Supposed to be an "easy" question about WORK


by mms05
Tags: supposed, work
mms05
mms05 is offline
#1
Jul11-06, 09:11 PM
P: 14
Hi! I need some help with this question:

Jim rides his skateboard down a ramp that is in the shape of a quarter circle with a radius of 5.00 meters. At the bottom of the ramp, Jim is moving at 9.00 m/s. Jim and his skateboard have a mass of 65.0 kg How much work is done by friction as the skateboard goes down the ramp?

What I used was
W=F(force) x r (displacement)
F= mv^2/r
= (65 kg x 9 m/s^2)/(5 m)
= 1035 N

W= (1035 N)(19.6 m) (To find 19.6, I used the circumference equation to find the distance down the ramp)

W= 20,286 J
20.3 kJ

I really do not think this is correct, and I'm trying to figure out what it means by "frictional" force.

Thanks for your help!
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Teegvin
Teegvin is offline
#2
Jul11-06, 10:18 PM
P: 37
Jim and his skateboard have gravitational potential energy at the top of the ramp, what is it? How much kinetic energy would he have if all of his potential energy were transformed into kinetic? How much kinetic energy does Jim have?

Remember
pe = mgh
ke = (mv^2)/2
mms05
mms05 is offline
#3
Jul11-06, 11:09 PM
P: 14
so i don't have to use the radius of the circular (1/4) ramp at all??

Teegvin
Teegvin is offline
#4
Jul11-06, 11:42 PM
P: 37

Supposed to be an "easy" question about WORK


The radius is also the height from which Jim starts.
mms05
mms05 is offline
#5
Jul12-06, 12:20 AM
P: 14
oh my goodness- I'm so utterly confused... It's just not clicking :(!
Teegvin
Teegvin is offline
#6
Jul12-06, 12:27 AM
P: 37
If it were a straight ramp 5m high, would it be easier to understand?
mms05
mms05 is offline
#7
Jul12-06, 12:32 AM
P: 14
yes, i think so- but that wouldn't be the same as it being curved, because the displacement on a curved ramp would be different than that on a straight ramp, no?
Teegvin
Teegvin is offline
#8
Jul12-06, 12:41 AM
P: 37
Not necessarily. If the ramp were a 45 degree incline, the displacement would be the same (5m down, 5m over); however, the way I figured this out does not need a displacement.

At the top of the ramp, Jim has no kinetic energy, but he has mgh potential energy. What does he have at the bottom?
Paquete
Paquete is offline
#9
Jul12-06, 12:56 AM
P: 9
mms05 is right, the displacement on a curved ramp would be different than that on a straight ramp, but, in this particular problem you can use a different approach , just like Teegvin said, in order to calculate work.

W = - (Uf - Ui)

where Uf is the final potential energy and Ui is initial potential energy.
thiotimoline
thiotimoline is offline
#10
Jul12-06, 05:00 AM
P: 51
Quote Quote by mms05
Hi! I need some help with this question:

Jim rides his skateboard down a ramp that is in the shape of a quarter circle with a radius of 5.00 meters. At the bottom of the ramp, Jim is moving at 9.00 m/s. Jim and his skateboard have a mass of 65.0 kg How much work is done by friction as the skateboard goes down the ramp?
Find the total energy at the top and at the bottom. The difference in energy is the energy wasted to do work against friction when rolling down the ramp. :)
mms05
mms05 is offline
#11
Jul12-06, 09:52 AM
P: 14
thank you all! :)
thiotimoline
thiotimoline is offline
#12
Jul20-06, 10:24 AM
P: 51
By conservation of total energy, initial total energy = final total energy + friction.


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