# Physics Problem

1. Oct 6, 2004

### sb_4000

Hi,
I think I posted this in the wrong place, so im posting it here. I'm taking Physics w/ cal 1, and Im having problems solving a couple of question, I was hoping some one could help, I would appreciated it..

1) block of mass m1=8kg is connected over an idead (masless and frictionless) pulley to block 2, of a mass m2=4kg. The magnitude of blocks acceleration is 3 m/s^2 and the angle of incline Theta=30 deg.
a) find the coefficient of the kinetic friction between block 2 and the plane of the incline.
b) what is the magnitude of the tension in the rope.

and also this problem which is projectile motion -
2) Projectile is fired from the top of a 40m tower at an angle of 60 deg above the horizontal hits the ground at the point 100m from the base of the tower.
a) find the speed at which the stone was thrown.
b) find the speed of the stone just before it hits the ground.
c)find the time at which the projectile hits the ground.
d) find the maximum height.

for a) i got 35 m/s

Thank You.

2. Oct 6, 2004

### Pyrrhus

Do a freebody diagram for the first problem, If the pulley is ideal then it has the same tension throughout it.

For the second problem consider.

Vy = 0 at max height
Y = 0 when it hits the ground.

3. Oct 6, 2004

### sb_4000

do you know how to get Part a in problem 1. Im having problems setting up the equation.

4. Oct 6, 2004

### Pyrrhus

Yes, i know.

Here's a hint:

Put your coordinate system with the same inclination as theta, so the normal force will have one non-zero component, while the gravity will have two non-zero components. Also consider the other two forces Tension and friction on the coordinate system.

5. Oct 6, 2004

### sb_4000

Thanks for responding to my question.

In problem two part "b" Is it F= w*sin(30)= 4N..this is what I get, Im probably doing it wrong. and In part one I still dont undrestand how to solve the problem.

6. Oct 6, 2004

### Pyrrhus

Draw a freebody diagram, you can use the hint i said, to solve it.

Do you know your geometry (specially trigonometry) well?

If you still don't understand, let me know.

7. Oct 6, 2004

### Gokul43201

Staff Emeritus
I can't seem to say this enough (as, I imagine does cyclo) ...yet it seems, no one listens : Draw the free-body diagram. That's half the work in the problem.

8. Oct 6, 2004

### sb_4000

Was my answer for "b" right?
Im still working on part a.

9. Oct 6, 2004

### Gokul43201

Staff Emeritus
Did you draw the free body diagram ?

10. Oct 6, 2004

### sb_4000

Yes I did.

I used the book as guide to draw it.
I also worked on part a, and I get 0.38 N.
is this right?

Last edited: Oct 6, 2004
11. Oct 6, 2004

### Pyrrhus

no, that's not correct.

12. Oct 6, 2004

### sb_4000

I also get 0.64 when I use cos(theta).

Im using the formula F=Mu_k*mg

13. Oct 6, 2004

### Pyrrhus

Try to make the equations again remembering this

$$\sum_{i=1}^{n} \vec{F}_{i} = m \vec{a}$$

14. Oct 6, 2004

### sb_4000

i used N=mg*cos(theta), F=mg*sin(theta)

and then mu_k = F/N and I get 0.57 is this one right?

15. Oct 6, 2004

### Pyrrhus

Ok let's try this with a different approach, you're beginning to understand, but still not there.

What are the forces acting on the x-axis on the first and second block?

What are the forces acting on the y-axis on the first and second block?

16. Oct 6, 2004

### sb_4000

i also did it using F_k = ma, N= mg, and F/N, and for this one i get 0.30

17. Oct 6, 2004

### Pyrrhus

18. Oct 6, 2004

### sb_4000

for the second block, the forces acting on it are Gravity and Normal force.
for the first block g, N, T

19. Oct 6, 2004

### Pyrrhus

Aren't both blocks tied together to a string?

Do you know what's an ideal pulley?

What is the normal force?

What is friction and how does it affect movement?

20. Oct 6, 2004

### sb_4000

Yes both blocks are tied to a string. I do not know what the ideal pulley is. The normal force is m*a, is the the upward motion of the block. The friction I believe slows down the movement of the block.