Physics Experiment

Aug 2012
131
1
new orleans
During a physics experiment, a fan is used to apply a push to a cart on a hortizontal track. A student would like the net force on the cart to be 12 N to the left. The student knows that the friction force on the cart by the track will be 3 N to the left. The fan should exert a force of

A. 15 N to the right
B. 15 N to the left
C 9 N to the right
D 9 N to the left
E None of these
 
Dec 2011
2,314
916
St. Augustine, FL.
If there is a net force of 12 N to the left on the cart, meaning it will move to the left, the force of kinetic friction will oppose the motion, that is, will point to the right.

Did you give the problem exactly as stated? Normally, friction does not contribute to motion, but rather retards it.
 
Aug 2012
131
1
new orleans
there was a typo
During a physics experiment a fan is used to apply a push to a cart on a horizontal track. A student would like the net force on the cart to be 12 N to the right. The student knows that the friction force on the cart by the track will be 3 n to the left.
 
Dec 2011
2,314
916
St. Augustine, FL.
If we choose to the right to be positive, we may then state:

\(\displaystyle F_{\text{net}}=F_{\text{fan}}-f_{k}\)

\(\displaystyle F_{\text{fan}}=F_{\text{net}}+f_{k}\)

We are told we want/have:

\(\displaystyle F_{\text{net}}=12\text{ N}\)

\(\displaystyle f_{k}=3\text{ N}\)

Thus, we have:

\(\displaystyle F_{\text{fan}}=12\text{ N}+3\text{ N}=15\text{ N}\)

Since this is positive, we know it is to the right by our definition.