### PROBLEM:

### Suppose two children push horizontally, but in exactly opposite directions, on a third child in a wagon. The first child exerts a force of 75.0 N, the second a force of 90.0 N, friction is 12.0 N, and the mass of the third child plus wagon is 23.0 kg.

### (a) What is the system of interest if the acceleration of the child in the wagon is to be calculated?

### (b) Draw a free-body diagram, including all forces acting on the system.

### (c) Calculate the acceleration.

### (d) What would the acceleration be if friction were 15.0 N?

**SOLUTION:**

PART A

If the acceleration of the child in the wagon is to be calculated, the system of interest is the wagon with the child in it.

PART B

Considering all the forces acting on the wagon and child with mass *m. *there will be 5 forces. The horizontal forces are the two forces coming from the two other children, denoted as and . We also have friction going against the direction of motion.

The vertical forces are the weight of the wagon and the child and the normal force. These forces are shown in the free-body diagram shown.

PART C

From Newton’s second law of motion, we know that

Considering the horizontal direction to the right as positive, the forces involved are . Therefore, we have

Solving for acceleration, we have

The direction of the motion is towards the direction of the child pushing at 90.0 N.

PART D

If the friction is 15 N, the acceleration would be

There would be no acceleration, meaning that the system is in equilibrium. The system is either not moving or it’s moving at a constant velocity.