Transcript Chapter 7: Work and Kinetic Energy-1 Reading assignment: Chapter 7.6-7.9 Homework: due Monday, Sept 26, 2005 Chapter 7: 30, 41, 43, 58, 65, 68 •

Chapter 7:
Work and Kinetic Energy-1
Reading assignment: Chapter 7.6-7.9
Homework: due Monday, Sept 26, 2005
Chapter 7: 30, 41, 43, 58, 65, 68
• 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.
Work
(as defined by a physicist)
Definition:
The work done on an object by an external force is
- the ____________ of the component of the force in the
direction of the displacement and the magnitude of the
________________.
W  F  d  cos 
How much work is done
when just holding up an
object?
W  F  d  cos
W
How much work is done
when the displacement is
perpendicular to the
force?
W  F  d  cos
W
F = 10 N
What is the work done by
 = 60°
- the gravitational force
- the normal force
- the force F
when the block is displaced
along the horizontal.
The total work is:
- the sum of the work done by all forces
- or: Wtotal


 Fnet  dnet
d = 10m
What is the work done when lifting?
- By the gravitational force?
- By the applied force?
W  F  d  cos
W
Strongest man lifting 140 kg boulder by 1 m.
Sign convention:
W is positive:
If F and d are parallel
If energy is transferred into the system
W is positive:
If F and d are antiparallel
If energy is transferred out of the system
Work is a ________ quantity. (not a vector)
Work has units of
newton·meter (N·m) = the joule (J)
Black board example 7.5
A man loads a refrigerator onto a truck using a ramp. Ignore
friction.
He claims he would be doing less work if the length of the ramp
would be longer. Is this true?
Black board example 7.1
A donkey is pulling a cart
with a force of magnitude
F = 500 N at an angle of
30º with the horizontal.
Calculate the work done by
the donkey as the cart is
pulled for one mile (1648
m).
500 N
Definition of dot product and work
Work is the scalar product (or ____ product)
of the force F and the displacement d.
 
W  F  d  F  d  cos
F and d are vectors
W is a __________ quantity
Scalar product between
vector A and B
Definition:
 
A  B  AB  cos
Scalar product is ______________:
   
A B  B  A
_________________ law of multiplication:
  
   
A  ( B  C)  A  B  A  C
Scalar Product using unit vectors:
We have the vectors A and B:




A  Ax i  Ay j  Az k




B  Bx i  By j  Bz k
Then:
 
A  B  __________
__________
__
 
A  A  __________
__________
_  A2
Black board example 7.2
A particle moving in the x-y plane undergoes a
displacement d = (2.0i + 3.0j) m as a constant force
F = (5.0i + 2.0j) N acts on the particle. Calculate
(a) The magnitude of the displacement and the force.
(b) The work done by F.
(c) The angle between F and d.