Phys101
Term:122
Online HW-Ch07-Lec02
Q1:
A package is released from the top of a building. Over its entire flight the work done
by gravity and the work done by air resistance, respectively, are:
A. positive; positive
B. positive; negative
C. negative; positive
D. negative; negative
E. unknown since vital information is lacking
Ans:
B, because gravitational force is with the direction of motion, hence increasing the
speed of the package (positive work), while air resistance is the opposite to the
direction of motion, hence reducing the speed of the package (negative work).
Q2:
A 2 kg block is attached to a horizontal ideal spring with a spring constant of 2000
N/m. The block can slid on a horizontal frictionless surface. If the block is
compressed by 4 cm against the spring and released from rest, what is the maximum
speed (in m/s) of the block? (Give your answer in three significant figures form)
Ans:
xi = 4 cm = 0.04 m ; vi = 0
Maximum speed at xf = 0 m; vf = ?
Work − Kinetic Energy Theorem:
∆K = Wspring
Kf − Ki =
1
1
kxi 2 − kxf 2
2
2
0
1
1
1
1
⇒ mvf 2 − mvi 2 = kxi 2 − kxf 2
2
2
2
2
0
⇒ vf = �
kxi 2
2000 × 0.042
= �
= 1.26 m/s
m
2
KFUPM-Physics Department
1
Phys101
Term:122
Online HW-Ch07-Lec02
Q3:
An escalator is used to move 20 people (60 kg each) per minute from the first floor of
a department store to the second floor as shown in the figure. Neglecting friction, find
the required power (in Watt). (Give your answer in three significant figures form)
𝐀𝐧𝐬:
Power: P =
W
∶ time = 1 min = 60 sec
time
Wrequried = −Wgravity = −(−(mg)(h)) = +mgh
Where h = 5 m; & 𝑚 = 20 × 60 = 1200 𝑘g
mgh
W
=
time
time
1200 × 9.8 × 5
=
= 980 Watt
60
So: P =
KFUPM-Physics Department
2