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

1. Apr 4, 2011

### 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?
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.
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?
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...
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 ...
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?
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.

2. Apr 4, 2011

### Kuryakin

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

Is number 11 written correctly?

3. Apr 4, 2011

### Pengwuino

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

The answer is there, I think it is.

4. Apr 4, 2011

### Pengwuino

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

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.

5. Apr 4, 2011

### Kuryakin

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

Thought it said 2cm :).

6. Apr 4, 2011

### fphysicsclass

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

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.

7. Apr 4, 2011

### Pengwuino

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

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?

8. Apr 4, 2011

### fphysicsclass

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

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

9. Apr 4, 2011

### Pengwuino

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

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

10. Apr 4, 2011

### fphysicsclass

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

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 ????

11. Apr 4, 2011

### fphysicsclass

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

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

12. Apr 4, 2011

### fphysicsclass

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

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.

13. Apr 4, 2011

### Pengwuino

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

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?

14. Apr 4, 2011

### fphysicsclass

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

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.

15. Apr 4, 2011

### fphysicsclass

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

the one with the greater mass?

16. Apr 4, 2011

### Pengwuino

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

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

17. Apr 4, 2011

### fphysicsclass

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

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

18. Apr 4, 2011

### Pengwuino

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

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.

19. Apr 4, 2011

### fphysicsclass

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

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.

20. Apr 4, 2011

### Pengwuino

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

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?