# Newton's Second Law Problems

• Stianos
In summary, the conversation is about a loaded penguin sled on an inclined plane. The coefficients of static and kinetic friction are given, and the question is about finding the minimum force needed to prevent the sled from slipping down the plane. The person asking for help shares their work and asks for clarification on the direction of the friction force. The expert confirms that the friction force should be pointing up the slope, not down as shown in the work.

#### Stianos

1. A loaded penguin sled weighing 70 N rests on a plane inclined at 20° to the horizontal. Between the sled and the plane the coefficient of static friction is 0.25, and the coefficient of kinetic friction is 0.16.
(a) What is the minimum magnitude of the force F, parallel to the plane, that will prevent the sled from slipping down the plane?

3. I cannot figure out what I'm doing wrong. I draw my free body diagram, write out an x/y chart, and then solve.
I write a lot, so I'm just post an imgur link with my work in it.
http://imgur.com/pCNNQ

Can anyone show me what I'm doing wrong?

Hello, Stianos.

If the sled is on the verge of slipping down the slope, what is the direction of the friction force?

Also, looks like you're treating the coefficient of friction as being the same as the force of friction.

If the sled is on the verge of slipping down the slope, then the friction is pointing up the slope?
Should I not do that? What should I do instead?
Thank you for responding, sorry I couldn't not respond immediately.

Stianos said:
If the sled is on the verge of slipping down the slope, then the friction is pointing up the slope?
Yes. It looked to me that you had the friction pointing down the slope.