# Horizontal force is needed to pull the sled

1. Mar 21, 2005

### vicervixen

A 12 kg sled is pulled along at a constant velocity on a horizintal surface by a horizontal force of 11N. How much horizontal force is needed to pull the sled at a constant velocity it two 57 kg girls are sitting in it?
How would I go about solving this?
Thanks, Amanda

2. Mar 21, 2005

### myself

Draw a free body diagram for the first case. Notice that the sled is in equilibrim because it is moving at a constant velocity. Therefore, all forces sum up to zero. Then, it is possible to figure out the coefficient of friction, which is used to solve the problem.

3. Mar 22, 2005

### Brian Earley

4. Mar 22, 2005

### whozum

Total mass = 12kg
Force of Gravity = 12x9.8 = 117.6N
Normal force = -Fg = -117.6N

Net vertical Force = Force of Gravity + Normal force = 117.6 + (-117.6) = 0N
Net Horizontal Force = Pull force + Friction force = 11N + (mu_k)*N = 0N since velocity is constant, there is no acceleration.

11 - mu_k*117.6 = 0
-117.6mu_k = -11

mu_k = -11/-117.6 = 0.1

New mass is 12+2x57 = 126kg
Gravity Force = - Normal force = 1234.8N
Fy = 0
Fx = Pull force + Friction Force = 0
Friction force = mu_k * N = 123.4N
Pull force + 123.4 = 0
Pull force = -123.4N.

The negative sign just means its in the opposite direction of the friction force.

*Editted for new mass

Last edited: Mar 22, 2005
5. Mar 22, 2005

### Brian Earley

New mass is 126kg.