TBR Physics Work/Energy Review Passage 1

[DocMC]

Q2) By how much would the potential energy of the television change if a 3m ramp were used to get it up to the platform instead of a 2m ramp?

The answer is stated as (3m - 2m)(100kg)(10m/s2) = 1000J increase.

Isn't this wrong? The ramp LENGTH was extended by 1 m. The actual height from the earth's surface isn't increased by 1m. I found the angle by sin(theta) = 1m/2m (from the original set-up), so theta is 30 degrees. Then to find the new height, I did sin(30)=h(new)/3m = 1.5m. The change in PE is therefore (1.5m - 1.0m)(10m/s2)(100kg) = 500J. Is my reasoning not correct?

Thanks!

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[postbacpremed87]

Q2) By how much would the potential energy of the television change if a 3m ramp were used to get it up to the platform instead of a 2m ramp?

The answer is stated as (3m - 2m)(100kg)(10m/s2) = 1000J increase.

Isn't this wrong? The ramp LENGTH was extended by 1 m. The actual height from the earth's surface isn't increased by 1m. I found the angle by sin(theta) = 1m/2m (from the original set-up), so theta is 30 degrees. Then to find the new height, I did sin(30)=h(new)/3m = 1.5m. The change in PE is therefore (1.5m - 1.0m)(10m/s2)(100kg) = 500J. Is my reasoning not correct?

Thanks!

No it is right. Difference between the mgh's...done.

 

[DocMC]

No it is right. Difference between the mgh's...done.

No. Take a ruler (1m long) and make an angle of 30 degrees with the floor. Take another ruler, and attach it to the end of the existing ruler that is at an angle with the floor. Does the height increase by 1m? Are both rulers a total of 2m off the ground? No... In the question, the ramp is extended by 1m.

 

[QrtrLifeCrisis]

The question is ambiguously worded. Does 2m and 3m refer to the length of the ramp or the height of the ramp? The answer would be 1000 J if it refers to height. If it refers to the length, you couldn't really figure out the change in height (and thus, potential energy) without knowing what the angle of the ramp is. Where did you get 30 degrees from?

 

[DocMC]

The question is ambiguously worded. Does 2m and 3m refer to the length of the ramp or the height of the ramp? The answer would be 1000 J if it refers to height. If it refers to the length, you couldn't really figure out the change in height (and thus, potential energy) without knowing what the angle of the ramp is. Where did you get 30 degrees from?

Yes, you are extending the length from 2m to 3m. Not directly the height. The original ramp had a height of 1m and ramp length of 2m, so the sin(theta)=1/2, so ramp angle is 30 degrees. I used that angle to find the new height after the ramp length was extended.

 

[QrtrLifeCrisis]

Yep, looks like the answer is incorrect. It should be 500 J based on the setup you provided. However, did the passage give any other type of information or was that a standalone? I'm thinking maybe we're not reading the question stem correctly. Either way, it sounds like you know the concept, which is more important for being ready for the MCAT.

 

[MCATMountain]

Based on the wording of the question you provided, it's rather clear that they're referring to the height of the ramp simply because they referred to the height as the comparison point. "Using 3M vs. 2 M". Unless the length was also 2m, I don't see the confusion in the question. They referenced the height, therefore the new item they introduced was also height.

 

[DocMC]

Based on the wording of the question you provided, it's rather clear that they're referring to the height of the ramp simply because they referred to the height as the comparison point. "Using 3M vs. 2 M". Unless the length was also 2m, I don't see the confusion in the question. They referenced the height, therefore the new item they introduced was also height.

The height of the ramp is initially 1m. The length is initially 2m. They extended the length of the ramp to 3m. They equate this to raising the height of the ramp by 1m, which is incorrect.

 

[jjunior]

i was very confused with this question while i was doing it. but i figured out why. bc while i was working on it, i thought to myself "the potential energy shouldnt have changed because the height is still the same" so i was looking for answer of zero but its not there. and i later figure it its bc the CHANGE that they referred to is not actually the change in potential energy between 3m and 2m ramp, its the CHANGE IN POTENTIAL ENERGY as u move up from 0 to 1m height using the 3m ramp; which is the same as moving up from 0 to 1m height using the 2m ramp.
but yea, i agree, the wording is confusing.... hope that helps

 

[DocMC]

i was very confused with this question while i was doing it. but i figured out why. bc while i was working on it, i thought to myself "the potential energy shouldnt have changed because the height is still the same" so i was looking for answer of zero but its not there. and i later figure it its bc the CHANGE that they referred to is not actually the change in potential energy between 3m and 2m ramp, its the CHANGE IN POTENTIAL ENERGY as u move up from 0 to 1m height using the 3m ramp; which is the same as moving up from 0 to 1m height using the 2m ramp.
but yea, i agree, the wording is confusing.... hope that helps

Wow. They definitely didn't make that clear

 

[jjunior]

the point is that the height is one meter regardless of the length of the ramp used (if the length of the ramp changes the angle changes) but the height is still fixed at 1m.

 

[jjunior]

this is directly written in the book
Choice B is the best answer . The change in potential energy is independent of the pathway, so whether it's a 2 m ramp or a 3 m ramp, the change in height for the object is still 1 m. This means that the change in potential energy for the system is found by APE = mgAh.
APE = mgAh = (100 kg)( 10.0 m/s2)( 1.0 m) = 1000 J
Only a change in mass or a change in elevation can change the potential energy of an object on the surface of the Earth. The
best answer is choice B.
perhaps the version you are using is diff from mine, but the 1.0m written in there represents the height, not from 3m subtract 2m

 

[drechie]

yeah the wording really just was really bad for this question. hope questions are more clear IRL

 

[BerkReviewTeach]

the point is that the height is one meter regardless of the length of the ramp used (if the length of the ramp changes the angle changes) but the height is still fixed at 1m.

THAT is the point of the question. I will agree that the wording is suspect, but keep in mind that there are some questions (about 5% of what you find in each of the books) that are designed to make you think and argue with the answer. The point of this question is exactly what Junior wrote here. After doing this question, and arguing about the ambiguity of the wording, hopefully you walk away cognoscente that changing the path without changing the height will not change the deltaPE for any lift or drop of an object.