Register to reply

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

by fphysicsclass
Tags: energy, kinetic, physics, potential, power
Share this thread:
fphysicsclass
#19
Apr4-11, 10:10 PM
P: 37
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
#20
Apr4-11, 10:11 PM
PF Gold
Pengwuino's Avatar
P: 7,120
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
#21
Apr4-11, 10:12 PM
PF Gold
Pengwuino's Avatar
P: 7,120
Quote Quote by fphysicsclass View Post
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
#22
Apr4-11, 10:15 PM
P: 37
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
#23
Apr4-11, 10:20 PM
PF Gold
Pengwuino's Avatar
P: 7,120
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
#24
Apr4-11, 10:22 PM
P: 37
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
#25
Apr4-11, 10:32 PM
PF Gold
Pengwuino's Avatar
P: 7,120
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
#26
Apr4-11, 10:33 PM
P: 37
Thanks for that description! Yay! I got something right. Now help me with 8! jkjk. But seriously, I need help
flyingpig
#27
Apr4-11, 10:40 PM
P: 2,568
Jesus Christ, you can't even do number 4? Here, think about what the WORK is being done.
fphysicsclass
#28
Apr4-11, 10:43 PM
P: 37
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
#29
Apr4-11, 10:48 PM
P: 37
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
#30
Apr4-11, 10:57 PM
PF Gold
Pengwuino's Avatar
P: 7,120
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 [tex]v^2 = 2gh[/tex] 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
#31
Apr4-11, 11:05 PM
P: 37
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
#32
Apr4-11, 11:06 PM
P: 37
My answer is closest to .4 m. Do you think that's right?
flyingpig
#33
Apr4-11, 11:11 PM
P: 2,568
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
#34
Apr4-11, 11:18 PM
P: 37
I mean 2.4 m because you must add 2 to the .4m.
fphysicsclass
#35
Apr4-11, 11:20 PM
P: 37
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
Pengwuino
#36
Apr4-11, 11:23 PM
PF Gold
Pengwuino's Avatar
P: 7,120
Yup, so remember they want the height from the floor so the answer should be 2.4m


Register to reply

Related Discussions
Awesome! I'm as smart as a high schooler! General Discussion 8
Interview with a High Schooler Academic Guidance 1
Questions about fusion (from a high-schooler)... Nuclear Engineering 31
Little Advice for a High Schooler? Academic Guidance 11
Please read, my grade depends on it Nuclear Engineering 5