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8.4 Kinetic Energy
SWBAT describe and calculate kinetic
energy
1. A force of 30 N pushes a block 30 m. How
much work is done by the 30 N force?
2. A force of 30 N East and a force of 10 N West
act concurrently on a 10 kg block initially at
rest.
a) What is the net force acting on the block?
b) What is the acceleration of the block?
c) What is the speed of the block after 5 seconds?
Definition: Energy is the capacity of a physical
system to perform work. Energy exists in several
forms. Energy is a scalar quantity.
Units: Joules (J)
According to the Law of Conservation of Energy, the total
energy of a system remains constant, although energy may
transform into another form.
A physical system is a portion of the physical universe chosen for
analysis. Everything outside the system is known as the environment.
The environment is ignored except for its effects on itself.
Mechanical Energy
Kinetic Energy (KE)
Non-Mechanical Energy
Internal Energy/Heat (Q)
Gravitational Potential
Electrical Energy (W)
Energy (GPE or ΔPE)
GPE can change into KE, but
the Chemical
Total Mechanical
Energy Energy
Elastic Potential Energy remains constant!
(EPE or PEs)
Nuclear Energy
Definition: The energy of an object that is due
to the object’s motion.
(If something is moving, it has KE)
Units: Joules (J)
Equation:
KE =
1
2
mv
2
(Kinetic Energy) = ½ (mass) (velocity)2
Where is the formula for Kinetic Energy?
A block has a mass of 20kg and is being pushed
at a constant speed of 5m/s. How much kinetic
energy does the block have?
5m/s
What is the relationship between the kinetic
energy and mass of an object?
KE =
KE
Direct Relationship
m
1
mv 2
2
What is the relationship between the kinetic
energy and velocity of an object?
KE =
KE
Exponential Relationship
v
1
mv 2
2
Work can be negative, even though it’s a scalar.
Think of it as putting work into (+) or out of (-) a
system.
Money is also a scalar. You can put money into a
bank account (+), or take money out of it (-)
A 100 kg block initially traveling with a velocity of 2 m/s is
accelerated by a net force of 50 N over a distance of 10 meters.
a. How much work is done on the block?
b. What is the change in KE? Is it equal to the work?
a. Calculate the initial Kinetic Energy
b. Calculate the final velocity.
c. Calculate the final Kinetic Energy, then the change in KE.
vi = 2 m/s
vf = ?
50 N
10 m
Read and Outline p. 164-168
Questions #1-5 on p. 166