No work is done by a force on an object if
a) the force is always perpendicular to its velocity
b) the force is always perpendicular to its acceleration
c) the object is stationary but the point of application of the force moves on the object
d) the object moves in such a way that the point of application of the force remains fixed.

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all are correct

a, b & d are correct

a, c & d are correct

b, c & d are correct

answer is C.

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Detailed Solution

W = F d \cos θ

.
For (a), θ = $90^{0}$

W = 0
For (d), displacement

W = 0
For (c), point of application is not fixed relative to the object, hence

if the surface of the object is smooth and in this case W = 0

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