# Need help with a centripetal force question

1. Oct 6, 2016

### sens

1. The problem statement, all variables and given/known data

2. Relevant equations

mv^2 / r = F

3. The attempt at a solution
This is my attempt and reasoning for this problem, is this the correct way to approach the problem or am i missing something?

the hanging mass is motionless the tension in string equals the force of gravity on mass m2, therefore
T = 4*9.8N
The horizontal force on the puck (which is the centripetal force) is the same as the tension.
F(horizontal) = 4*9.8 N
(c) mv^2 / r = F(centripetal) ===> (0.8)(v^2) / (1.5) = 4*9.8 ===> v = 8.57321 m/s

2. Oct 6, 2016

### Staff: Mentor

Your logic and method look good. You'll want to display the end result with the correct number of significant figures though.

3. Oct 6, 2016

### sens

Are you sure the answer is correct? The solution key that the professor just posted said the answer was 10.5 m/s so i'm trying to figure out where i went wrong.

4. Oct 6, 2016

### Staff: Mentor

I don't see any problem with your solution. Your result is correct for the problem given.

You might make your answer 10 m/s if you were rounding to one significant digit, but your professor's result has three. So that won't account for the difference.

5. Oct 6, 2016

### sens

ok thank you for your help