Transcript Work

Chapter 6
Work and Energy
6.1 – Work
Work Formula & Units
Positive & Negative Work
6.2 – Work-Energy Theorem & Kinetic Energy
KE Formula & Units
6.3 – Gravitational Potential Energy
GPE Formula
Positive & Negative Work
6.4 – Conservation of Energy
Total Mechanical Energy
6.5 – Power
Power Formula
6.1 – Work Done by a Constant Force
Work is done on an object whenever a force is
applied parallel to the displacement.
Work = Force x Displacement
Less work is done on the object in bottom figure.
W  ( F cos  ) s
work
(N·m or Joule)
force (N)
displacement (m)
W  ( F cos  ) s
θ = 90°; cosθ = 0
W=0
θ = 180°; cosθ = -1
W = - F(s)
θ = 0°; cosθ =1
W = F(s)
Block is moving
this way 
θ = 270°; cosθ = 0
W=0
Person is doing positive work
on the barbell when lifting.
Person is doing negative work
on the barbell when lowering
Work can be positive or negative, but it
is NOT a vector.
Work is measured in Joules (Newtonmeters) or ft-lbs
Are you doing work on the object?
1. Lifting a weight up off the floor.
YES
2. Pushing a truck as hard as you can but the
truck doesn’t move
NO
3. Carrying books across a room.
NO
4. Lowering a barbell during a bench-press rep.
YES, negative work
5. Gravity pulling a ball down to earth.
YES
6. Gravity pulling on a book resting a table.
NO
For now, a good way to know if work is
done is to see if the PE or KE of the
object is changed.
Work will cause a change in energy of the
object.
Ch. 6 Homework #1
Ch. 6
Problems #1-5 (p. 180)
6.2 – Work-Energy Theorem & KE
Energy -
The ability to do work; measured in Joules
Kinetic Energy - Energy due to motion
1 2
KE  mv
2
velocity(m/s)
mass (kg)
 F  ma
v  v  2 ad
 Fd  W  mad
W m
 
v 2f
 v02
2
W  mv  mv
1
2
2
f
1
2
W  KE f  KEi
2
0
2
f
2
0
v 2f  v 02
2
 ad
The Work-Energy Theorem A net external force on an object changes
the KE of the object.
The change in KE of the object equals
the work that was done on the object
W = ΔKE
W  KE f  KEi
Ch. 6 Homework #2
Ch. 6
Problems #12,13,15,17
p. 181
Potential Energy Energy due to relative position
Elastic Potential Energy
Electrical Potential Energy
Gravitational Potential Energy
6. 3 - Gravitational Potential Energy
Work done by the force of gravity
Wgrav  mgh
height difference (m)
W  ( F cos  ) s
Wgrav  (mg cos 0 )(h0  hf )
Gravitational Potential Energy
PE  mgh
height (m)
The work done by gravity does not
depend on the path taken, only the
height difference.
6. 4 – Conservation of Mechanical Energy
The total mechanical energy (E) of an object
remains constant, neglecting frictional forces.
E = KE + PE
Einitial = Efinal
The Kingda Ka is a giant roller coaster
with a vertical drop of 127 m. Suppose
that the coaster has a speed of 6.0 m/s
at the top of the drop. Neglect friction
and air resistance and find the speed
of the riders at the bottom in miles/hour
Chapter 6 Homework #3
Ch. 6
Problems #25,26,28,35,32,36
page 182
6. 5 – Power
Power - the rate at which work is done.
Work (joules)
Average Power (watts) =
time (sec)
1 horsepower = 550 ft-lbs/sec = 745.7 watts
Conservation of Energy Lab
When block is
moving up or
down at constant
velocity, the net
force is zero.
Fup = Fgrav + fk
Fdown = Fgrav - fk
Fup + Fdown = 2 (Fgrav )
Conservation of Energy Lab
1. W = mg
2. Fgrav = (Fup + Fdown) /2
3. Fgrav = Wsinθ
4. Work = Fgrav x length
5. ΔPE = mgh
6. Workactual = Fup x length
Ch. 6 Equations
W  ( F cos  ) s
1 2
KE  mv
2
W  KE f  KEi
Wgrav  mgh
Work (joules)
Average Power (watts) =
time (sec)
E = KE + PE
Einitial = Efinal