# Normal force of a mass sliding down a quarter circle

1. Mar 16, 2016

### Jamest39

1. The problem statement, all variables and given/known data
A 22-gram mass is released from rest at position A on a stationary frictionless surface whose shape is that of a quarter circle of radius R = 0.66 m.
(b) Determine the magnitude of the normal force that acts on the mass when it is at position B (it is at position B when the imaginary radii form a 48° on the top sector of the quarter circle and a 42° angle on the bottom half).

2. Relevant equations
N = mgcosθ

3. The attempt at a solution
In order to use the equation above to determine the normal force, you need would need an angle between the normal force vector and the mg vector but couldn't find any way to do so. The answer was given to us as 0.48 N, and I plugged that into the equation above to get θ = 77.14°, but I don't know any way I would be able to get that angle.

2. Mar 16, 2016

### RedDelicious

Is this the full question?

I notice you went from the question to part (b). Please include all information, if you haven't.

3. Mar 16, 2016

### Jamest39

Yeah, question (a) was "Determine the speed of the mass at the instant when the mass is at position B."
mgh = (1/2)mv^2
v = sqrt(2gh)
h = R sin θ So,
v = sqrt(2*9.8*0.66*sin48°) = 3.10 m/s

4. Mar 16, 2016

### RedDelicious

That's what I thought.

Do you think that might be related to part b in any way? Keep in mind this is circular motion. Any forces that might affect the normal force?

5. Mar 16, 2016

### Jamest39

The centripetal acceleration? a = v^2/r

6. Mar 16, 2016

### RedDelicious

Yep. Now you just need to use Newton's 2nd law again but with all the forces this time.

7. Mar 16, 2016

### Jamest39

So ΣF = ma,
and the forces acting on it are gravity and the normal force. But how can I express mg in that formula when its at that position on the curve?

8. Mar 16, 2016

### RedDelicious

You do it the same way you did it the first time. The first time you set it equal to the normal force because it was the only force acting on it, but this time it isn't, so you don't, but its component is still the same.

9. Mar 16, 2016

### Jamest39

I can set ΣF = ma equal to the magnitude of the normal force?

10. Mar 16, 2016

### RedDelicious

Before you get there you're going to need the correct velocity.

You set PE = KE, which would be fine if it were true. What that implies is that all of the potential energy it initially had has been converted into kinetic energy, which we know is not true because you found its height at point B and it definitely wasn't zero, meaning it has potential energy that you're not accounting for. Starting from the basic fact that the conservation of energy tells us that, in an isolated system, the initial energy equals the final energy.

$$Ei=E_f\\ mgh_i+\frac{1}{2}mv_i^2=mgh_f+\frac{1}{2}mv_f^2$$

$$mgR=mgRsin\left(\theta \right)+\frac{1}{2}mv_f^2$$