Q1. A 20-N crate sits on a horizontal floor. The coefficient of kinetic friction

Question

Q1. A 20-N crate sits on a horizontal floor. The coefficient of kinetic friction between the crate and the floor is 0.40. A 40-N force is applied to the crate acting to the right, and the crate moves 2 meters. Which one of the following statements are true?

(a) The work done by the friction is -16 J.

(b) The work done by the 40-N force is -80 J.

(c) The work done by teh gravity is 20 J.

(d) The acceleration of the crate is 18 m/s^2.

Q2. An object is in a uniform circular motion. Which one of the following statements is true?

(a) Its acceleration must be constant

(b) Its velocity is toward the center of rotation

(c) The net force can be along the tangential direction

(d) If its velocity increases twic, the magnitude of the centripetal force must increase four times

(e) Only (a) and (b) are correct

Q3. In order to throw a ball as far as possible, you swing your arm behind your head. Which one of the following statements are true?

(a) this saves energy

(b) this increases the force on the ball;

(c) the work done on the ball is larger;

(d) this ensures the mechanical energy is conserved;

(e) all the forces involved are nonconservative forces

Q4. The minimum speed to orbit around the earth is?

(a) 11 km/s

(b) 8 km/s

(c) 7 km/s

(d) 6 km/s

Q5. If a car of 1000 kg moves from 1 m/s to 2 m/s within 10 seconds, how large is the average power of this car?

(a) 0 W

(b) 50 W

(c) 100 W

(d) 150 W

Explanation / Answer

First Question -

Weight of the crate, W = 20 N so mass m = W/g = 20/9.81 = 2.04 kg

Frictional force, Ff = u*W = 0.40*20 = 8 N

Now write the expression -

F - Ff = m*a

=> 40 - 8 = 2.04*a

=> a = 32/2.04 = 16 m/s^2

Now work done by the friction, Wf = Ff * d = 8 *(-2) = -16 J (direction of displacement is opposite to that of force).

Work done by 40 N force = 40*2 = 80 J

Hence the only correct answer is (a).

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