Help Cure High Schooler's Anxiety: Solve 12 Energy Questions!

[fphysicsclass]

Help me do this assignment. My grade depends on it! *Cure a high schooler's anxiety*

1.I am stuck on these 12 questions. The given data and equations are provided in each question.
2. 1. A bowling ball hangs from a 2-meter rope attached to the ceiling. Keeping the rope taut, you pull the ball up until the rope makes an angle of 60° with respect to the vertical. You let the ball go. About how fast will it be moving at the bottom of the swing?

Cannot be determined without the mass of the ball.
4.5 m/s
6.3 m/s
3.2 m/s
5.9 m/s

2. A brick is dropped from a second story window. Next, a second brick, which weighs twice as much as the first brick, is dropped from the same window. When the second brick strikes the ground it has...

Half as much KE as the first brick.
Same KE as the first brick.
Twice as much KE as the first brick.
Four times as much KE as the first brick.
Not enough information to compare KE's.


3. An engine delivers 32 hp. About how long will it take for the engine to do 700 kJ of work? Remember, 1 hp = 746 W.

0.03 s
0.3 s
3 s
30 s
300 s

4. Kiera carries a 5-kg box up the stairs. If the second floor is 4 meters above the first, about how much work does Kiera do on the box? Give your answer in Joules.

5
20
200
Not enough information to calculate the work.
40

5. Kyle has to move a 1500-N piano onto a porch 1.2 meters above the ground. He finds a board to use as a ramp and rolls the piano up to the porch. What average force was needed if the ramp was 3.1 meters long? Give your answer in Newtons and neglect friction.

484
581
750
3880
5810

6. Consider the energy transitions that occur during a pole-vault event. Disregard rotational motion and air resistance. Choose the best energy description for each given action. Make a number selection for each letter:

1)Min. KE, max grav. PE
2)All KE
3)KE decreases, elastic PE increases
4)All elastic PE
5)KE and elastic PE decrease, Gravitational PE increases
6)Grav. PE decreases, KE increases

a.The pole vaulter moves down the runway.
b.The pole vaulter plants the end of his pole into the box. He slows and the pole bends.
c.The pole vaulter leaves the ground. He goes up, slowing and unbending his pole
d.The pole vaulter clears the bar.
e.Having cleared the bar, the pole vaulter is falling toward the pit.

7. The PE of an object in Earth's gravitational field can be written as:

PEgrav=-G*Mearth*(m/r)

Where is the potential energy equal 0?
Choose one answer.
When r = 0.
When r = the radius of the earth.
When r is very small but not 0.
When r very large, approaching infinity.
When r = the radius of the Earth + 300,000 meters. 8. A marble rolls down a ramp that sits on table. The marble leaves the base of the ramp with a horizontal velocity. The base of the ramp is 2 meters above the floor. The marble strikes the floor at a distance of 1.8 meters from where it left the ramp. Give the height of the ramp with respect to the floor.
Choose one answer.
2.4 m
0.4 m
4.4 m
1.4 m
2.2 m

9. In which situation described below is Taylor positive work?
Choose one answer.
He pushes on the wall which does not move.
He carries a book across the room at a constant height.
Losing at tug-a-war, he is pulled toward his opponent.
He ties a rope to an object and swings it in a horizontal circle at a uniform rate.
He walks up the stairs. 10. The units for power are...
Choose one answer.
Newton
Pascal
Joule
Watt
None of these

11. A spring with stiffness coefficient K = 9 N/cm is stretched 20 cm. Measured in Joules, the elastic potential energy is ...
Choose one answer.
18
36
90
900
1800

12. If a pole vaulter moves at 8.0 m/s down the runway, what is the maximum height that she can clear?
Choose one answer.
3.2 m
32 m
6.4 m
8 m
1.6 m
3. I have attempted each question but energy transformations are almost impossible for me.

 

[Kuryakin]

Is number 11 written correctly?

 

[Pengwuino]

Is number 11 written correctly?

The answer is there, I think it is.

 

[Pengwuino]

To the poster, we're not here to do your homework, and we know your grade depends on it (duh?). However, you need to show some effort for every single problem you want help with.

 

[Kuryakin]

The answer is there, I think it is.

Thought it said 2cm :).

 

[fphysicsclass]

To the poster, we're not here to do your homework, and we know your grade depends on it (duh?). However, you need to show some effort for every single problem you want help with.

What's the minimum effort I need to show? I have work on the handout given to me, but my answers don't lead me to any of the options. Conceptual questions like 6 are impossible to show effort on paper, anyways.

 

[Pengwuino]

You need to convince us that you didnt just post this with the expectation that someone would give you all the answers. You have to have some idea how to do most of these problems.

For example, for the first question, obviously some sort of energy conservation applies. What is it?

 

[fphysicsclass]

You need to convince us that you didnt just post this with the expectation that someone would give you all the answers. You have to have some idea how to do most of these problems.

For example, for the first question, obviously some sort of energy conservation applies. What is it?

Well I know that it will be in complete KE, with 0 PE. I don't know how to find it though.

 

[Pengwuino]

Well I know that it will be in complete KE, with 0 PE. I don't know how to find it though.

So if an object falls a certain distance, h, what is the change in potential energy?

 

[fphysicsclass]

h = 2-2sin30 = 1 The kinetic energy at the bottom of the swing will be equal to mgh at 60 degrees => mg=0.5mv^2 => 2g = v^2 => v = 4.4m/s ?

 

[fphysicsclass]

Ok so I got a couple more of those. Now I only need help with these numbers: 8, 9, 11, and 12.

Can you tell me if I did 6 correctly?
c1
a2
b3
d4
e6

 

[fphysicsclass]

Nevermind. I got the last 2. Now I'm just stuck on numbers 8 and 9. Oh and please double check my number 6. Thanks.

