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PHY 2048C
General Physics I with lab
Spring 2011
CRNs 11154, 11161 & 11165
Dr. Derrick Boucher
Assoc. Prof. of Physics
Session 13, Chapter 11
Chapter 11 Homework
Due Sunday 2/27 @ midnight
(previously Friday 2/25)
UPCOMING EVENTS:
Monday 2/28 Finish Chap. 11 and
review for:
EXAM 2 on Wed. 3/2
chapters 8-11
Chapter 11
Practice Problems
1-7 odd, 23, 25, 33, 49, 51, 63, 67
Unless otherwise indicated, all practice
material is from the “Exercises and Problems”
section at the end of the chapter. (Not
“Questions.”)
The Basic Energy Model
W > 0: The environment does work on the system
and the system’s energy increases.
W < 0: The system does work on the environment
and the system’s energy decreases.
Defining Work
Consider a force acting on a particle as the particle moves
along the s-axis from si to sf. The force component Fs
parallel to the s-axis causes the particle to speed up or
slow down, thus transferring energy to or from the particle.
We say that the force does work on the particle by this
force. is.
The unit of work is J.
We must evaluate the integral either geometrically,
by finding the area under the curve, or by actually
doing the integration. (last resort!)
The Work Done by a Variable
Force
To calculate the work done on an object by a force
that either changes in magnitude or direction as the
object moves, we use the following:
We must evaluate the integral either geometrically,
by finding the area under the cure, or by actually
doing the integration.
Work Done by a Constant Force
Consider a particle which experiences a constant
force which makes an angle θ with respect to the
particle’s displacement. The work done is
Both F and θ are constant, so they can be taken
outside the integral. Thus
You should recognize this as the dot product of the
force vector and the displacement vector:
EXAMPLE 11.1 Pulling a suitcase
QUESTION:
EXAMPLE 11.1 Pulling a suitcase
EXAMPLE 11.1 Pulling a suitcase
EXAMPLE 11.1 Pulling a suitcase
EXAMPLE 11.1 Pulling a suitcase
Example #1, Problem 11-41, p. 332
Example #2, Problem 11-43a, p.
332
Example #3, Problem 11-16, p. 330
Example #4, Problem (made-up)
calculus and work
The BIG equations
Example #5, Problem 11-25, p. 331
Example #6, Problem 11-46, p. 332
Power
The rate at which energy is transferred or
transformed is called the power, P, and it is
defined as
The unit of power is the watt, which is
defined as 1 watt = 1 W = 1 J/s.
Other unit: horsepower (hp) 1 hp = 746 W
Example #7, Problem (made-up)
power
Review Problem #1: 10 minutes
A 5-g bullet is fired horizontally into a 3-kg wooden block resting
on a horizontal surface. The coefficient of kinetic friction
between the block and the surface is 0.20. The bullet remains
embedded in the block, which slides 25 cm along the surface
before stopping. What was the velocity of the bullet in m/s?
Review Problem #2a: 7 minutes
The radius of a Ferris wheel is 5.0 m and it makes one rotation in
10.0 s. (a) What is the apparent weight, in N, of a 65-kg
passenger at the top of the wheel? (Assume she sits on top of
the seat, which always remains upright as the wheel rotates..)
Review Problem #2b: 5 minutes
The radius of a Ferris wheel is 5.0 m and it makes one rotation in
10.0 s. (a) What is the apparent weight, in N, of a 65-kg
passenger at the bottom of the wheel? (Assume she sits on top
of the seat, which always remains upright as the wheel rotates..)
Review Problem #3: 15 minutes
A pump is required to lift 800 kg of water per minute from a well
and eject it with a speed of 20 m/s. Assume the top of the water
in the well is a constant 10 m below the point at which the water
is ejected. What horsepower engine is needed to run this pump?