Download Work, Power, and Energy

Work, Power, and Energy:
Explaining the causes of motion
without Newton
KIN335 Spring 2005
What you should know
• Definition of work and its characteristics
• Definition of energy (including kinetic
energy and potential energy)
• Work-energy relationship
• Conservation of mechanical energy
• Definition of power
• Other expression of power and its meaning
Work
• Mechanical definition
– The product of force and the amount of
__________ in the direction of that _____ (it is
the means by which energy is transferred from
one object or system to another (Webster’s New
World Dictionary, p. 1538)
– W = F·d
• Unit : joule (__) = 1N·1m
– English Unit : ft·lb
Work
• Three things to know for determining the amount
of work done on an object
– The average ________ exerted on the objects
– The _________ of this force
– The ___________ of the object along the line of action
of the force during the time the force acts on the object.
• Positive or Negative Work? Examples
Energy
• Definition
– ______________________
• Different form of energy
– Chemical E, Thermal E, Mechanical E
• Two forms of mechanical energy
– _________ energy – due to the motion (v)
– _________ energy – due to the position (h)
Mechanical E = Kinetic E (KE) + Potential E (PE)
Kinetic Energy
• A moving object has the capacity to do work due
to its motion
• Quantification : KE = ½·m·v2
• Unit : kg·(m/s)2 = kg·(m/s2)·m = N·m = J
• Q1. How much kinetic energy does a 2-kg discus
have if its velocity is 20 m/s?
Potential Energy
•
•
The energy (capacity to do work) that an object
has due to its __________
Two types of potential energy
–
Gravitational potential energy – position relative to
the earth
PE = W·h =mg·h
– Strain energy – due to the deformation of an object
SE = ½·k·∆x2
– Examples
•
The greater the deformation of the object, the
greater the strain energy
Strain Energy
•
•
•
Energy due to the ____________ of an object
Ex) Fiberglass vaulting pole bent Î ↑ SE
Related with the object stiffness, its material
properties, and its deformation
SE = ½·k·∆x2
• Muscle mechanics and Tissue mechanics
Mechanical Energy
• At the top of a giant swing on the high bar,
a 50-kg gymnast’s velocity is 1 m/s and he
is 3.5 m high. What is the gymnast’s total
mechanical energy at this instant?
Work-Energy Relationship
•
•
•
Work = Energy ? (Y/N)
Energy = capacity to work
Unit? When you see units for both
–
–
–
•
Work = F·d = (kg·m/s2)·(m)=kg·m2/s2
Kinetic energy = ½·m·v2 = kg·(m/s)2 = kg·m2/s2
Potential energy = m·g·h = (kg)·(m/s2)·(m) = kg·m2/s2
Work done= ∆E = Ef – Ei =∆KE + ∆PE
Why is the relationship between
work and energy so important?
•
•
•
Change of velocity Î ___________________
Work-energy relationship indicate how kinetic
energy can be changed by doing work
Energy = capacity to work
↑Work done = _______
A large change in kinetic energy Î a _______ force
applied over a long distance
cf) Impulse-Momentum
(ΣF)·∆t = m·(Vf –Vi)
–
–
Doing work to decrease energy
•
•
•
•
•
•
Catching a ball (negative work, absorbing
energy)
Landing from a jump or fall
Absorbing energy Î _________________
These actions increase distance over which the
force acts, thus decreasing the average value of
the force.
Follow-throw in pitching
Safety and protective equipments
Conservation of Mechanical Energy
Mechanical E = Kinetic E (KE) + Potential E (PE)
•
•
•
•
•
When no external force acts other than _______
Î No work done because of no external forces
act
W = ∆E = 0 Î 0 = ∆KE + ∆PE
∆E = Ef – Ei =0 Î Ef = Ei
The total mechanical energy of an object is
constant if no external forces other than
________ act on the object.
Q. When we drop a baseball (m = 0.5 kg) at
10.41m at Computer Common, what would it be
a calculated drop velocity on the ground?
Power
•
Definition
– Ability to do a given work in a short time
– Rate of doing work
– How much work done in a specific amount of time
– How quickly or slowly work is done
•
•
•
P = ___________
Unit : Watt (___) = 1 J/ 1 s
Q8. In vertical jump-and-reach test, a 60-kg
student jumps 60 cm, whereas a 90-kg student
jumps 45 cm. Assuming both jumps took the
same time, which jumper was more powerful?
Another expression of Power
•
•
•
P = W/∆t = (F·d)/∆t = F·(d/∆t) = ______
Power is the product of ____________ and
____________ along the line of action of that
force
Examples
–
–
–
–
Moving a stack of books
Pedaling a bicycle
Running kinematics (SR X SL)
Muscle contraction
Q7. An Olympic weight lifter snatches 100 kg. In a snatch, the
barbell is moved from a stationary position on the floor to a
stationary position over the athlete’s head. Only 0.5 s elapsed
from the first movement of the barbell until it was overhead, and
the barbell moved through a vertical displacement of 2.0 m.
What was the weight lifter’s average power output during the
lift?
Q5. A baseball strikers the catcher’s glove with a horizontal
velocity of 40 m/s. The mass of the baseball is 0.15 kg. The
displacement of the baseball due to the deformation of the
catcher’s glove and the movement of the catcher’s hand is 8
cm from the instant if first makes contact with the glove until
it stops.
a. How much kinetic energy dose the baseball possess just
before it strikes the glove?
b. How much work does the catcher do to the baseball during
the catch?
c. Is the work done positive or negative?
d. What is the average impact force exerted by the glove on the
baseball?