the full model. Note that the inertial model purposely excludes a large number of "real world" causative actions. It is assumed that the modeller has examined the situation being modelled and has determined that an inertial model is satisfactory for their purpose. __________________ ~\o/~

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