A dockworker loading crates on a ship finds that a 21-kg crate, initbaly at rest

Question

A dockworker loading crates on a ship finds that a 21-kg crate, initbaly at rest on a horizontal surface, requires a 70-N honizontal force to set it in motion. However, after the crate is in motion, a horizontal force of s2 N is required to keep it moving with a constant speed. Find the coefficients of static and kinetic friction between crate and floor. static friction inetic friction Two blocks are fastened to the ceiling of an elevator as in Figure P4.19. The elevator accelerates upward at 1.90 m/s2. The blocks both have a mass of 13.5 kg. Find the tension in each rope. top rope bottom rope The coefficient of static frict on between the m 3.00 kg crate and the 35.0ncline of Figure P4,41 s 0.290, what minimum force F must be applied to the crate perpendicular to the incline to prevent the crate from sliding down the incline? 5.0

Explanation / Answer

Given,

m = 21 kg ; F = 70 N ; F' = 52 N ;

We know that the frictional force is given by:

Ff = u N

Considering the sum of forces in horizontal direction, when the mass is stationary(at rest) static frictional force is into considertation,

Fx = F - Ff(s) = 0

Ff(s) = F

us (mg) = F => us = F/mg

us = 70/21 x 9.81 = 0.339

Hence, us = 0.339

Once the mass starts moving,

F' - Ff(k) = 0

uk (mg) = F' => uk = F'/(mg)

uk = 52/21 x 9.81 = 0.252

Hence, uk = 0.252

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