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I need a physics tutor, NOW! Will pay $20/hr paypal.

yhelothar

yhelothar

Lifer
If you can solve these two problems, please be my tutor. I will probably need about an hour or two worth of help. I'm taking the first semester of calculus based physics.

1st question:
A bowler throws a bowling ball of radius R=.11m down the lane with initial speed v0 = 8.5m/s. The ball is thrown in such a way that it skids for a certain distance before it starts to roll. It is not rotating at all when it first hits the lane, its motion being pure translation. The coefficient of kinetic friction between the ball and the lane is 0.210.

a. What length of time does the ball skid?
answer: 1.18s
b. How far down the lane does it skid?
answer: 8.6m
c. how many revolutions does it make before it starts to roll?
answer: 5.18rev
d. how fast is it moving when it starts to roll?
answer: 6.07m/s

This is how I tried to solve the problem, but didn't come up with the correct answer:
For part a, I know that the ball stops skidding when all of the linear motion translates to rolling motion. Thus, I can conclude that v=r?, or ?=v/r.
I can find a, by using a net torque equation.. Ia=µmgr
The bowling ball is a thin spherical shell, so the moment of inertia, or I, should be 2/3mr^2. I'm not sure if this is the correct moment of inertia though.
I end up getting ?=77.27rad/s and a=12.485rad/s^2.
From using the ?0=?+at equation, I get that t=6.19s.. quite off from 1.18s.

2nd problem:
A physical pendulum consists of a meter stick that is pivoted at a small hole drilled through the stick a distance x from the 50 cm mark. The period of oscillation is observed to be 4.2 s. Find the distance x.
 
This is going to be quick because I have to go to class, but hopefully this will help.

w = (alpha)*t = ((torque)/I)*t = (F*R/I)*t = (v_0 -at)/R

Where F = force of friction = (mu)*Normal force.

v_0 = initial velocity; I = moment of inertia. I'm pretty sure your formula is correct, though check it in some book/online.

a = linear acc = F/m
R = radius of sphere.

I think the mistake you made is not subtracting out the linear deecerlation from the velocity when doing the w = v/R thing. As the angular velocity is increasing, the regular velocity is decreasing.

For the second part, lookup the formula for the period of a physical pendulum(in terms of the moment of inertia). From there it should be trivial. I'll try to help you once I get back from class (a couple of hours).

Hope this helps, and good luck
 
I just got back from the final.. studying didn't help at all, in fact, I could've probably answered most of those questions before the start of that class.
The only thing I haven't studied was on the test.. a pendulum problem. I ended up bombing the last problem.
I think I aced everything else though.
 
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