# I need help with hw (my book hasnt came in yet)

1. Jun 27, 2010

### JJ89

1.A mass m = 20 kg is moving along a surface with coefficient of kinetic friction mk = 0.3.

A)What is the force of kinetic friction on the mass?

2.A wooden block of mass m = 9 kg starts from rest on an inclined plane sloped at an angle q from the horizontal. The block is originally located 5m from the bottom of the plane.

a)If the block, undergoing constant acceleration down the ramp, slides to the bottom in t = 2 s, and q = 30°, what is the magnitude of the kinetic frictional force on the block?

3A block of mass m1 = 2 kg and a block of mass m2 = 3 kg are tied together and are pulled from rest across the floor by a force of Fp = 21 N. The coefficient of friction of the blocks with the floor is µ = 0.13.
a) What is the acceleration of the two blocks?

a = ?m/s2
b) What is the tension in the string between the blocks?

T = N ?

4 A block of mass mblock = 1.9 kg rests on a cart of mass mcart = 3.1 kg, which moves without friction on a horizontal surface. The coefficient of static friction between the block and the cart is µ = 0.76. The cart is accelerating to the right.

a) What is the maximum acceleration such that the block does not slide on the cart?

amax = ? m/s2
b) What is the magnitude of the force on the cart required to provide the acceleration amax?

F = N ?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jun 27, 2010

### Uku

3. Jun 27, 2010

### JJ89

im on #2 right now

4. Jun 27, 2010

### JJ89

still need help tho

5. Jun 27, 2010

### Uku

Well show what you have going, don't know where to help.

6. Jun 27, 2010

### JJ89

whats your email, ill send you the graph

7. Jun 27, 2010

### Uku

Just put the graph here using the image function...

8. Jun 27, 2010

### JJ89

this is for #3

#### Attached Files:

• ###### m1.gif
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9. Jun 27, 2010

### Uku

So how far are you on that nr. 3? The golden rule of all homework forums, no-one is going to solve the problem for you.
Hint: write out Newtons second law for every body involved.

10. Jun 27, 2010

### JJ89

Oh alright im on B, the acceleration is 2.94 and i cant find the tension in between.

11. Jun 27, 2010

### JJ89

2.92

12. Jun 27, 2010

### Uku

So you mean you are on exercise 3). For these kind of tension problems there are two powerful tools:
a) Make a good drawing of the setup, put all the forces on the diagram and also add a coordinate axis (in pulley problems you simply bend the axis around the pulley). The axis will give you a clear view on the signs of things.
b) Write out Newtons second law for every body included, then solve the equation system you get.