Question

True or False, please answer only if you are sure:

A) The work done by a constant force acting on an object is independent of the orientation of the force relative to the motion of the object
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 of a moving object is always positive.
D) The kinetic energy of an object is independent of the direction of the motion involved.
E) Work cannot be done in the absence of motion.
F) The work-energy principle is perfectly valid even when there are frictional forces acting

Answer 1

A) the kinetic of an object is independent of the direction of motion involved.

TRUE...KE = 1/2 mV^2; where V is the magnitude of the velocity.

B) THe work done to raise a box onto a platform depends on how much time it takes to raise it . (neglect friction, assume it starts on the ground at rest, and ends up resting on the platform)

FALSE...WE/t = Power = mgh/t; where WE is the work and t is the time. Power, not work, depends on the time interval over which work was expended. The work WE = mgh depends only on the mass m, g, and the height h the box was raised.

C)The work-energy principle is perfectly valid even when there are frictional forces acting.

TRUE...in fact this is the normal case, as friction is always there. Thus there is no such thing as a perpetural motion machine.

D) The kinetic energy can never be negative.

TRUE...sort of. If total energy TE = KE + PE + WE; we can find the potential energy PE = TE - KE - WE in which case there is a minus sign in front of the kinetic energy term KE. But, and this is a big BUT, we are simply subtracting a positive KE from the total energy. KE = 1/2 mV^2 >= 0 because m and V^2 are always positive.

E)Work cannot be done is absence of motion.

TRUE...sort of. Mechanical work cannot be done without motion. The work function WE = FS; where F is force and S is the distance pushed or pulled by that force. However, work is done when chemical change occurs Unless we count the molecular level of motion, there is no motion in the traditional sense of mechanical work.

F)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.

TRUE. In WE = FS; the force in question is aligned with the distance moved. For example, if we pull a box with a force P at 30 deg above the horizon, F = P cos(30) to algn the pull force F with the distance the box slides along the surface.