Download Energy, Work, and Power - Science with Ms. Tantri

Kinetic Energy &
Gravitational Potential Energy
Kinetic Energy
 Moving objects can apply forces
to other objects & move them
 The amount of KE an object
possess is, quantitatively:
Ek = ½ mv2
Ek = kinetic energy (J)
m = mass (kg)
v = velocity (m/s)
Pg. 231 #1, 2, 3
KE Conceptual Questions
 A driver in a car accelerates until the
speed doubles. By what factor did the
car’s EK increase?
 Two cars are travelling
at the same speed. Car A is
twice the mass of car B.
How does car A’s KE
compare to car B’s?
Kinetic Energy
 The work done to change an object’s speed is
equal to its change kinetic energy:
W = ΔEk
W = Ekf - Eki
 Try Pg. 232 #1, 2
Gravitational Potential Energy
 When placed at a higher position,
above a reference level, objects can
apply forces and move other objects.
Fg
 The amount of GPE an object
possesses is, quantitatively:
Δd
Δ Eg = mg Δh
Δ Eg = change in gravitational potential energy (J)
m = mass (kg)
Δh = change in height (m)
Gravitational Potential Energy
 GPE is relative, and depends on the height of the
object to the reference point chosen.
 E.g. A 75 kg diver jumps from a 10.0 m diving board
into the pool below. What is her GPE with respect to:
a) the 5.0 m board?
b) the ground?
The amount of GPE an object has is relative.
Pg. 234 #1
GPE Conceptual Questions
 Object A has twice the mass of object B.
B is 4.0 m above the ground, and A is 2.0 m
above the ground.
Which has the greater amount of GPE?
 For A and B of the previous question:
Both A and B are lowered 1.0 m.
Which has the greater amount of GPE now?
GPE Conceptual Questions
 You carry a heavy box up a flight of
stairs at constant velocity. Your friend
carries an identical box on an elevator
to reach the same floor as you.
Who did the greatest amount of work
(against gravity)? Explain.
Mechanical Energy
Mechanical energy is the sum of an object’s kinetic
energy and gravitational potential energy
ET = Ek + Eg
KE Example
A 95 g donut falls from a table and hits the floor.
The maximum KE of the donut during its fall is 2.6 J.
At what point does the donut achieve max KE?
Calculate the donut’s maximum speed.
Energy
Pg. 235 #1, 2, 3, 5
KE Conceptual Questions
 Examine the pictures which follow, and answer the
following questions:





Which object has the greatest velocity?
Work is being done on which of the objects in the photos?
What force is doing the work in each case?
Which objects are probably losing kinetic energy?
Which object has the greatest amount of kinetic energy?
KE Conceptual Questions
Relating GPE and Work
W = ΔEg
 When work is done on an object, it
changes its GPE when:



Work done gives system GPE only
Object is lifted at constant velocity
Force and displacement are parallel (θ = 0º)
 The formula for gravitational potential energy is
derived using work.
GPE Conceptual Questions
 Examine the pictures which follow, and answer the
following questions:



What object do you think has the greatest amount of GPE?
Which objects clearly illustrate that GPE may be
converted into KE?
What other forms of potential energy can you think of,
besides GPE?
GPE Conceptual Questions
GPE Examples
 Jose Guerra, an 85 kg Olympic diver from Cuba, climbs
to the top of a diving platform at constant velocity. As a
result, he gains 15 kJ of energy, with respect to the
ground. How high is the platform from the ground?