# Homework Help: Friction problem.

1. Nov 29, 2004

### Vector86

A block slides down an inclined plane 9m. long that makes an angle of 38 degrees with the horizontal. The coefficient of sliding friction is 0.25 If the block starts from rest, find the time required for it to reach the foot of the plane

Thats all the info i got, plz i need help.

2. Nov 29, 2004

### cepheid

Staff Emeritus
Strategy:

1. List the given variables so that you can easily see what information you have.

2. Draw a Free Body Diagram (FBD) for the block. Include all of the forces acting on the block and from that find the net force acting on the block.

3. Are you sure the mass of the block is not given? It must be. Use it, and the net force, to find the acceleration of the block.

4. Knowing the acceleration of the block, find the time required to traverse the incline.

3. Nov 29, 2004

### HawKMX2004

I dont think mass is required, it should cancel itself out shouldnt it? Also i want to add, keep your X variables different from your Y variables. Dont forget that there is a Force of Gravity in the X direction which is sin(38)(9.8) in this case, and a Force of Gravity in the Y direction which is cos(38)(9.8) both pulling down on the block, after you have your forces and such figured out, it is helpful to me anyway, to forget about the incline, and draw it flat, and use the incline forces...hope that helps if you need some formulas or get stuck, let me know I'll try to help ya

4. Nov 29, 2004

### cepheid

Staff Emeritus
Could you please explain how either component of the object's weight could possibly be independent of its mass?

Edit: Never mind...those quantities you have listed are the accelerations...mgsin(38) / m = gsin(38) etc. I got it now...thanks.

5. Nov 29, 2004

### Vector86

thanks for the help but i still dont know how to start the problem. if u could help me with that i could probably finish it

6. Nov 29, 2004

### cepheid

Staff Emeritus
Step 2 above is how to start the problem

7. Nov 29, 2004

### Vector86

well it still doesnt make sense to me

8. Nov 29, 2004

### cepheid

Staff Emeritus
Did you try drawing the free body diagram? What forces are acting on the block? There is a component of the blocks weight pushing it against the incline. There is the normal force of the incline against the block. Those two forces in that direction (perpendicular to the incline) cancel out, so don't worry about them. Call that the y-direction. What about forces parallel to the incline (call that the x-direction)? Well, there is a component of the object's weight dragging it down the incline, and there is a frictional force in the opposite direction, opposing this motion. So there is a net force of F (weight along incline) - F (friction). This net force accelerates the block down the ramp. Once you know the net force, you know the accleration. With the accleration, finding the time required to reach the bottom is a matter of simple kinematics.

9. Nov 29, 2004

### Vector86

can anyone give me the answer so i can check my work

Last edited: Nov 29, 2004
10. Nov 29, 2004

### Zlex

Vector,

If you want your work checked, feel free to post and folks here will be happy to point out where you have made a mistake, if any.

11. Nov 29, 2004

### Vector86

12. Nov 29, 2004

### Vector86

sorry

sorry i meant the equation for time. the equation i plan on using is distance = original velocity times time plus acceleration times time squared over two.

13. Nov 29, 2004

### Vector86

all i need to know is how to get either the force of weight or the mass because from there i can find everything else. please

14. Nov 29, 2004

### cepheid

Staff Emeritus
OK...since you have the general idea now...I'll help you along further. That triangle you drew...GOOD. It's hypoteneuse is the weight. Just call it mg for now. the x-component of the weight (i.e. down the ramp) is mg(sin38). that's the force pulling the block down the ramp. Now, what is the formula for the force of friction?

$$f = \mu_f N$$, where N is the normal force. Can you calculate the normal force and substitute that in? Once you have a formula for f, you subtract it from the downward force (since it is directed up the ramp, opposing the motion). So:

$$F_{NET} = mg\sin38^o - f$$

HINT: as Hawkman already stated, it doesn't matter that you don't know m.

15. Nov 29, 2004

### Zlex

Double Hint:

Fnet = ma

16. Nov 29, 2004

### Tabe

Ok, I have a similar problem. I am trying to solve for the coefficient of kinetic friction, and I am give theta, and acceleration. Does anybody have a clue how to solve this problem without the mass or forces?