Question

state two condition where workdone by a force is zero

Answer 1

when we push a wall

wh n we carry load perpendicular to body or on head

Answer 2

A force with no motion or a force perpendicular to the motion does no work. Case 1 : When a force is exerted on an object which does not move,no work is done on the object. Explanation : No matter how hard or how long you have pushed, if the object does not move, then you have done no work on the object. It is sitting still in the same place! Why then do you feel fatigued (as well as frustrated, etc. ). The resolution to this dilemma comes in considering that when your muscles are used to exert a force on something, the individual muscle fibers are in a continual process of contracting and releasing to maintain the net collective result of a steady force on an external object. That contracting and releasing involves force and motion, and constitutes internal work in your body. The energy shows up as warming in your muscle tissue, but if the object doesn't move, there is still no net work on the object. ---------------------------------------...
Case 2 : When an object is carried by the person at constant velocity by a force which acts at the right angles to the motion,no work is done on the object. Explanation : If the object is being carried at constant velocity, then no net force is necessary to keep it in motion. The force exerted by the person is an upward force equal to the weight of the object, and that force is perpendicular to the motion. If there is no motion in the direction of the force, then no work in done by that force. Yet you certainly feel like you are doing work if you carry a heavy box. The resolution of the paradox is similar to the first example - your muscles must maintain an extra tension to stay upright under the load. This requires a greater amount of internal contraction and release of our muscle fibers, and hence internal work in our bodies. But the work done on the object is zero since by moving in a straight line at constant speed, it's energy is remaining the same.