 

[Pengwuino]

There you go, although I'm guessing your teacher is having you use g = 10m/s^2? With g = 10 m/s^2, you would round up to 4.5m/s whereas the correct value of g = 9.8m/s^2 gives you the answer you got.

Pause for a second and look at what you have. Any problem where you have an object falling a distance h from rest will have what you wrote down, that is $$mgh = .5mv^2$$. If you manipulate this a little, you find basically what you found: $$v = \sqrt{2gh}$$ (you had figured h = 1m). So as you can see, objects falling from rest will reach a speed after falling a distance "h" that is completely independent of the mass of the objects.

So, with that in mind for the second question, you'll look at the kinetic energy, $$KE = .5mv^2$$. Both objects fall from the same height. They'll both have the same velocity. However, the kinetic energy does depend on the mass, so which should have the higher kinetic energy?

 

[fphysicsclass]

I have a feeling for 9 that positive work occurs when he walks up the stairs, but I'm not sure why. Still clueless on 8, and unsure about my answers for 6.

 

[fphysicsclass]

the one with the greater mass?

 

[Pengwuino]

C1 and D4 do not go together for problem 6. The maximum gravitational potential energy is at the top of the jump right?

 

[fphysicsclass]

Am I right that c should be 6 and d should be 2?

 

[Pengwuino]

No, for c) how does the elastic potential energy change? And why should 2 be D? At the top, the pole vaulter is almost at a standstill.

 

[fphysicsclass]

I'd just like to let you know that the actual q's for six are the letters. Any of the numbers could match up with it.

 

[Pengwuino]

For the work problem, remember, when you do work to something, you store some sort of potential energy into it. What description has an increase in potential energy?

 

[Pengwuino]

I'd just like to let you know that the actual q's for six are the letters. Any of the numbers could match up with it.

Yes, I realize that. I should have said why should d) be 2.

 

[fphysicsclass]

In c, the elastic PE is decreasing and the KE is increasing as the pole vaulter gains speed and passes over the pole.

For d, I thought the KE increased because he begins to move downwards since the direction of the movement changes.

 

[Pengwuino]

The key for c) is that the statement says he is slowing and the pole is unbending. In other words, his kinetic energy is decreasing and the pole's elastic potential energy is decreasing as well which looks more like 5.

For d), yes, the kinetic energy will eventually increase, but when you just clear the bar, you're almost at a standstill and your gravitational potential energy is at it's highest which sounds more like 1.

 

[fphysicsclass]

Oh! I was NOT thinking of 'instantaneous' descriptions of energy. Maybe I should remember that for the exam in the future.

For 9, why is walking upstairs not positive work?

For 8, where do I start?

 

[Pengwuino]

For 9, it actually is! Bringing an object to a greater height gives it more potential energy. Giving an object energy is the result of doing a positive amount of work to it. The object now has the ability to perform work on something else because it has an energy associated with it.

 

[fphysicsclass]

Thanks for that description! Yay! I got something right. Now help me with 8! jkjk. But seriously, I need help

 

[flyingpig]

Jesus Christ, you can't even do number 4? Here, think about what the WORK is being done.

 

[fphysicsclass]

No flyingpig. I can do number 4. If you read the comments I'm stuck on number 8. Just help me on this last one.

 

[fphysicsclass]

Here it is if you don't want to go back to page 1. 8. A marble rolls down a ramp that sits on table. The marble leaves the base of the ramp with a horizontal velocity. The base of the ramp is 2 meters above the floor. The marble strikes the floor at a distance of 1.8 meters from where it left the ramp. Give the height of the ramp with respect to the floor.
Choose one answer.
2.4 m
0.4 m
4.4 m
1.4 m
2.2 m

 

[Pengwuino]

8 is actually a bit involved compared to the other problems. You really are looking for how high the ramp is here. So you again use your energy conservation laws and the ol $$v^2 = 2gh$$ guy. You want to find the height of the ramp. However you don't immediately know the velocity but you do have two things that can help you. Whatever this velocity is, it allowed the marble to travel 1.8 meters in the horizontal direction while simultaneously falling 2 meters. Use your kinematic equations to figure how what velocity would allow this to happen.

 

[fphysicsclass]

Oh god. I got a 95 on the kinematics test, studied forever hoping that this stuff wouldn't come back to haunt me... and now it has. We just started the work/energy/optics unit. I want to understand this.

So from deriving several equations I got v = deltax/sqrt(deltay/.5g)

Assuming that the initial horiz. speed was 5.9 m/s, I got deltay = .45m

 

[fphysicsclass]

My answer is closest to .4 m. Do you think that's right?

 

[flyingpig]

Huh, interesting.

Try this

x = vt (x is horizontal distance)

y = -gt^2/2 + 2 (vertical distance, check that t = 0, y = 2)

There is no velocity before because it is freefalling.

Your answer is wrong, because the base of the ramp is 2m high, why would it be 0.4m...? But I am not really reading your posts so I am not much of help, but I have set up some eqtns for you

 

[fphysicsclass]

I mean 2.4 m because you must add 2 to the .4m.

 

[fphysicsclass]

See, my derived equation allows me to neglect time! :D
I found it by myself... so proud.

However, I think I neglected to add 2 m to .4 m because it asks for with respect to the floor. Look:

8. A marble rolls down a ramp that sits on table. The marble leaves the base of the ramp with a horizontal velocity. The base of the ramp is 2 meters above the floor. The marble strikes the floor at a distance of 1.8 meters from where it left the ramp. Give the height of the ramp with respect to the floor.
Choose one answer.
2.4 m
0.4 m
4.4 m
1.4 m
2.2 m