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When we move forward in a car, things around us move backward. I understand some force is being applied on the car that is the reason for its forward movement. But what is the source of force on the things moving backward that we see when moving forward? If I am accelerating, things outside my car are also accelerating, so there must be some forces in action on the outside things. Or not? If yes what are those forces?

gpuguy
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    You are asking about fictitious forces. They are a bit confusing, in part because of the poor name. Here are a couple posts where I have talked about them. https://physics.stackexchange.com/q/93599/37364 https://physics.stackexchange.com/q/509288/37364 – mmesser314 May 16 '20 at 03:47

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In your first example if the car is moving forward at constant velocity then the net force on the car is zero. As such the car is an inertial (non accelerating) frame of reference. As a good approximation the road (earth) can locally be considered an inertial frame as well. It is equally valid to say it is the car that is moving forward in the reference frame of the road as to say it’s the road that is moving backward in the reference frame of car. That’s because all inertial frames are equivalent.

In your second example, however, the car is accelerating forward but the road is not accelerating backwards. The road only appears to be accelerating to the person in the car because the person is in a non inertial (accelerating) reference frame. In a non inertial frame a pseudo (fictitious) force is needed to explain the apparent acceleration of the road.

Hope this helps,

Bob D
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The key idea is frames of reference. The ideas of Physics are always related to a particular observer, and they mostly work best if the observer is in an "inertial frame of reference", ie the observer is not being accelerated (or experiencing an unbalanced force). An observer who is not in an inertial frame of reference will see effects which are actually caused by their own acceleration, but will often try to explain them as being due to what we call fictitious forces.

If you choose a frame of reference of a person standing on the ground watching a car accelerate, you see the car accelerate, but you yourself experience no force and you don't see other things accelerating on their own.

If you are sitting in the car you experience a force on your back (from the car accelerating). This means you see other objects accelerating (backwards) with no detectable force on them. I am not aware of any name for this particular fictitious force.

Peter
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