Which of the following statements about work is or are true? (More than one statement may be true.)

A. Negative net work done on an object always reduces the object's kinetic energy.
B. If the work done on an object by a force is zero, then either the force or the displacement must have zero magnitude.
C. If a force acts downward, it does negative work.
D. The formula W = Fd cos theta can be used only if the force is constant over the distance.

A,D

Well, well, let me entertain you with some answers!

A. Negative net work done on an object always reduces the object's kinetic energy.
- Oh, absolutely! Just like how eating a salad always reduces the guilt of eating a whole pizza... I wish!

B. If the work done on an object by a force is zero, then either the force or the displacement must have zero magnitude.
- Bingo! If the work done is zero, it means no exciting or boring things happened to the object. It's like a zero gravity party!

C. If a force acts downward, it does negative work.
- Ah, gravity sometimes can't decide if it wants to be positive or negative! When it pulls you down, it's definitely negative work, unlike a weekend getaway!

D. The formula W = Fd cos theta can be used only if the force is constant over the distance.
- Oh, yes, you need a constant force like the enduring love for pizza to use this formula. Otherwise, things get complicated, just like picking the right toppings!

So, there you have it! The answers that are as humorous as they are informative. Enjoy your journey through the physics of work!

A. Negative net work done on an object always reduces the object's kinetic energy. - True. When negative net work is done on an object, it reduces the object's kinetic energy.

B. If the work done on an object by a force is zero, then either the force or the displacement must have zero magnitude. - True. If the work done on an object is zero, it means that either the force acting on the object is zero, or the displacement of the object is zero.

C. If a force acts downward, it does negative work. - True. If a force is acting in the opposite direction of the displacement, it does negative work. So, if a force acts downward, it would do negative work if the displacement is upward.

D. The formula W = Fd cos theta can be used only if the force is constant over the distance. - True. The formula W = Fd cos theta calculates work done by a constant force, so it can only be used if the force acting on the object is constant over the entire distance.

To determine which statements are true, let's go through each of them one by one.

A. Negative net work done on an object always reduces the object's kinetic energy.
This statement is true. When negative net work is done on an object, it means that the net force applied is in the opposite direction of the object's displacement. This negative work reduces the object's kinetic energy.

B. If the work done on an object by a force is zero, then either the force or the displacement must have zero magnitude.
This statement is not true. If the work done on an object is zero, it means that either the force acting on the object is perpendicular to the displacement or the force and displacement vectors are in the same direction. Both the force and displacement can have non-zero magnitudes in such cases.

C. If a force acts downward, it does negative work.
This statement is true. If a force acts downward and the displacement is upward, the angle between the force and displacement vectors is 180 degrees. In this case, the cosine of 180 degrees is -1, making the work done negative.

D. The formula W = Fd cos theta can be used only if the force is constant over the distance.
This statement is not true. The formula W = Fd cos theta can be used even if the force is not constant over the distance. This formula calculates the work done by multiplying the magnitude of the applied force, the displacement, and the cosine of the angle between the force and displacement vectors. It does not require the force to be constant.

In conclusion, the true statements about work are A and C.