Transcript Physics_U7

Unit 7:
Work and Energy
Section A: Work
 Corresponding Book Sections:
 7.1
 PA Assessment Anchors:
 S11.C.3.1
What is “Work” ?

Work occurs when three conditions are met:
1.
2.
3.
A force is applied to an object
The object moves
At least some of the force being applied is in the
direction of the motion of the object
General Equation for Work
W = Fd
Unit: Joule (J)
Practice Problem
 Find the work necessary to accomplish what is
shown in the picture.
m = 98 kg
Am I doing work?
 Let’s say I go shopping at Weis:
 Picking out items from the shelf
 Placing the groceries on the belt
 Holding the bag of groceries
 Carrying the bag of groceries to my car
Work, version 2.0
 What happens in this situation?
 Does our equation for work “work” ?
The best equation for Work
W = Fd cos θ
Practice Problem
 Find the work done by gravity in this situation:
mass = 4970 kg
distance = 5 m
Positive, Negative, Zero
Work
Work is positive
if the force has a
component in
the direction of
motion
Work is zero
if the force has
no component in
the direction of
motion
Work is negative
if the force has a
component opposite
the direction of motion
Finding Total Work
 Work can be added together, just like forces:
 Wtotal = W1 + W2 + W3 + … = ∑W
 Wtotal = Ftotald cos θ
Sum of the Work
Practice Problem
 Find the work done in this situation:
Section B: Work & Energy
 Corresponding Book Sections:
 7.2
 PA Assessment Anchors:
 S11.C.3.1
Work-Energy Theorem
 The total work done on an object is equal to the
change in its kinetic energy.
 Wtotal = ΔK =

1 2 1 2
mv f  mvi
2
2
Practice Problem
 A truck moving at 15 m/s has a kinetic energy of
140,000 J. What is the mass of the truck?
Practice Problem #2
 How much work is required for a 74 kg sprinkler to
accelerate from rest to 2.2m/s ?
Pratice Problem #3
 A boy pulls a sled as shown. Find the work done by
the boy and the final speed of the sled after it moves
2 m, assuming initial speed of 0.5 m/s.
Let’s take another look at
PP#3
 Could we solve this using the kinematics equations
and Newton’s 2nd Law?
 The answer is YES.
 Should we try?
Work on a Spring
1 2
W  kx
2
 “k” is referred to a the spring constant
 Remember…from the last unit…

Practice Problem
 In the chase scene from Toy Story the Slinky Dog is
stretched 1m, which requires 2J of work. Find the
spring constant.
Practice Problem, Part 2
 How much work is required to stretch the dog from
1m to 2m?
Power
 A measure of how quickly work is done
 Units:
 Joule / second: J/s
 Watt: W (preferred unit)
W
P
or P = Fv
t
Typical values of power
Practice Problem #1
 Calculate the power needed to accelerate from 13.4
m/s to 17.9 m/s in 3.00 s if your car has a mass of
1,300 kg.
Practice Problem #2
 What is the average power needed to accelerate a
950 kg car from 0 m/s to 26.8 m/s (60 mph) in 6 s.
Ignore friction.
Section C: Energy
 Corresponding Book Sections:
 8.1, 8.2, 8.3
 PA Assessment Anchors:
 S11.C.3.1
Two main types of energy
 Kinetic Energy
 Energy an object has while it’s in motion
 Potential Energy
 Energy an object has while it’s not moving
Kinetic Energy
 Energy an object has while in motion
1 2
KE  mv
2
 Unit: Joule (J)
Practice Problem #1
 A truck moving at 15 m/s has KE of 14,000 J. Find
the mass.
Potential Energy
 Energy available to be converted to kinetic energy
(energy of non-motion)
 Unit: Joule (J)
Gravitational Potential
Energy
PE  mgh
 Your book uses “U” to represent Potential Energy -I’ll use “PE”
Two types of forces:
 Conservative
 The work done by a conservative force is stored as
energy that can be released later
 Example: Lifting a box from the floor
 As you lift the box, you exert force and do work
 If you let go of the box, gravity exerts a force and does
work
Two types of forces:
 Nonconservative
 The work done by a nonconservative force cannot be
recovered later as KE
 Example: Sliding box across floor
 The work done to slide the box can’t be restored as KE
 Instead, the energy changes forms into heat
Examples of Conservative
& Nonconservative Forces
 Conservative
 Nonconservative
 Springs
 Friction
 Gravity
 Tension
Sections D & E:
Momentum
 Corresponding Book Sections:
 9.1, 9.2, 9.3
 PA Assessment Anchors:
 S11.C.3.1
What is momentum?
 Linear momentum
 The product of an object’s mass and velocity
p  mv
 Units: kg m/s
So, this means…
 If mass increases, momentum increases
 If speed increases, momentum increases
 Vice-versa if speed or mass decrease
Sample Problem #1
 A 1180 kg car drives along a street at 13.4 m/s. Find
the momentum.
Sample Problem #2
 A major league pitcher can throw a 0.142 kg
baseball at 45.1 m/s. Find the momentum.
Change in Momentum
 Just like the change in speed, distance, etc.
 Final - initial
 Equation:
p  p f  pi
Adding momentum
 Since momentum is a vector quantity, it will add
like vectors add
 We’ll keep it simple and say that:
ptotal  p1  p2  p3  ...
  or  
ptotal   p
Practice Problem #1
 Two 4.00 kg ducks and 9.00 kg goose swim toward
some bread that was thrown in the pond. The
ducks each have a speed of 1.10 m/s while the
goose has a speed of 1.30 m/s. Find the total
momentum.
Momentum and
Newton’s 2nd Law
 Remember that Newton’s 2nd Law is ƩF=ma
 We can relate this to momentum:
p
F 
t
Impulse
 Relationship between applied force and time
I  Favgt
What is impulse?
 Vector quantity
 Units: kg m/s
 Points in same direction as average force
Another way to represent
Impulse:
p
 If:
F 
t
 Then:
 And if:


 Then:
Ft  p
I  Favgt
I  p
Practice Problem #1
 A 0.144 kg baseball is moving toward home plate at
43.0 m/s when it is hit. The bat exerts a force of
6,500 N for 0.0013s. Find the final speed of the ball.
Practice Problem #2
 After winning a prize on a game show, a 72 kg
contestant jumps for joy with a speed of 2.1 m/s.
Find the impulse experienced.
Rain vs. Hail
 As you’re holding an
umbrella, does it require
more force, less force, or
the same force to hold up
the umbrella if the
raindrops turn to hail?
Conservation of
momentum
 If the net force acting on an object is zero, its
momentum is conserved
 In other words, the momentum before a collision is
the same as the momentum after a collision
 pf = pi
Practice Problem #1
 A honeybee with a mass of 0.150g lands on a 4.75g
popsicle stick. The bee runs toward the opposite
end of the stick. The stick moves with a speed of
0.120 cm/s relative to the water. Find the speed of
the bee.
Elastic vs. Inelastic
Collsions
 Elastic
 Inelastic
 Momentum is conserved
 Momentum is conserved
 Kinetic energy is conserved
 Kinetic Energy is NOT
conserved
 In other words:
 Objects bounce off each
other
 In other words:
 Objects either stick or
stop