Question

If you jump inside of a moving elevator, does the mass of the elevator lessen?

Answer 1

Draw a free body diagram of the elevator cage.

There are a number of scenarios:

Elevator is moving with constant speed
Elevator is accelerating

Person is standing unmoving on floor
Person is 'jumping' i.e. in the process of pushing themselves into the air
Person is in the air
Person is 'landing' i.e. in the process of absorbing the jump in their knees.

Remember that every force has an opposite reaction
Remember that acceleration requires a force
Remember that constant linear motion (i.e. no acceleration does not require a net force)

It is also interesting to note that your question has a different answer to "if a bird flies off a perch in a moving elevator...."

Answer 2

The scenario is, elevator is moving with constant speed, and person is in the air. I forget how to draw a FBD - I haven't taken physics in many years now. I was just curious

Answer 3

Why is it different if a bird flies off a perch in a moving elevator? It seems like the same thing, but with a smaller mass at work here.

Answer 4

At the instant the body is in the air - does he exert any force on the elevator floor?

Whilst he is standing on the floor does he exert any force on the floor?

Answer 5

The reason it is different is because a person who has jumped will come down again.

A bird flying is in continuous motion and hence must be experiencing an upward force to keep it in the air.

Answer 6

Hmm..when the body is in the air, no, he is not exerting any force on the elevator. When he is standing on the floor, he is exerting force on the floor.

Answer 7

Therefore, I will conclude that during the time the body is in the air, after he/she has pushed themselves off the ground, the mass of the elevator lessens by the amount of the mass of the person

Answer 8

While in the case of a bird flying off a perch, as you said, there is a continuous downward force produced by the bird to keep it in the air, therefore, the mass of the elevator remains continuous

Answer 9

If the elevator is in constant motion then it is the SAME as if it is stationary.

If he was standing on scales on the floor of a building - then jumped - the effect is the same.

Your conclusion is correct.

Answer 10

Oh I see, excellent, thanks MrNood!

Answer 11

To be a little pedantic - it is the WEIGHT that changes - I.e. the force required in the cable to support the elevator.
The mass of the total system does not change.