[marshallch]

My daughter is 9 and doing a science project with free fall on the trampoline. She wants to attach a scale to herself and see what she weighs when on the trampoline and what she weighs when in the air or microgravity. I assume this all has to do with Newton's 2nd law of motion. However when I read all the physics stuff, I get confused. Can anyone give me a nice clear definition of Newton's 2nd law of motion and how it related to free fall on the trampoline--I need it to be able to be understood by a 4th grader. Thanks, Cari

[jiten55]

Newton' Second Law

Application of external force on a body causes acceleration in the latter that is proprtional to the applied force.

Remember external force when falling is mg (mass times g, acceleration due to gravity).

Acceleration when falling = g (32 ft/sec^2)

This means that the velocity of a freely falling body continuously increases at the rate of 32ft per second.

[marshallch]

Ok, thanks. Can you give me some good examples of this and how I might explain it on a fourth grade level?

[jiten55]

Gravity is due to another Law, Law of gravitation which says that every body attracts every other body with a certain force.

In case of a falling body, the external force is the attraction by the Earth.

This external force acts on a freely falling body which falls because of this force.

According to Newton's Second Law, the external force causes acceleration of the body, which is the change in velocity of the body.

As a consequence, the velocity of falling body keeps increasing all the time as it falls.

As to checking what she weighs when falling: This may not work.

Reason: Weighing scales indicated pressure on the scale while a person is standing on the scales. When both are falling, pressure on the scale will be zero!

I hope some of the above will be clear to the student.

[PolluxCastor]

The weight while in the air is the easy part, as stated by Jiten55, that is zero.

The hard part is measuring the weight while in contact with the trampoline, the further the trampoline is indented the more force it exerts upward (the weight is constantly changing while in contact with the trampoline).

Maybe the maximum weight could be measured.

Maybe try this, get a fish scale and put some weight on it (something soft so it won't hurt if it comes off and lands on a foot). Then videotape the scale as she is jumping on the trampoline. That could give readings every 1/30th of a second.

I'd be interested in the results.

[marshallch]

We did the experiment and just measured her weight in free fall with a spring scale which weighed zero in free fall but 75 on the ground. We are saying her weight (as defined in physics with force) is zero but mass stays the same. My husband took several pictures so we can put those on our display as well as the scale which has the tennis shoes (an old pair) ducted taped to it.
Thanks for all your help. Cari:)

[trampolines]

Well nice but I want to know more about trampolines and all thanks