Question

Tactics Box 5.1 Working with Friction Forces

Learning Goal:

To practice Tactics Box 5.1 Working with friction forces.

Although friction is a complicated force, many aspects of it can be described with a simple model. This Tactics Box presents the conditions under which these models are valid and outlines the essential steps in solving friction problems.

TACTICS BOX 5.1 Working with friction forces

1. If the object is not moving relative to the surface it's in contact with, the friction force is static friction. Draw a free-body diagram of the object. The direction of the friction force is such as to oppose sliding of the object relative to the surface. Then use ProblemSolving Strategy 5.1 to solve for fs° If fs is greater than fs \max =μ n, then static friction cannot hold the object in place. The assumption that the object is at rest is not valid, and you need to redo the problem using kinetic friction.

2. If the object is sliding relative to the surface, then kinetic friction is acting. From Newton's second law, find the normal force n. The friction force is then directed opposite to the motion, and its magnitude is fk=μk n.

3. If the object is rolling along the surface, then rolling friction is acting. From Newtor's second law, find the normal force n. The friction force is then directed opposite to the motion, and its magnitude is fr=μr n.

Suppose you have to move a heavy crate of weight 875 N by sliding it along a horizontal concrete floor. You push the crate to the right with a horizontal force of magnitude 300 N, but friction prevents the crate from sliding.

Part A

Draw a free-body diagram of the crate in the diagram below. Use the dot as the particle representing the crate and make sure to draw your vectors so that they have the correct orientation and their magnitudes, in newtons, are consistent with the conditions of the problem.

Draw the vectors starting at the black dot. The location, orientation, and length of the vectors will be graded.

Part B

What is the magnitude Fp of the minimum force you need to exert on the crate to make it start sliding along the floor? Let the coefficient of static friction μs between the crate and the floor be 0.56 and that of kinetic friction, μk, be 0.47.

Express your answer numerically in newtons to three significant figures.

Part C

Now assume that the crate is sliding along the floor with constant velocity. What is the magnitude Fp of the force you need to exert on the crate to make it continue to slide along the floor with constant velocity?

Express your answer numerically in newtons to three significant figures.

Answer 1