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Chapter 5
Work and Energy
Work and Energy
• Work• Energy• Conservation of Energy• Power
Everyday vs. Scientific Work
Definition of Work
Work = component of force in the direction of displacement ×
magnitude of displacement
Units of Work
1 Joule (J) = 1 Newton × 1 meter
Definition of Work
𝑊=𝐹 cos(𝜃)Δ𝑥
𝜃
𝐹
Δ𝑥
Which show positive work? Which show negative work?
If the object moves 5 m to the right, and the applied force is 2 N, how much work is done on the object?
60°60°
5 N 10 N
PracticeTwo forces act on the 10-kg block below, causing it to accelerate from rest over the period of 3 seconds. What is the work done on the block if these are the only two forces acting on the block?
Quick Check
Is work being done?• Picking up a book off the floor• Carrying a lunch tray across the cafeteria• Pushing a parked car• Pulling a suitcase across the airport
Energy
Kinetic Potential
Mechanical
?
??
Connecting work to energy…
Start with:
and
Kinetic Energy
the energy of an object due to its motion
Units = ?
𝐾𝐸=12𝑚𝑣2
PracticeCalculate the speed of an 8.0×104 kg airliner with a kinetic energy of 1.1×109 J.
Work-Kinetic Energy Theorem
• Work done by the object or work done on the object?
𝑊 net=Δ𝐾𝐸
PracticeOn a frozen pond, a person kicks a 10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is 0.10?
Ans=2.5 m
Potential Energythe energy associated with an object due to its
position, shape, or condition
*Potential energy is relative
Gravitational Elastic
Energy
Kinetic
Mechanical
Potential
Gravitational Potential Energy
is the height, relative to a zero level
Units = ?
𝑃 𝐸𝑔= h𝑚𝑔
60 m
25 m
500 kgA
B
C
Elastic Potential Energy
is the spring constantis the compressed or stretched distance
Units = ?
𝑃 𝐸elastic=12𝑘𝑥2
Elastic Potential Energy
REVIEW
Where is the maximum KE?
Where is the maximum PE?
Mechanical energy = ?
Conservation
• What does it mean that something is conserved?
• Can energy be created or destroyed?
Conservation of Mechanical Energy
𝑀𝐸 𝑖=𝑀𝐸 𝑓
True only if there is no friction
60 m
25 m
500 kgA
B
C
the rate at which work is done
Units = ?
Another form of the equation?
Power
𝑃=𝑊Δt
PracticeTwo horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.
Calculate the power delivered by the horses.
How much work is done by the two horses?
