Chapter 7-8 Pretest

1. A motorcycle plus rider has a mass of 1000 kg. They travel inside a loop with a radius of 6 m. At the lowest point, the speed is 20 m/s. What force does the track exert on the motorcycle?

FC = mv2/r FC = 1000 x 202/6

FC = 67000 N

2. Use the universal law of gravitation to find the force between a 100 kg person standing on the surface of the earth and the earth itself.

F = Gm1m2/d2

F = (6.67 x 10-11)( 6 x 1024) (100)/(6.37 x 106)2

F = 986 N

3. A uniform horizontal bar 1.00 m long is pivoted at the left end. If the bar weighs 1.50 N, the clockwise torque is: A) 0.50 m-N, B) 0.75 m-N, C) 1.00 m-N, D) 1.50 m-N.

T = F x lT = 1.5 x 0.5 (center of gravity is ½ of 1 m)T = 0.75 Nm (B)

4. To produce rotational equilibrium in the bar of question 3 what upward force must be exerted 0.75 m from the pivoted end? A) 0.50 N, B) 0.75 N, C) 1.00 N, D) 1.50 N.

T = F x l0.75 Nm = F x 0.75 F = 1 N

5. A uniform bar weighing 30.0 N is 5.00 m long. Attached At the left end is a weight of 50.0 N, and attached at the right end is a weight of 20.0 N. To prevent translational motion of the bar, what upward motion must be applied? A) 30.0 N, B) 70.0 N, C) 80.0 N, D) 100.0 N

30 + 50 + 20 = 100 N

6. To prevent both translational and rotational motion of the bar in number 5, a single upward equilibrant force must be applied between the: A) left end and center of the the bar, but nearer the left end, B) left end and center of the bar, but nearer the center, C) right end and center of the bar, but nearer the right end, D) right end and center of the bar, but nearer the center.

Pick the left end as the pivot point.

CW CCW30 x 2.5 + 20 x 5 = 100 x d 75 + 100 = 100 x d 175 / 100 = d 1.75 = d B) between left end and center of the bar, but nearer the center

7. A beam 4.00 m long weighs 1400 N. Its center of gravity is 1.50 m from the heavy end. At the heavy end a weight of 1000 N is attached, while at the light end there is a weight of 1350 N. Find the magnitude and point of application of the equilibrant force.

1400 + 1350 + 1000 = 3750 N up

Pick the left end as the pivot point.CW CCW

1400 x 1.5 + 1350 x 4 = 3750 x d 2100 + 5400 = 3750 x d 7500 = 3750 x d

d = 2 m from the left end

8. The gravitational force between two masses is 36 N. What is the gravitational force if the distance between them is tripled?a. 4.0 N b. 9.0 Nc. 18 N d. 27 N

8. The gravitational force between two masses is 36 N. What is the gravitational force if the distance between them is tripled?a. 4.0 N b. 9.0 Nc. 18 N d. 27 N

9. If a net torque is applied to an object, that object will experience which of the following?a. a constant angular speedb. an angular accelerationc. a constant moment of inertiad. an increasing moment of inertia

9. If a net torque is applied to an object, that object will experience which of the following?a. a constant angular speedb. an angular accelerationc. a constant moment of inertiad. an increasing moment of inertia

10. Where should a force be applied on a lever arm to produce the most torque?a. closest to the axis of rotationb. farthest from the axis of rotationc. in the middle of the lever armd. It doesn’t matter where the force is applied.

10. Where should a force be applied on a lever arm to produce the most torque?a. closest to the axis of rotationb. farthest from the axis of rotationc. in the middle of the lever armd. It doesn’t matter where the force is applied.

11. Suppose a doorknob is placed at the center of a door. Compared with a door whose knob is located at the edge, what amount of force must be applied to this door to produce the torque exerted on the other door?a. one-half as muchb. two times as much c. one-fourth as muchd. four times as much

11. Suppose a doorknob is placed at the center of a door. Compared with a door whose knob is located at the edge, what amount of force must be applied to this door to produce the torque exerted on the other door?a. one-half as muchb. two times as much c. one-fourth as muchd. four times as much

12. If you cannot exert enough force to loosen a bolt with a wrench, which of the following should you do?

a. Use a wrench with a longer handle. b. Tie a rope to the end of the wrench and pull on the rope.c. Use a wrench with a shorter handle. d. You should exert a force on the wrench closer to the bolt.

12. If you cannot exert enough force to loosen a bolt with a wrench, which of the following should you do?

a. Use a wrench with a longer handle. b. Tie a rope to the end of the wrench and pull on the rope.c. Use a wrench with a shorter handle. d. You should exert a force on the wrench closer to the bolt.

13. A ball is whirled in a horizontal path on the end of a string. Predict the path of the ball when the string breaks, and explain your answer.

The ball continues in a straight line in the direction it was going when the string broke. (inertia)

14. How does the use of a machine alter the work done on the object?

The use of a machine does not alter the work done on an object.