Question

Object sliding down a ramp .... PE and KE

Answer 1

oh, and time it takes to slide to the bottom of the ramp is 1.638 seconds

Potential energy = .0025*9.8*.46

Kinetic energy at the bottom of the ramp is?

Answer 2

this problem involves friction

Answer 3

I think we can use work-kinetic energy theorem.

Answer 4

hopefully :)

I seem to come up with 2 different answers; and i know one of them is wrong.

Answer 5

The aceleration must be constant right?

Answer 6

\[a=\frac{2d}{t^2}\]
\[v=at=\frac{2d}{t}\]
\[K_e=\frac12mv^2\]
energy lost to friction

\[P_e-K_e\]

Answer 7

assume a constant acceleration yes

Answer 8

You're did it correct i think

Answer 9

do you know the co-efficient of sliding friction?

Answer 10

not outright; but that can be calculated with given information i believe

Answer 11

Hmm, actually we don't need \(\mu\) since time is known.

Answer 12

mmm true.

Answer 13

\[\mu_k=\frac{g~sin\theta-a}{g~cos\theta}\]
\[F_k=\mu_k*\vec n=\frac{g~sin\theta-a}{g~cos\theta}*mg~cos\theta\]
\[F_k=m(g~sin\theta-a)\]
\[W_f=F_k*d=md~(g~sin\theta-a)\]

Answer 14

hey you know that v=2d/t^2.
So KE is just 1/2 mv^2.

Answer 15

i mean, what else do you want?

Answer 16

just working it thru a couple of ways. When I first tried to work this out; I used:

Pe = mgh = 0.01127 J

in a frictionless system; Ke = Pe at the bottom of the ramp

mgh = mv^2/2

gh = v^2/2

sqrt(2gh) = v

v = 3.0027 m/s right?

Answer 17

The problem maybe 1.638 sec isn't the correct time :)

Answer 18

yes, v=3m/s in that case.

Answer 19

its pretty good, we slid the penny 5 times and averaged it out

Answer 20

at first the velocity of the Ke i just worked out threw me for a loop

Answer 21

you actually measured 3m/s!?

Answer 22

i couldnt see why the velocity from v=2d/t wasnt matching up :)

Answer 23

the 3 m/s was not measured, it was calculated

Answer 24

oh i thought you performed an experiment with a penny..

Answer 25

i spose my question is; is using the velocity of 2d/t a proper way to determine the kinetic energy in the system at the bottom of the ramp?

Answer 26

yes. This works no matter what the friction is (even 0).

Answer 27

thanks :)

so if it was a frictionless system, then the kinetic energy at the bottom of the ramp would still equal the potential energy from the start of it; and all that would change is the final velocity of the object then.

Answer 28

..and a change in time, but thats a given :)

Answer 29

yes.

Answer 30

Here's a nice video of an extreme case where the object slides with uniform velocity:
http://www.youtube.com/watch?v=84DXjumAxNQ&feature=plcp

Answer 31

lol, great; i minute ten watching a sock slide down a banister ....

Answer 32

x)