Question

MULTIPLE CHOICE, Show your work in the space provided, Circle the correct answer 1. An object is moving with constant non-zero velocity. Which of the following statements about it must be true? As shown in the figure, a woman is straining to lift a large crate, but without success because it is too heavy. We denote the forces on the crate as follows: P is the magnitude of the upward force being exerted on the crate by the person, C is the magnitude of the vertical contact force on the crate by the floor, and Wis the weight of the crate. How are the magnitudes of these forces related while the person is trying unsuccessfully to lift the crate? A) P+C=w B) P+C<W C) P+C>w D) P=C 2. A force of 10 N keeps a wooden box moving at a steady speed of 5 m/s, on a wooden floor. If the mass of the box is 5 kg, calculate the coefficient of kinetic friction? 3. A 55-kg box rests on a horizontal surface. The coefficient of static friction between the box and the surface is 0.30. A horizontal 140-N force is applied to the box. What is the friction force on the box?

Answer 1

1. The forces acting in the upward direction are P and C. While the only force working in the downward direction is the weight of the object, which is W.

If the woman can't lift the object, it means that,

                       P + C < W

Hence, the correct option is 'B'.

2.

(Normal Force) 10 N 5 kg f(Friction) W

Force of friction is given by

                   $f = \mu N$

where $\mu$ is the coefficient of friction, and N is normal force.

From the free body diagram of the object, we can write

                    $N = W = 5 \times 9.8 \,\, (\text{acceleration due to gravity}) = 49 \,\, Newton$

So, the force of friction is

                   $f = 49 \mu$

Again from the free body diagram, it is clear that, if the box is moving with constant velocity then

                

             or, $49 \mu = 10$

            or, $\mu = 0.204$

Hence, the coefficient of the kinetic friction between the wooden box and the surface is 0.204.

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