Transcript Chapter 7: Work and Kinetic Energy-1 Reading assignment: Chapter 8.1-8.4 Homework: due Wednesday, Sept.

Chapter 7:
Work and Kinetic Energy-1
Reading assignment:
Chapter 8.1-8.4
Homework: due Wednesday, Sept. 28, 2005
Chapter 8: Q23, 4, 19, 24, 34, 36
• The concept of energy (and the conservation of energy)
is one of the most important topics in physics.
• Work
• Kinetic energy
• Energy approach is simpler than Newton’s second law.
What if the force varies? We have to __________ the force along x
xf
Work done by a varying force:
W   F  x dx
xi
Thus, the work is equal to the ________ under the F(x) vs. x curve.
Black board example 7.3
A force acting on a particle
varies as shown in the
Figure.
What is the total work done on the particle as it is moved from x =
0 to x = 8 m?
Hint: It is the area under the curve.
Consider a spring
law:
(Force required to stretch or
compress a spring by x):
Fs  k  x
k is the _________________ constant of a spring.
Stiff springs have a ____________ k value.
Work done by a spring
xi
xf
1
2
2
W  k ( x________  x _____ )
2
Black board example 7.3
A 0.500 kg mass is hung
from a spring extending
the spring by a distance
x = 0.2 m
(a) What is the spring constant of the spring?
(b) How much work was done on the mass by the gravitational force
(c) How much work was done on the mass by the spring force?
The kinetic energy of a particle is:
1
2
K  mv
2
A bullet of mass m = 0.020 kg moves at 500 m/s.
A truck of mass m = 1000 kg moves at 5 m/s
Which has more kinetic energy?
Work due to friction
If friction is involved in moving objects, work has to be
done against the kinetic frictional force.
This work is:
Wf  f k  d
Black board example 7.4
Angus is pulling a 10,000 kg truck with all his
might (2000N) on a frictionless surface
for 10.0 m.
(a) How much work is the man doing?
(b) What is the speed of the truck after 10 m.
(c) What is the speed of the truck after 10 m
if there is friction?
(friction coefficient: 0.0153)
Power
Power is the
done:
Average power:
(work done per time
interval Dt)
at which work is
dW
P
dt
W
P
Dt
The power can also be expressed as:

 ds  
dW
P
F
 F v
dt
dt
(Dot product)
The units of power are __________ (__/__) = watt (W)
Black board example 7.7
An elevator having a total mass of
3000 kg moves upward against
the gravitational force at a
constant speed of 9.13 m/s.
(a) What is the power delivered by
the motor?