Download Packet 3 - Work Energy Power

Work Energy Power - Practice Packet #3
Name:
.
List vocabulary here:
List equations here (star the equations to memorize):
27. A disc of mass m slides with negligible friction along a flat surface with a velocity υ. The disc strikes a wall head-on and bounces
back in the opposite direction with a kinetic energy one- fourth of its initial kinetic energy. What is the final velocity of the disc?
(Α) υ/4 (Β) υ/2 (C) –υ (D) -υ/2
(Ε) -υ/4
28. A 2000 kg car, initially at rest, is accelerated along a horizontal roadway at 3 m/s 2. What is the average power required to bring
the car to a final speed of 20 m/s?
(A) 6 × 103 W
(B) 2 × 104 W
(C) 3 × 104 W
(D) 4 × l04 W
(E) 6 × 104 W
22. When a block slides a certain distance down an incline, the work done by gravity is 300 J. What is the work done by gravity if
this block slides the same distance up the incline?
(A) 300 J
(B) Zero
(C) -300 J
(D) It cannot be determined without knowing the distance the block slides.
(E) It cannot be determined without knowing the coefficient of friction.
23. A student holds one end of a string in a fixed position. A ball of mass 0.2 kg attached to the other end of the string moves in a
horizontal circle of radius 0.5 m with a constant speed of 5 m/s. How much work is done on the ball by the string during each
revolution?
(A) 0 J
(B) 0.5 J
(C) 1.0 J
(D) 2π J
(E) 5π J
24. For a particular nonlinear spring, the relationship between the magnitude of the applied force F and the resultant displacement x
from equilibrium is given by the equation F = kx2. What is the amount of work done by stretching the spring a distance xo ?
(A) kxo3
(B) ½ kxo
(C) ½ kxo3
(D) 1/3 kxo2
(E) 1/3 kxo3
25. A 1000 W electric motor lifts a 100 kg safe at constant velocity. The vertical distance through which the motor can raise the safe
in 10 s is most nearly
(A) 1 m
(B) 3 m
(C) 10 m
(D) 32 m
(E) 100 m
18. If the potential energy function is given by U(r) = br –3/2 + c, where b and c are constants, which of the following is an expression
for the force on the particle?
3b 5 / 2
3b 1 / 2
3 1/ 2
2 b 5 / 2
r
r
r
 cr
(A)
(B)
(C) r
(D) 2br 1/ 2  cr
(E)
5
2
2
2
Questions 17-18 refer to the following graph, which represents a hypothetical potential energy curve for a particle of mass m.
17. If the particle is released from rest at position r o, its speed at position 2ro is most nearly
8U 0
6U 0
4U 0
2U 0
U0
(A)
(B)
(C)
(D)
(E)
m
m
m
m
m
15. A force F is exerted by a broom handle on the head of the broom, which has a mass m. The handle is at an angle  to the
horizontal, as shown above. The work done by the force on the head of the broom as it moves a distance d across a horizontal
floor is
(A) Fd sin 
(B) Fd cos
(C) Fm cos
(D) Fm tan
(E) Fmd sin
16. A spring has a force constant of 100 N/m and an unstretched length of 0.07 m. One end is attached to a post that is free to rotate in
the center of a smooth table, as shown in the top view above. The other end is attached to a 1 kg disc moving in uniform circular
motion on the table, which stretches the spring by 0.03 m. Friction is negligible. What is the work done on the disc by the spring
during one full circle?
(A) 0 J
(B) 94 J
(C) 186 J
(D) 314 J
(E) 628 J
13. A conservative force has the potential energy function U(x), shown by the graph above. A particle moving in one dimension
under the influence of this force has kinetic energy 1.0 joule when it is at position x 1 Which of the following is a correct statement
about the motion of the particle?
(A) It oscillates with maximum position x2 and minimum position x0.
(B) It moves to the right of x3 and does not return.
(C) It moves to the left of x0 and does not return.
(D) It comes to rest at either x0 or x2.
(E) It cannot reach either x0 or x2.
71. A 2 kg object initially moving with a constant velocity is subjected to a force of
magnitude F in the direction of motion. A graph of F as a function of time t is shown.
What is the increase, if any, in the velocity of the object during the time the force is
applied?
(A) 0 m/s (B) 2.0 m/s (C) 3.0 m/s (D) 4.0 m/s (E) 6.0 m/s
1973M1. A horizontal force F is applied to a small block of mass m1 to make it slide along the top of a larger block of mass m2 and
length l. The coefficient of friction between the blocks is . The larger block slides without friction along a horizontal surface.
The blocks start from rest with the small block at one end of the larger block, as shown.
a. On the diagrams below draw all of the forces acting on each block. Identify each force.
b.
c.
d.
Find the acceleration of each block. a1 and a2, relative to the horizontal surface.
In terms of l, a1, and a2, find the time t needed for the small block to slide off the end of the larger block.
Find an expression for the energy dissipated as heat because of the friction between the two blocks.
1981M2. A swing seat of mass M is connected to a fixed point P by a massless cord of length L. A child also of mass M sits on the
seat and begins to swing with zero velocity at a position at which the cord makes a 60° angle with the vertical is shown in Figure
I. The swing continues down until the cord is exactly vertical at which time the child jumps off in a horizontal direction. The
swing continues in the same direction until its cord makes a 45° angle with the vertical as shown in Figure II: at that point it
begins to swing in the reverse direction. With what velocity relative to the ground did the child leave the swing?
1982M1. A 20 kg mass, released from rest, slides 6 meters down a frictionless plane inclined at an angle of 30º with the horizontal
and strikes a spring of spring constant K = 200 newtons/meter as shown in the diagram above. Assume that the spring is ideal,
that the mass of the spring is negligible, and that mechanical energy is conserved. Use g = 10 m/s², (sin30º = ½, cos 30º = 0.866)
a. Determine the speed of the block just before it hits the spring.
b. Determine the distance the spring has been compressed when the block comes to rest.
c. Is the speed of the block a maximum at the instant the block strikes the spring? Justify your answer.