Physics Honors-Inclines-Car Rolling Down a Hill.

[Medgirl314]

Homework Statement

A 900 kg car is parked on a hill when its brakes fail. It begins to roll down the hill, which is at a 15 degree angle above the horizontal. What is the car's speed after it has rolled 20m? Assum no friction.

Homework Equations

F=ma
v²=2aΔx

The Attempt at a Solution

a=g(sin)(theta)-9.8(cos)(theta)
a=2.54-9.47
a=6.93

v²=2aΔx
v²=3(6.93)(20)
v²=277.2
v=√277
v=16.6 m/s

That looks right to me! Could someone please confirm?

 

[BvU]

Nope. But if you have a question on something specific, go ahead.

Point is, we lose credibility with educational institutions if we start handing out "approved" stamps. That way everybody gets A++ if they ask PF.

You are by now quite qualified to check your own work. Do so and be confident!

 

[Medgirl314]

I'll hold back on posting so much, then, if credibility is lost. I disagree somewhat that everybody would get all A's, as it takes a certain level of skill to come to an answer to begin with, and since I still do most of the work even if the answer is wrong and someone helps fix it.
Thanks!

 

[BvU]

All right, all right. I agree with a=g(sin)(theta). Along the slope, downwards. The 9.8(cos)(theta) is perpendicular to that. (You did make a drawing, right ?)

In an almost similar case, but with apiano in the car and some friction, I objected against the v²=2aΔx. I am prepared to repent if you convince me that it's relevant and correct here.

For more information, coonsult the other thread (with friction and ...)

 

[Medgirl314]

I consulted a previous, extremely similar, diagram. I'm still not sure why you're asking about a piano in the car. Did I state there was a piano in a car in one of my problems? If so, it was a major typo. It's relevant here because I'm trying to find the final speed, which is the velocity, of the car after it has rolled 20m. The equation I gave included all the information needed to determine the velocity. Unless, of course, we need to take the mass into account? Up until this point, we've discarded it, but maybe since we're working on an incline now it's needed.

 

[BvU]

No I was just kidding. Recognized the 900 kg as the weight of the piano in an earlier, quite similar thread. Weird sense of humour, I agree. Poor you.

Let's get to work. Where on Earth did you find v²=2aΔx.

Do you really want me to copy the relevant equations for linear motion with a constant acceleration from another earlier thread or can you collect hem yourself ?

 

[Medgirl314]

I already collected them. Please see the link in my other thread.

 

[BvU]

Great. Why don't we meet in the other thread. Once we have an answer we can set ##\mu=0## and then we have bagged this one too !