Q06. Work Energy & Power

1. Two objects with masses, m1 and m2, have the same

kinetic energy and are both moving to the right. The

same constant force is applied to the left to both masses.

If m1 = 4m2, the ratio of the stopping distance of m1 to

that of m2 is:

1. 1:4

2. 4:1

3. 1:2

4. 2:1

5. 1:1

F x K K

Kx

F

1

2

1x

x

vF

Alternatively:

2 20

2

v vx

a

2 2

0

2

m v v

F

K

F

2. At time t = 0 a particle starts moving along the x axis.

If its kinetic energy increases uniformly with t the net

force acting on it must be:

1. constant

2. proportional to t

3. inversely

proportional to t

4. proportional to t

5. proportional to 1 /

t

21

2K mv t

v t

1d v

F ad t t

3. Camping equipment weighing 6000 N is pulled across a

frozen lake by means of a horizontal rope. The coefficient

of kinetic friction is 0.05. How much work is done by the

campers in pulling the equipment 1000 m if its speed is

increasing at the constant rate of 0.20 m/s2 ?

1. –1.2 106 J

2. 1.8 105 J

3. 1.5 105 J

4. 4.2 105 J

5. 1.5 106 J

F w ma

aF w

g

aW w d

g

2

2

0.20 /6000 0.05 1000

10 /

m sN m

m s

54.2 10 J

4. A man pushes an 80-N crate a distance of 5.0 m upward

along a frictionless slope that makes an angle of 30° with

the horizontal. His force is parallel to the slope. If

the speed of the crate decreases at a rate of 1.5 m/s2, then

the work done by the man is:

1. –400 J

2. –254 J

3. –140 J

4. 140 J

5. 286 J

W

Fa

30

sin 30w

F w ag

sin 30a

F wg

sin 30a

W w dg

2

2

1 1.5 /80 5.0

2 10 /

m sN m

m s

140 N

5. Three identical springs (X,Y,Z) are arranged as shown.

When a 4.0-kg mass is hung on X, the mass descends 3.0 cm.

When a 6.0-kg mass is hung on Y, the mass descends:

1. 0.9 cm

2. 2.3 cm

3. 4.5 cm

4. 6.0 cm

5. 9.0 cm

z

y

k x m g

m g

kx

4.0

3.0

kg g

cm

k z k y z

k y z M g

2

yz

2y M g

k

2 3.0

6.04.0

cmkg g

kg g 9.0 cm

6. A 50-N force is the only force on a 2-kg crate that starts

from rest. At the instant the object has gone 2 m the

rate at which the force is doing work is:

1. 25 W

2. 75 W

3. 100 W

4. 500 W

5. 1000 W

P F v v at

Fa

m

21

2x at 2x

ta

2v xa

2xFv

m

50 10 /P N m s

2 2 50

2

m N

kg 10 /m s

500 W