Work and Friction: Boy Sliding Down Hill

[flynostrich08]

Homework Statement

a little boy, with a mass of 40 kg, gets in his sled and slides down from the top of a 30 m high snowy slope. he reaches the bottom of the hill with a speed of 20 m/s. was the hill frictionless and if not, how much work was done by friction while the boy was sliding?

The Attempt at a Solution

all i can do is draw what is happening i don't know what equations to start with

 

[Doc Al]

Hint: Compare the initial mechanical energy with the final mechanical energy.

 

[flynostrich08]

ok so i enter Wnc= Ef-Ei which = (1/2mvfinal^2 + mgh)-(1/2mvintial^2+mgh)=-3760 is that the work of friction then since friction is the only nc force? or is there another equation to solve for from there. Thanks for your help by the way

 

[Doc Al]

ok so i enter Wnc= Ef-Ei which = (1/2mvfinal^2 + mgh)-(1/2mvintial^2+mgh)=-3760 is that the work of friction then since friction is the only nc force? or is there another equation to solve for from there.

Yes, that's the work done by friction. That's all you need.

 

[flynostrich08]

you have been extremely helpful thanks!

i have another question i need help with that i posted and can't get an answer to. if you don't mind-A satellite is orbiting above the earth. By what factor must the orbital radius be changed in order to double the period of the orbit?

I have to solve for two variables T-period and r-radius of satellite but i can't solve for two variables if they are both in the same equations.

 

[Doc Al]

i have another question i need help with that i posted and can't get an answer to. if you don't mind-A satellite is orbiting above the earth. By what factor must the orbital radius be changed in order to double the period of the orbit?

I have to solve for two variables T-period and r-radius of satellite but i can't solve for two variables if they are both in the same equations.

You've already started a thread on that new problem (https://www.physicsforums.com/showthread.php?t=410242). See my comments there.

Hint: You can express T in terms of r.