As per newton's first law, only an external force can bring any change in the acceleration of a body, internal forces cannot. So, when we apply brakes to an accelerating car, aren't those brakes(opposing force) part of internal forces? How can they put the car to a halt?


The car is slowing due to the force exerted by the road on the tyres. All the brakes do is transfer that force from the tyres to the body of the car.

  • $\begingroup$ That means if the road were friction-less, the brakes would overturn the car? $\endgroup$ – Swami May 29 '14 at 5:45
  • 3
    $\begingroup$ No, if roads were frictionless, I guess, cars would never have been invented ;) $\endgroup$ – tpb261 May 29 '14 at 5:53
  • $\begingroup$ hypothetically, what would be the answer? Friction exists between car-parts but not between road-car. $\endgroup$ – Swami May 29 '14 at 6:01
  • $\begingroup$ @Swami Without friction, there would be no way to get the car moving nor to change its state of motion using driven wheels/brakes. Jet motors and aerodynamic forces to start / stop would then be the only option. $\endgroup$ – Selene Routley May 29 '14 at 6:08
  • $\begingroup$ @Swami: if the road were frictionless the brakes would not be able to slow the car. If you have ever driven on ice you would have experienced exactly this phenomenon (and I have to say it's quite scary :-). $\endgroup$ – John Rennie May 29 '14 at 6:11

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