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Centre of mass of a system cannot change its state of motion, unless there is an external force acting on it. Yet the internal force of brakes can bring a car to rest. Then what stops the car?

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Using your terminology, the brakes aren't a "internal force". You can think of the wheels as attached to the road rather than attached to the car. The brake pads are attached to the car and rub against the wheels attached to the road. –  Brandon Enright Dec 22 '13 at 5:55

1 Answer 1

Friction. The wheels are one system, the brakes are another. Relative motion between the wheels and the brakes is what causes friction, and this reduces the angular speed of the wheels. A reduction in the angular speed of the wheels requires the car to slow down too, assuming the wheels don't slip.

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if we consider our system to be (Car+wheels+breaks) then without skidding of tires how the system comes to rest with only internal friction forces? –  user31782 Dec 22 '13 at 12:50
    
Who said the entire thing is one system? I'm saying the wheels are one system, the brakes are the second system –  Pranav Hosangadi Dec 22 '13 at 18:38
    
could you explain my point in your answer ,for i have the same question in my mind as of OP depicted. –  user31782 Dec 23 '13 at 12:57
    
@anupam, see Brandon's comment to the original question above. –  Pranav Hosangadi Dec 24 '13 at 18:33
    
Should i ask the same question as that of OP by mentioning that the system is (Car+wheels+breaks)? –  user31782 Dec 24 '13 at 18:39

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