KD-5 Power Point - Moline High School
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Transcript KD-5 Power Point - Moline High School
Chapter 7; Chapter 8
KD5 CONSERVATION OF
MOMENTUM AND ENERGY
Review: Momentum
Momentum - Mass in motion
momentum = mass X velocity
Vector quantity (magnitude and direction)
Review: Momentum
What are the 2 ways to
increase momentum?
If the velocity of an object
doubles, what happens to
the momentum?
Which has more momentum
a car moving down a hill or a
roller skate moving with the
same speed?
Which has more momentum
a skateboard moving at 2
m/s or a truck at a red
Increase mass OR
increase velocity
Momentum Doubles
Car due to more mass
with the same velocities
Skateboard due to the
Truck has no velocity
Thus no momentum
Review: Momentum
Calculate the momentum of a
90 kg football player running
at 6 m/s.
540 kg m/s
Law of Conservation
Physical properties in an isolated
system don’t change
Just transferred from 1 object to another
Examples:
Law of Conservation of Momentum
Law of Conservation of Energy
Law of Conservation of Mass
Law of Conservation of
Momentum
Example: 2 cars collide
The momentum of the 2 vehicles before the
collision is equal to the total momentum of
the 2 vehicles after the collision
The momentums of each car might change but
the total momentum remains the same
Law of Conservation of
Momentum
The total momentum of any closed,
isolated system does not change
Momentum is NEVER lost!
Just transferred
Law of Conservation of
Momentum
p = p or
m1v1 + m2v2 = m1v1 + m2v2
Example
A ball with a mass of 0.10 kg moves to
the right with a speed of 2.0 m/s. It
hits a .040 kg ball which is standing
still. After the balls hit, the 2nd ball
moves to the right with a speed of 0.80
m/s. What is the velocity of the 1st
ball?
Answer
A ball with a mass of 0.10 kg moves to the right
with a speed of 2.0 m/s. It hits a .040 kg ball
which is standing still. After the balls hit, the
2nd ball moves to the right with a speed of 0.80
m/s. What is the velocity of the 1st ball?
m(ball 1) v(ball 1) + m(ball 2) v(ball 2) = m(ball 1) v(ball 1) + m(ball 2) v(ball 2)
.1kg (2.0 m/s) + .04kg (0 m/s) = .1 kg (v) + .04 (.8 m/s)
.2 + 0 = .1 (v) + .032
.168 = .1 (v)
1.68 = v
Collisions
p (before a collision or explosion) = p
(after a collision or explosion)
2 Types of Collisions
1. Elastic Collision-Objects hit and
bounce off
Both momentum and kinetic energy are
conserved
Ex) Pool balls collide
2 Types of Collisions
2. Inelastic Collision-Objects hit and
stick together
Only momentum is conserved
Ex) 2 cars collide and stick together
Energy
Energy-The ability to do work
Forms of energy:
Electrical
Chemical
Solar
Thermal
Mechanical
Mechanical Energy
1. Kinetic energy
2. Gravitational Potential Energy
3. Elastic Potential Energy
Mechanical Energy: Kinetic
Kinetic Energy-Energy of motion
KE = ½ mv²
m=mass (kg)
v=velocity (m/s)
KE=kinetic energy (J)
Mechanical Energy: Gravitational
Gravitational Potential Energy-Stored
energy of position
GPE = m g h
m=mass (kg)
g=9.8 m/s²
h=height (m)
GPE=gravitational potential energy (J)
Mechanical Energy: Elastic
Elastic Potential Energy-Stored energy
of stretch or compression
EPE = ½ k x²
k=elastic constant (N/m)
x=distance stretched/compressed from
rest (m)
EPE=elastic potential energy (J)
Elastic Potential Energy
Law of Conservation of Energy
The total energy of a system remains
constant
Energy is neither created or destroyed
(only transferred to another form)
Total Energy Before = Total Energy After
KE + GPE + EPE = KE´ + GPE´ + EPE´
“The Process” to Solve Energy
Problems
1. Sketch a picture of the problem
2. Identify the types of mechanical
energy present at the important stages
of the problem
3. Set the before energies equal to the
after
4. Substitute
5. Solve and Label
Example:
Bill throws a 0.1 kg ball straight up with
a speed of 7.5 m/s. How high did the
ball go?
Remember
Bill throws a 0.1 kg ball straight up with
a speed of 7.5 m/s. How high did the
ball go?
KE + GPE + EPE = KE´ + GPE´ + EPE´
½ mv2 + mgh + 1/2 kx2 = ½ mv2 + mgh + 1/2 kx2
Only velocity to start with (only KE)
= Only height to end with (only GPE)
½ m v2 = mgh
½ (.1 kg) (7.5 m/s)2 = .1kg (9.8 m/s2) v2