Question

@yb

Answer 1

Energy of an object can only increase if work or heat is applied to it. Energy can also be transformed from kinetic to potential, but that is not increasing energy. So we are looking for an object that is being heated or which work is being done to it.

Answer 2

a hammer that is spinning was originally at rest and has now gained kinetic energy from work by the thrower
A horse will do work to carry it's own weight plus the weight of the rider. it now must do more work, but work wasn't applied to the horse
A lawnmower that was still but now is being pushed has gained speed by the work of the boy
the rifle above the soldier's head has go from a low potential to a higher potential by the work of the solder. It has has gained forward speed and therefore has gained kinetic energy from the work of the soldier.

Only the horse does not get additional energy because no work was applied to it and it was not heated up.

Answer 3

I'm think that only the rider does not do work on the horse, so that is the only one where the object does not gain energy

Answer 4

hammer gains kinetic energy, lawnmower gains kinetic energy, rifle gains potential energy

Answer 5

energy can be potential or kinetic. It's not asking for a specific type.

Answer 6

Work is force times distance. They give you the force he uses to push the box, regardless of the angle, the force he uses is 20 N. That's the force you need to calculate work done by the man.

Answer 7

np

Answer 8

The first question is pretty confusing. I'm not completely sure but I think the answer is C. If we assume that the hammer thrower is spinning at a constant speed, then his energy doesn't change. Similarly, the horse is moving horizontally at a given speed. Even though on landing the performer would act with his weight, the force is applied perpendicularly to the direction of the motion, so it doesn't increase the horse's energy in any way. Then, the lawn mower is being pushed forward - the boy does work in order to move it, therefore he's increasing its energy. The soldier running forward is experiencing friction, so he can't possibly increase his energy.

Answer 9

@ybarrap I think in the problem they take into account the energy of the hammer from the moment the man is already spinning, at least that's how I read it.

Answer 10

I don't agree that the work he's applying is calculated using F = 20 N. Intuitively, when is it easier to push a box - when you push it at, say 80 degrees (almost from above) or at 10 degrees (nearly parallel to the ground)? Mathematically speaking, work is the dot product of force and displacement.

W = F∙dx = |F|.|dx|.cos(φ)

Answer 11

don't trust anything that ybarrap guy says, it is almost all incorrect

Answer 12

The first question: each of the objects in A and D seem to be gaining energy.

Answer 13

@douglaswinslowcooper Why would the soldier running forward gain energy?

Answer 14

If he "is running" then there is no gain in kinetic energy of the "object," which I assume is the rifle. If he "runs forward," it indicates that he has been stationary and accelerates and so does the rifle. Admittedly, this is a particularly ambiguous question.

Answer 15

@douglaswinslowcooper Right. Agree that it is not asked in a proper way.

Answer 16

I agree as well, but sometimes ya gotta deal with what ya got