# Blocks A & B

Bama
Find the tension ( stretching force) on the string whie system is accelerating. I know that the pull on block "A" for acceleration is gm2 / m1 + m2. so what is the source of the kinetic energy store in block "A".

#### Attachments

• block A & B.doc
19 KB · Views: 149

Mentor
Apply Newton's 2nd law to both blocks. That will allow you to solve for the acceleration and the tension in the string. (I assume that pulley in the middle is frictionless and massless; if not, analyze it as well.)

The "source" of the KE of both blocks is the gravitational PE that is transformed into KE as the block falls.

Bama
To see if I under KE correct, let me give you an example of what i understand. We will use a television for example. Normally when you turn your television off, the power or energy is not there until you turn it on again, however if you have a standby option, would that stand by power be consider KE?

Homework Helper
? No, that has nothing to do with kinetic energy. Kinetic energy is the energy in a moving body: (1/2)mv2.

Last edited by a moderator:
Bama
So in other words, KE is like jump starting the energy in a moving body once it has been jump started, it is only energy until something cause it to stop abruptly of which PE is loss affect and and later transform back to KE. So the answer to my question in as far as the source of the KE in block A, should be

" As block "A" moves to the right, it losses PE that transfers to kinetic energy."

See attachment

Mentor
As block A moves to the right and block B drops down, the system loses gravitational potential energy (since B is lowering) and gains kinetic energy.

Bama
Here is what confusing me. There are choices that states, PE is loss as B is lower, however it is worded in such a way as all the PE loss is transferred to A as Kinetic Energy or only parts of the loss of PE is tranferred to block A as Kinetic energy. As I understand your comments is the Kinetic Energy gain in B?

Mentor
I'm not sure what you are saying. As B lowers, the loss of PE is transformed into a gain of the KE of the system (since they are tied together, B cannot speed up without A also speeding up--unless the string breaks). If the blocks weren't tied together, things would be different.

Bama
Doc, here is what I have. This problem seems simple but nerving as well. Here is the question and choices I am given. What you say makes a lot of sense however the choices I have are hard for me to understand. Question: What is the source of the kinetic energy stored in block A? Now I know it isn't number one but the others are a bit confusing for me. It seems as if it's a play on words but if it isn't, what key element should I have notice?

1) In falling, block B loss kinetic energy that transfers to block A.

2) As it moves to the right, block A loses potential energy that transfers to kinetic energy.

3) The PE assiociated with the block B is reduced as block B falls and all of the PE loss is transferred to block A as KE.

4) The PE assiociated with the block B is reduced as block B falls,but only part of this loss of PE is transferred to block A as KE.

Mentor
Bama said:
Doc, here is what I have. This problem seems simple but nerving as well. Here is the question and choices I am given. What you say makes a lot of sense however the choices I have are hard for me to understand. Question: What is the source of the kinetic energy stored in block A? Now I know it isn't number one but the others are a bit confusing for me. It seems as if it's a play on words but if it isn't, what key element should I have notice?
The key thing to understand is that this is a conservation of energy problem. Whatever PE the system loses will equal the KE that the system gains. Also realize that the system has two parts: block A and block B. Even though only one block actually falls, both blocks share the KE since they are attached.

1) In falling, block B loss kinetic energy that transfers to block A.
The blocks gain KE, so this one doesn't make sense.

2) As it moves to the right, block A loses potential energy that transfers to kinetic energy.
Block A moves horizontally so its PE doesn't change, so this one doesn't make sense.

3) The PE assiociated with the block B is reduced as block B falls and all of the PE loss is transferred to block A as KE.
OK, partly right: The PE of block B is reduced since B is the block that falls. But if all of the PE loss went into the KE of block A, then block B wouldn't move! So this answer can't be right. (The blocks share the KE.)

4) The PE assiociated with the block B is reduced as block B falls,but only part of this loss of PE is transferred to block A as KE.
That's the only one that makes sense.

Let me know if any of this is still confusing.

Bama
So with number 2 moving to the right should have never been consider because there isn't no left or right only horizontal and vertical. Do I feel like a fool on this one. No further explanation is needed but if you were me, where would you start over at? In order for me to eliminate or understand what I am being ask, I need to do what?

Mentor
Bama said:
So with number 2 moving to the right should have never been consider because there isn't no left or right only horizontal and vertical.
In your diagram, block A moving to the right is moving horizontally. So left-right is just another way of saying horizontal. What's wrong with answer 2 is that it states "block A loses potential energy": that's not true. If something only moves horizontally, its gravitational PE does not change.