Transcript Kinetic Energy

Chapter 3
Energy
The Goal of this activity is to
• Introduce the student to the terms work,
kinetic energy and gravitational potential
energy,
• Illustrate the concepts with simple
example problems, and
• Test the students understanding with
pertinent problems.
Your responsibilities…
• Read Chapter 3, Section 3.1 and 3.2, pages 62 to 69. It would
be good to do it right now, before proceeding – its only 7 pages
– with pictures – read it now.
• View this presentation (adapted from Professor Martino’s
presentations available on my faculty web page).
• On a single sheet of paper describe, and define each of the
following energy terms illustrating the concept with one
numerical example: Work, Kinetic Energy, (Gravitational)
Potential Energy.
• Submit written solutions to the 7 “Practice Problems” in this
presentation following the standard HW problem format.
Work
• An applied force acting
through a distance
parallel to the force
• Units of work (and
energy) = joule (J)
• Zero // distance, no work
• Displacement
perpendicular to applied
force, no work
Fig 3.3 Work
Against Gravity
Practice Problem #1
• A weightlifter bench presses 80kg (approx.
175lbs.) 0.75m straight up.
• a. How much work does she do, assuming
constant velocity, in one lift (just the 0.75m
straight up)?
• b. How many repetitions would she have to
do to burn off 1 hamburger (400 Calories).
Note 1 Calorie = 4.186 J.
Practice Problem #2
• A person pushes a stalled 2000 kg car from
rest to a final speed of 2.0 m/s. During this
time the car moves 20 m. Neglecting friction
v
between the car and the road, find:
• a. the final acceleration of the car
• b. the horizontal force exerted on the car
f
(Hint: Use Newton’s 2nd Law to find the force using the acceleration.)
• C. the work done on the car
Practice Problem #3
A person pushes a 10 kg box at a constant velocity over a distance of 4
m. The frictional force between the box and the floor is 30% the weight
of the box. How much work does the person do in pushing the box?
Power
• The rate at which work
is done
• Units: watts (W) ;
horsepower (hp)
• Example: Walking
versus running upstairs
• The “power bill” - you
actually pay for “work”
(kWh), which is related
to energy
Units of Power
• Horsepower
(1 hp = 550 ft·lb/s)
• Watts
(N·m/s or J/s)
• 1 hp = 746 W
Fig 3.5
Convert 9.8 kW to hp
Practice Problem #4
• An elevator m=800 kg has a maximum load
of 8 people or 600 kg. The elevator goes up
10 stories = 30 m at a constant speed of 4
m/s. What is the average power output of the
elevator motor if the elevator is fully loaded
with its maximum weight? (neglect friction)
(Hint: First determine the time the elevator takes to go up 10 stories, then determine the work the elevator motor
exerted. Finally find the power of the motor.)
Motion, Position and Energy
• Work and energy
are related
• Energy = ability to
do work
• When work is done
on something, a
change occurs in
its energy level
Next:
• Relationship
between work and
energy associated
with position
• Relationship
between work and
energy of motion
Potential Energy (PE)
• Energy associated with
“position”
– “Potential” to then do work
• Gravitational Potential Energy
(GPE)
– Measuring h: need a
reference position (or
reference height)
• Work can “change” PE
• Potential Energy can
“change” into Kinetic Energy
PE = mgh
Practice Problem #6
• A cart is loaded with a brick and pulled at
constant speed along an inclined plane
to the height of a seat-top. If the mass of
the loaded cart is 3.0 kg and the height
of the seat top is 0.45 meters, then what
is the potential energy of the loaded cart
at the height of the seat-top?
Kinetic Energy (KE)
KE = 1 mv 2
2
• Energy associated with
“motion”
• Results from work or
change in potential energy
• Speed squared!
• Double the speed,
KE increases by 4
Practice Problem #5
• A 500 kilogram car is driving at 15
meters/second. What's its kinetic
energy?
Practice Problem #7
A forklift lifting a crate of mass 100 kg at a
constant velocity to a height of 8 m over a time
of 4 s. The forklift then holds the crate in place
for 20 s.
a. How much power does the forklift exert in
lifting the crate?
b. How much power does the forklift exert in
holding the crate in place?
Practice Problems adapted
from…
• http://zebu.uoregon.edu/~probs/mech/w
ork.html
• http://www.sparknotes.com/testprep/boo
ks/sat2/physics/chapter7section6.rhtml
• http://www.physicsclassroom.com/class/
energy/u5l1b.cfm