# Help with physics homework regarding friction, tension, springs and centrifugal force

1. Sep 5, 2012

### Shinster007

1. The problem statement, all variables and given/known data
The first problem I am having trouble with is as follows:

A child pushes a 40kg sled across the ice at a constant speed. If μk = 0.05 calculate the force applied to the sled.

2. Relevant equations
I know that f=$\mu$FN

3. The attempt at a solution

I know that the free body diagram would consist of a point with a direction in the x coordinate, but I have no idea how to go about this problem. Math and physics is not my strong subject; this is my first ever physics class
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 5, 2012

### CAF123

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Have you tried drawing the free body diagram? The sled is moving at constant speed so the net force acting on it is zero.

3. Sep 5, 2012

### Shinster007

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Here is what I came up with:

Ʃy=FN-mg=0. Therefore, FN=mg=(40kg)*(9.8 m/s2)=392N.

Ʃx=F-f=0. Therefore, F=f, which equals $\mu$s*FN, which equals (0.05)*(392N), giving me an answer of 19.6N. Did I do this correctly?

4. Sep 5, 2012

### CAF123

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Yes.

5. Sep 5, 2012

### Shinster007

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Okay thanks for the help, I am feeling kinda dumb now that I know it is that simple.

Next question:

A 60kg snowboarder accelerates down a 32 degree slope at 3.0m/s2. Calculate μk.

Attempt:

Obviously since there is an acceleration, I know that the forces in the x direction acting on the snowboarder do not equal zero. When setting up a free body diagram I have FN acting upward, Fg acting "downward" (but not on the y axis), then Fx in the positive x direction, and fx in the negative direction.

I then get the following:

Ʃy= FN-mg*cos∅=0;
so FN=(60kg*9.8m/s2)*(cos 32)=498N.

Ʃx=mg*sin∅-$\mu$kFN=ma.

Is this correct so far? Would it then just be a matter of moving everything around algebraically and solving for $\mu$k?

6. Sep 5, 2012

### Shinster007

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Oops, *centripetal

7. Sep 5, 2012

### CAF123

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Yes, simply solve for $\mu_k$

8. Sep 5, 2012

### Shinster007

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Okay, getting the hang of this.

Last question:

A baseball player initially running at 3.4m/s slides to a stop at third base in 1.2 seconds. Calculate the μk between him and the ground.

Attempt:

I'm not sure how to approach this problem. 3.4m/s divided by the 1.2s will give me the acceleration I think (or I guess deceleration in this case). Other than that I am at a loss

9. Sep 5, 2012

### CAF123

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

The frictional force provides the negative acceleration, necessary for him to come to a stop. Using the negative acceleration calculated above, you can find $\mu_k.$

10. Sep 5, 2012

### Shinster007

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Well I know that f=$\mu$k*FN, but I don't have a mass in order to calculate FN, nor do I have f. What equation can I use to solve the problem?

11. Sep 5, 2012

### CAF123

Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Simply use $F = ma.$ You know the only force acting horizontally on the baseball player as he slides (force of friction). This is your $F.$ Now sub in what $F$ is equal to and what do you notice about $m$?

12. Sep 5, 2012

### vela

Staff Emeritus
Re: Help with physics homework regarding friction, tension, springs and centrifugal f

Your teacher probably encourages you to work problems out using symbols first and plug numbers in at the end. This is one of the reasons why.