# Homework Help: Force and Kinetic friction problem

1. Nov 2, 2008

A sled weighing 100 N is pulled horizontally across snow so that the coefficient of kinetic friction between sled and snow is .275. A penguin weighing 50 N rides on the sled

to start with the penguin digs in his claws so he is firmly attached to the sled. What value of F do you need for the sled and penguin to move at constant speed??

after a while, our penguin gets tires od holding on with his claws. Now the coeffient of static friction between penguin and sled is .750 find the maximun horizontal force F that can be exerted on the sled before the penguin bgins to slide off?

2. Nov 2, 2008

### tiny-tim

Show us what you've tried, and where you're stuck, and then we'll know how to help.

3. Nov 2, 2008

Ok well i've been doing some diagrams
In my diagrams for 1st part I show that there is a force pulling the sled and opposite to that there is kinetic friction. Force due to gravity is pointing down and a normal pointing up.
for the penguin I show a force which is opposite to the one that pulls the sled. Fg downwards and a normal upward.

For sled
I found the normal to be 100 N since its the same as Fg which 100 N. I multiplied this number times the Kinetic friction to find the force due to kinetic friction (100 N * .275 = 27.5) I knwo that the sum of forces on the horizontal direction are going to equal ma
and mass is 100N/g = 10 Kg (g=10m/s^2)
so
sum of all forces in x = F - 27.5 = 10kg (a)

now im stuck

4. Nov 2, 2008

### tiny-tim

Excellent …

and constant speed means the acceleration, a, is … ?

5. Nov 2, 2008

acceleration = 0

6. Nov 2, 2008

does that mean that the minimum value for the sled to move will be 27.5 N????

7. Nov 2, 2008

### tiny-tim

… poor little penguin!

Yes … except you forgot the penguin!!

8. Nov 2, 2008

ok... I dont know how to deal with the penguin

9. Nov 2, 2008

### tiny-tim

Feed it fish, of course!

I meant, you forgot its weight!

10. Nov 2, 2008

lol

mmmm i not sure but do I add penguins weight to the force???

11. Nov 2, 2008

### tiny-tim

You add the penguin's weight to the sled's weight to calculate the normal force, to calculate the friction force.

12. Nov 2, 2008

oh so its the weight of the whole system
in that case the Force = 41.25 N
becasue Force due to friction is 150(.275) = 41.45 N

so then I use this force to solve part 2 right?

if im correct then the Static friciton between penguin and sled is
(.750*50)= 37.5 N

so the sum of forces in x direction = force of kinetic friction - force= 0??????

13. Nov 2, 2008

### tiny-tim

Yes.
No … it's a different F … you start again.
Yes.
Nooo … for the penguin, the sum of forces in x direction = force of kinetic friction = mass x acceleration.

14. Nov 2, 2008

so for part 2 the force will be different is that force going to be 27.5 or no??

and for the last part where you said no i dont know how to do that because there is no acceleration

15. Nov 2, 2008

### tiny-tim

No … the force is unknown … it's the (different) F which you have to calculate.
ah … yes there is … the penguin won't come off without acceleration, will it?

16. Nov 2, 2008

ok so can you help me figure this out because i have no clue how to do it

17. Nov 2, 2008

### tiny-tim

The penguin will start to slide when the acceleration is enough to balance the maximum possible friction force …

so that gives you the acceleration of the penguin …

the acceleration of the sled-plus-penguin is the same (just before it starts to slide) …

so that gives you the force F on the sled-plus-penguin.

Have a go!

18. Nov 2, 2008

sorry but i still dont get it

this is what i have
the Fs= 37.5 N
the sum of F is x = Fs - F = ma

but i dont know what F is so i cant solve for a

19. Nov 2, 2008

### tiny-tim

No (and what's x? ) … it's only Fs = ma … the friction is an internal force, between the penguin and the sled …

so the friction will only show up in a Newton's second law equation for either the penguin on its own, or the sled on its own …

in this case, do the penguin on its own, to find a, then do the penguin-plus-sled, to find F.

20. Nov 2, 2008