A 90-pound container has a coefficient of friction of 0.61. How much force is needed to start moving it?

A 90-pound container has a coefficient of friction of 0.61. How much force is needed to start moving it?

• A. 20 pounds
• B. 40 pounds
• C. 60 pounds
• D. 54.9 pounds

Answer: (D)

The force needed to start moving is the maximum static friction, which equals the coefficient of friction times the normal force. On a horizontal surface, the normal force equals the weight, so you multiply 0.61 by 90 pounds. That gives 0.61 × 90 = 54.9 pounds. This is the threshold push required to overcome sticking and just start motion. The other numbers don’t match the friction limit, so they don’t represent the force needed to initiate movement.

The force needed to start moving is the maximum static friction, which equals the coefficient of friction times the normal force. On a horizontal surface, the normal force equals the weight, so you multiply 0.61 by 90 pounds. That gives 0.61 × 90 = 54.9 pounds. This is the threshold push required to overcome sticking and just start motion. The other numbers don’t match the friction limit, so they don’t represent the force needed to initiate movement.