I know that when a force is exerted on an object by another one which causes it to move, work is performed. As a result energy is transferred to the object by the force from other object.

But consider friction, which does negative work. (As mentioned above, when a body A exerts a force on another body B energy is transferred from that body A to B.)

From where does heat energy come from when (say) the floor exerts the friction force on a body? By definition the floor should transfer its energy to the body.

  • $\begingroup$ The kinetic energy of the block is lost and is converted to heat. Heat energy does not "come" from anywhere. It goes into the ground $\endgroup$ Jan 3 '19 at 21:04
  • $\begingroup$ Heat is generated only if there is relative sliding between the surfaces. $\endgroup$ Jan 3 '19 at 23:14

When body A applies a force to body B in the same direction as the motion of B in the absence of friction, work is positive causing a positive change in kinetic energy of body B that equals the work done by body A (the work-energy principle).

Now let’s say the force is removed so that now body B in moving at constant velocity (constant kinetic energy). Body B then encounters floor friction. The direction of the floor friction force is opposite the direction of motion of body B, so work is negative. The negative work done by the floor causes a negative change in kinetic energy eventually bringing body B to a stop. The kinetic energy of body B is converted to friction heat on the contacting surfaces between body B and the floor.


I am confused about Newton's third law in this ,what force are considering this , the one which floor exerts on box or which box exerts on floor ( I read somewhere that the reaction force of friction is box applying same resistive force on surface )

To respond to your above comment on my answer:

The effect is due to the friction force that the floor exerts on the box causing it to decelerate (slow down) losing kinetic energy and generating heat. From Newton’s third law the friction force on the box due to the floor is equal and opposite of the friction force on the floor due to the box. However, from Newton’s second law, $a=\frac{F}{m}$. The floor being fixed in place resists any acceleration due to the friction force imposed on it by the box. The force the floor exerts on the box, however, causes it to decelerate.

Hope this helps.


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