College Physics
11th Edition
ISBN:9781305952300
Author:Raymond A. Serway, Chris Vuille
Publisher:Raymond A. Serway, Chris Vuille
Chapter1: Units, Trigonometry. And Vectors
Section: Chapter Questions
Problem 1CQ: Estimate the order of magnitude of the length, in meters, of each of the following; (a) a mouse, (b)...
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Question
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Why is -1/5g is nigative here
![Wa = Fa-d = mgd cos 180° = -mg.d
M Then we conclude that:
* the work done by F therefore can be negative, positive or zero if the motion
occurred horizontally.
{ 90 }
Chapter 7: Kinetic Energy and Work
ample
V An elevator with mass of m =
500 kg is descending (moving downward)
with speed vi = 4.0 m/s when its supporting cable begins to slip, allowing
it to fall down with constant acceleration of a = g/5 as shown in the figure.
(a) During the falling through a distance d = 12 m, what is the work Wg done
on the elevator by the gravitational force F , ?
(b) ) During the 12 m fall, what is the work Wr done on the elevator by the
upward pull (T) of the elevator cable?
(c) What is the net-work W done on the cab during the fall? The net-work is
the sum ( E )of the works done by the forces acting on the cab:
(d) What is the cab's kinetic energy at the end of the 12 m fall?
O Solution:
-Elevator
cable
a. The falling and gravitational force F are in the same
Does
direction ( 0
0), so
7 negative
Cab
work
Wg = Fg.d = mgd cos 0°
Does
positive
work
Wg = (500 kg) (9.8) (12 m) = 58800 J × 59 kJ
b. EF = ma →
Fr – Fg
= ma
FT = ma + Fg
%3D та + mg 3 т(а + g)
Wt = Frd cos 0 =
m(a + g)d cos 0
Does
7 negative
W, = m(-8+8)d
1
g +g)d cos 0 = m
g)d cos 0
Cab-
work
Does
(4
Wr
(500 kg) |
× 9.8 m/2)(12 m) cos 180° = -47 kJ
positive
work
c. W = Wg + Wr = 58800 J – 47040 J =
d. W + K. E.j = K.E.f
11760 J z 12 kJ
1
2
1
(500 kg) (4-
m
K.E.f = ¬mv;² + W =
+ 11760 J
K.E.f = 15760 J × 16 kJ
91
Chapter 7: Kinetic Energy and Work
ork done by a spring force
1. The spring force is an example of variable force not
constant, its change with the displacement.
Block
attached
to spring
x-0
F,= 0
2. When the spring at equilibrium, that means x = 0,
and it's at rest , there is no compressed nor extended.](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F7a13ec14-7762-4a7f-a063-c762b04b68f9%2F8284aff5-0803-4a45-81b2-bee7747581a0%2Fl97gea5_processed.jpeg&w=3840&q=75)
Transcribed Image Text:Wa = Fa-d = mgd cos 180° = -mg.d
M Then we conclude that:
* the work done by F therefore can be negative, positive or zero if the motion
occurred horizontally.
{ 90 }
Chapter 7: Kinetic Energy and Work
ample
V An elevator with mass of m =
500 kg is descending (moving downward)
with speed vi = 4.0 m/s when its supporting cable begins to slip, allowing
it to fall down with constant acceleration of a = g/5 as shown in the figure.
(a) During the falling through a distance d = 12 m, what is the work Wg done
on the elevator by the gravitational force F , ?
(b) ) During the 12 m fall, what is the work Wr done on the elevator by the
upward pull (T) of the elevator cable?
(c) What is the net-work W done on the cab during the fall? The net-work is
the sum ( E )of the works done by the forces acting on the cab:
(d) What is the cab's kinetic energy at the end of the 12 m fall?
O Solution:
-Elevator
cable
a. The falling and gravitational force F are in the same
Does
direction ( 0
0), so
7 negative
Cab
work
Wg = Fg.d = mgd cos 0°
Does
positive
work
Wg = (500 kg) (9.8) (12 m) = 58800 J × 59 kJ
b. EF = ma →
Fr – Fg
= ma
FT = ma + Fg
%3D та + mg 3 т(а + g)
Wt = Frd cos 0 =
m(a + g)d cos 0
Does
7 negative
W, = m(-8+8)d
1
g +g)d cos 0 = m
g)d cos 0
Cab-
work
Does
(4
Wr
(500 kg) |
× 9.8 m/2)(12 m) cos 180° = -47 kJ
positive
work
c. W = Wg + Wr = 58800 J – 47040 J =
d. W + K. E.j = K.E.f
11760 J z 12 kJ
1
2
1
(500 kg) (4-
m
K.E.f = ¬mv;² + W =
+ 11760 J
K.E.f = 15760 J × 16 kJ
91
Chapter 7: Kinetic Energy and Work
ork done by a spring force
1. The spring force is an example of variable force not
constant, its change with the displacement.
Block
attached
to spring
x-0
F,= 0
2. When the spring at equilibrium, that means x = 0,
and it's at rest , there is no compressed nor extended.
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