Teaching Physics with the Physics Suite Edward F. Redish

Pulling two boxes

(a) A worker is pulling a pair of heavy crates along the floor with a rope. The rope is attached to the lower crate, which has a mass M. The upper crate has a mass m and the coefficient of friction between the crate and the floor is m. If the rope is held at an angle q, what is the maximum force F that can be exerted without the box beginning to slide?

(b) The worker knows that the lower crate has a mass of 50 kg and the upper a mass of 10 kg. She finds that if she pulls with a force of 120 N at an angle of 60º she can keep the crates sliding at a constant speed. Can you use this information to find the coefficient of friction between the lower crate and the floor? If you can, do it. If you can't, explain why not.

(c) In a different situation, she finds that she can pull a lower crate of mass 30 kg and an upper crate of mass 7.5 kg with a constant velocity of 50 cm/s pulling at an angle of 45º. Can you use this information to find the coefficient of friction between the lower crate and the floor? If you can, do it. If you can't, explain why not.

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Page last modified October 21, 2002: D07