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paulb203

Member

Registered: 2023-02-24

Posts: 422

Physics; conservation of energy

I’m working my way through Khan Academy’s High School Physics and I want to check that I’m getting the concept of conservation of energy. Is the following at least roughly correct?

When a ball is rolled down a U-shaped ramp from the top of one side it will, ignoring friction, travel to the bottom of the ramp, up the other side to the top, back down , and up the first side to where it began, etc, etc, ad infinitum.

The U is converted to K as the ball descends, and the K is converted to U as it ascends.

At the respective tops the U ‘tank’ is full. At the bottom it is empty.

At the tops the K ‘tank’ is empty. At the bottom it is full.

Halfway down, or up, both tanks are half full.

**
When rolling it down, and taking friction into account, it won’t get quite to the top of the other side as some of K will be converted to thermal energy and sound energy. It will get part the way up the other side, to position h, descend, and then part the way up the first side, but not as far as h, etc.
The total energy at the outset = the total energy once the ball has come to rest at the bottom of the ramp. If there was x Joules of U(GPE) at the beginning there is x Joules of energy now, but in different forms.

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"The secret of getting ahead is getting started."
Mark Twain

Bob

Administrator

Registered: 2010-06-20

Posts: 10,774

Re: Physics; conservation of energy

Not roughly correct; but rather, completely correct. The stored gravitational energy at the top is called potential energy and the energy of movement is called kinetic energy.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

paulb203

Member

Registered: 2023-02-24

Posts: 422

Re: Physics; conservation of energy

Thanks, Bob.

I’m struggling a bit with the concept of kinetic energy.
Having always thought of energy as THE CAPACITY TO DO WORK (or cause change) to discover that kinetic energy is the energy an object has BY VIRTUE OF ITS MOTION puzzles me.
If I’m sitting at home, with potential chemical energy from the food I had earlier, I have the capacity to do work, e.g., go for a walk.
When I go for the walk I think of it in terms of spending that potential energy, by doing some work.
But I’m now told that energy can’t be destroyed, that it has to go somewhere, in this case, converted into kinetic energy. It seems more intuitive to me to think of it being converted into work. I had energy, and I spent it doing some work. And the more work I do, the less energy I have.

I’m not questioning it, as such, just trying to get my head around it.

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"The secret of getting ahead is getting started."
Mark Twain

Bob

Administrator

Registered: 2010-06-20

Posts: 10,774

Re: Physics; conservation of energy

Work done and energy are different ways to measure the same thing.

If a mass, m, is accelerated from rest to a velocity v then (using v^2 = u^2 + 2as)

  (u=0)

= force times distance moved.

If an object drops from rest under gravity a distance h, then

mgh is a measure of its potential energy (PE) before it is released.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob