16 Mini MCATs Physics #240

[sizillyd]

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I've attached a photo of the problem. This is #240 from Examkrackers 16 Mini MCATs.

The question asks: A 10kg block is moved from the ground to a height of 20 m. In which sections (A-D) is work being performed?

A. Work is done in all four sections
B. No work is done in any section
C. Work is done only in sections A and C
D. Work is done only in sections B and D.

In section A the block is moved 5 m to the right. In section B the block is moved 10 m up. In section C the block is moved 5 m to the left. Then finally, the block is moved 10 m up.

I thought work was accomplished in all sections because the displacement in all sections in positive.

The correct answer is D. In sections A and C, the object is moving parallel to the resisting force, so no work is performed. In sections B and D, however, the object is moving against the resisting force, so work must be done.

If I push a block on the ground, isn't the block moving parallel to the resisting force (friction)? I don't see how the first part is right. In the second part, "the object is moving against the resisting force, so work must be done". If something is moving AGAINST the resisting force, isn't it moving parallel to the resisting force? The explanations in the problem were extremely confusing. Can someone explain this to me a little better? Thank you very much.

 

[syoung]

Work in general definition, especially when it relates to gravity, is simply W=mgh (or the force through the distance). And this is for conservative forces (i think). The paragraph in that photo says this I think.
When you move in a horizontal direction, there's no force you're pushing against. Vertically, gravity attracts all objects towards the earth, so you must move it away from it.
With something like Friction like kyou are talking about, yes you are doing work against the friction force

 

[Melomare17]

on a side note--how good is the mini mcat as a study source?

 

[Buttafuoco]

I think what they're trying to relate here is that even though you need F*d to move the object parallel to gravity against friction, you haven't increased its internal energy, whereas when you move it a distance where it gains PE, you have. In A, you don't really get net work because every bit of energy you put into the block is dissipated by friction. I think the definitions and sign conventions can get pretty arbitrary, but whateva at least they give you a pretty clear definition of what they're looking for in the question and they aren't giving you an option for A, B, and D.

 

[MedPR]

I've attached a photo of the problem. This is #240 from Examkrackers 16 Mini MCATs.

The question asks: A 10kg block is moved from the ground to a height of 20 m. In which sections (A-D) is work being performed?

A. Work is done in all four sections
B. No work is done in any section
C. Work is done only in sections A and C
D. Work is done only in sections B and D.

In section A the block is moved 5 m to the right. In section B the block is moved 10 m up. In section C the block is moved 5 m to the left. Then finally, the block is moved 10 m up.

I thought work was accomplished in all sections because the displacement in all sections in positive.

The correct answer is D. In sections A and C, the object is moving parallel to the resisting force, so no work is performed. In sections B and D, however, the object is moving against the resisting force, so work must be done.

If I push a block on the ground, isn't the block moving parallel to the resisting force (friction)? I don't see how the first part is right. In the second part, "the object is moving against the resisting force, so work must be done". If something is moving AGAINST the resisting force, isn't it moving parallel to the resisting force? The explanations in the problem were extremely confusing. Can someone explain this to me a little better? Thank you very much.

Assuming that the force we are talking about is the one moving the object from the ground to 20m above the ground, then no work is done for any displacement perpendicular to that force (A and C).

When you push a box across the ground, the force of gravity is always acting, but there is no work done by gravity because the displacement is not in the direction of gravity.

 

[SaintJude]

Work done by a certain force is only in the direction of that particular force. The equation for work is W = Fdcos(theta).

When a person is pulling a block at an angle theta, you only count the force parallel to the displacement, b/c the "normal force" is basically not resisting the pull exerted by the person.

 

[aspiringdoc09]

I've attached a photo of the problem. This is #240 from Examkrackers 16 Mini MCATs.

The question asks: A 10kg block is moved from the ground to a height of 20 m. In which sections (A-D) is work being performed?

A. Work is done in all four sections
B. No work is done in any section
C. Work is done only in sections A and C
D. Work is done only in sections B and D.

In section A the block is moved 5 m to the right. In section B the block is moved 10 m up. In section C the block is moved 5 m to the left. Then finally, the block is moved 10 m up.

I thought work was accomplished in all sections because the displacement in all sections in positive.

The correct answer is D. In sections A and C, the object is moving parallel to the resisting force, so no work is performed. In sections B and D, however, the object is moving against the resisting force, so work must be done.

If I push a block on the ground, isn't the block moving parallel to the resisting force (friction)? I don't see how the first part is right. In the second part, "the object is moving against the resisting force, so work must be done". If something is moving AGAINST the resisting force, isn't it moving parallel to the resisting force? The explanations in the problem were extremely confusing. Can someone explain this to me a little better? Thank you very much.

I know others have answered your question but the answer is B. This tests your concept of the Law of Conservation of Energy. W=change PE + change in KE but KE is 0 so you are only concerned with PE=mgh. Because the height isn't increased in in A and C, their is no work being done. Work is alos equal to the force x distance but it is more important to pay attention to positional energy in this question. Part II in your picture deals specifically w/ W=Fd. Also, pay attention to wording. They specifically use PE in the question stem. Also, they said it was...moved to a height... If the person was carrying the block in sections A & C then they aren't doing work on it, because a force is not applied. Hope this helps.