A) The work-energy principle is not valid if there are frictional forces acting.

Question

A) The work-energy principle is not valid if there are frictional forces acting. B) The work done to raise a box onto a platform depends on how fast it is raised. (Neglect friction, assume it starts on the ground at rest, and ends up resting on the platform) C) The kinetic energy can be positive or negative. D) The kinetic energy of an object is independent of the direction of the motion involved. E) The work done by a constant force acting on an object depends on the orientation of the force relative to the motion of the object F) Work can be done in the absence of motion.

Explanation / Answer

Solution:

A) False Work energy principle is valid even when there is friction.

Example: If an object is moving with some initial velocity and is brought to rest due to friction , in a distance d,

Net force = friction force * distance = -fd (negative since it is friction)

according to work energy theorem, W= change in KE

=> Change in Kinetic energy = -fd

B) FalseThe work done in raising a box onto a platform depends on the height to which it has to be raised.It is given by the potential energy since work has to be done against gravity in lifting it. Work to lift = mgh

C) False The kinetic energy is always Positive. It is 1/2 mv^2, where m is the mass and v is the velocity of the object.

But Change in Kinetic energy can be positive or negative.

If an object has its initial velocity > final velocity , change in kinetic energy will be negative.

D) Yes Energy is a scalar. Kinetic energy depends only on mass ans velocity of object but not on its direction. True

E) Yes Work depends on orientation of force with respect to displacement. W = (Fcos theta )* d . TRUE

F) NO work can be done in the absence of displacement. False

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