Penguin and slide. Check my answers? I want to be prepared for the test

[zell_D]

Homework Statement

A sled weighing 60 N is pulled horizontally across snow so that the coefficient of kinetic friction between sled and snow is .100. A penguin of 70 N rides on the sled
a) The penguin digs claws into sled, what value of F do you need for the sled and penguin to move at constant speed
b) The penguin releases grip on the sled, coefficient of static friction between penguin and sled is .700, find the maximum horizontal force F that can be exerted on the sled before the penguin starts sliding off

Homework Equations

F=ma
Newton's 2nd and 3rd? law

The Attempt at a Solution

a) constant speed implied a=0
F=ma -> F=0
FN=Fg, FN=mg
Fapplied=friction
Fapplied=(.100)FN=.100*(60+70)=13N to move at constant speed

b) actually only 3 forces acting on the penguin FN Fg static friction
static friction = .700*mg
=.700(70)=49 N = max static friction threshold. So a force that's > 49 N needs to be exerted on the slide before the penguin falls off?

I am assuming kinetic friction has no role in this problem and that since acceleration is not asked for I don't have to set 49 to ma to find the max acceleration before it slides off and everything.

 

[Doc Al]

The Attempt at a Solution

a) constant speed implied a=0
F=ma -> F=0
FN=Fg, FN=mg
Fapplied=friction
Fapplied=(.100)FN=.100*(60+70)=13N to move at constant speed

Good.

b) actually only 3 forces acting on the penguin FN Fg static friction
static friction = .700*mg
=.700(70)=49 N = max static friction threshold. So a force that's > 49 N needs to be exerted on the slide before the penguin falls off?

You found the maximum static friction force that the sled can exert on the penguin. But what you need to find is the maximum force F you can exert on the sled before the penguin starts to slide off.

I am assuming kinetic friction has no role in this problem and that since acceleration is not asked for I don't have to set 49 to ma to find the max acceleration before it slides off and everything.

That's exactly what you have to do! (Just because they don't ask for the acceleration, doesn't mean you don't need to solve for it as a step towards the solution.)

 

[zell_D]

ok but when i set 49 to ma, i am kind of confused as to which mass to use to find the acceleration. the sled's mass or the total mass?

 

[Delphi51]

You are doing F = ma for the penguin only, so use the mass of the penguin.
The maximum force that can be exerted on the penguin by the sleigh is that friction force, thus the limit on the acceleration.

 

[zell_D]

so it would be 49=(70/9.8)a and then a = 6.86 m/s^2?
so that is the MAX acceleration this system can have before the penguin slides off right?

so in order to find the force that I need to apply on the sled to achieve this acceleration, I then use F=ma again? but this time, with the mass of the sled AND the penguin? not so sure but i am thinking about it like this:

the penguin will stay on the slide as long as the max acceleration is 6.86, so I am applying force on both masses. right?

 

[Delphi51]

Yes on all points!

 

[zell_D]

hmm I think I am starting to get forces, got to get into energy next argh >.<

but thanks for the help delphi51 and doc al