Explain the followings :-
1.work done by the braking force when brakes are applied to a vehicle-negative work
2.work done by friction-negative work

Dear Student ,
                          we know that work  always done by force. By force we can move body
                         and by force we can stop body .we know  work done said positive if displacement 
                        of body is in same direction as force but if force is in opposite direction to the 
                        displacement then this work done is called negative work done
                         we know friction force(sliding friction)  always opposite to the direction  of motion of body
                       in case of braeking there is sliding friction. that is why work done in breaking is negative 
                       same argument is correct for second also.
Regard