Transcript PHY160-17

TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review energy and discuss how
energy is conserved and changes forms
- Investigate how conserved energy
works in pulley systems
- Study the force of friction on a wooden
block
Suppose a barge carrying 100,000 Kg of coal (a bit more that 100 tons) is moving down
the Ohio river at 10 m/sec when it is notice that there is a fishing boat in the channel, 100
meters away. The boat guiding the barge goes into reverse and stops the barge.
What is the kinetic energy of the moving barge?
Kinetic energy = ½ mass × (velocity)2
= 0.5 × 100,000 kg × (10 m/sec)2
= 5,000,000 Joules
What force does the tugboat have to exert, to remove this much energy while the barge
moves 100 m?
Energy = Force × distance
Force = Energy / distance
= 5,000,000 Joules / 100 m
= 50,000 Newtons
This is exactly the same answer you got in the previous
activity using F = m × a ; there you had to explicitly determine
the acceleration; here you didn’t.
- In this (and other examples), we saw the energy at
the start was the energy at the finish.
- Another way to state this is to say energy was
conserved.
2 meters
You applied this principle in
the last lab, when raising and
dropping a metal ball.
A 0.005 kg (0.05 N) ball resting on
the ground has zero energy.
- In this (and other examples), we saw the energy at
the start was the energy at the finish.
- Another way to state this is to say energy was
conserved.
2 meters
You applied this principle in
the last lab, when raising and
dropping a metal ball.
Raising the ball 2 meters gives the
ball stored energy
energy = force × distance
= 0.05 N × 2 m = 0.1 Joules
- In this (and other examples), we saw the energy at
the start was the energy at the finish.
- Another way to state this is to say energy was
conserved.
You applied this principle in
2 meters
the last lab, when raising and
dropping a metal ball.
As the ball accelerates, the stored
energy becomes kinetic energy
Halfway down, the distance of the
ball is half what it started, so it has
only half its stored energy - the rest
is now kinetic energy (½mv2).
- In this (and other examples), we saw the energy at
the start was the energy at the finish.
- Another way to state this is to say energy was
conserved.
You applied this principle in
2 meters
the last lab, when raising and
dropping a metal ball.
At the bottom, the stored energy is
again zero - it has all become
kinetic energy.
So, kinetic energy = ½mv2 should
be equal to the stored energy of 0.1
Joules.
IDENTICAL CARTS CLIMBING INCLINES
A
B
Which cart requires more force to lift? B
Which cart travels a longer distance? A
When the carts reach the top, what quantity is equal
for both, despite the different forces and distances?
The force × distance = energy is the same
for both
What if the cart follows a
crazy ramp up the same
block? Does this change
the energy stored?
No!
IDENTICAL CARTS CLIMBING INCLINES
A
B
If both carts were released from the top, how would their kinetic
energies compare when they reached the bottom?
They would be the same, since the (equal) stored energies are converted
to kinetic energy.
How would their speeds compare?
Kinetic energy depends on velocity (or speed)
[ Kinetic energy = ½ mass × velocity2 ]
so the speeds would be the same, too.
This example assumes energy is conserved - is
energy always conserved? (That is, can you create or
destroy new energy?)
You’ve heard since you were a young child in
science class: ENERGY CANNOT BE CREATED OR
DESTROYED.
However - is energy always conserved within the
system you are measuring?
Back to the falling ball example:
2 meters
What happens AFTER the ball hits the
ground? Does it still have stored energy
or kinetic energy?
The ball bounced a bit, so it had
some kinetic energy left.
What about when the ball comes to a
complete rest? Where did the energy
“go”?
Some went into sound, some went into
heat. Energy in the whole room is
conserved, but things like friction can
cause energy to leave the system you
are measuring.
WHAT YOU ARE EXPECTED TO KNOW:
- Stored energy is given by force × distance, and
(in the absence of friction) does not depend on
the path taken
- Stored energy can be changed into kinetic
energy
- Solve problems involving force, mass and
distance using kinetic and potential energy
TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review energy and discuss how
energy is conserved and changes
forms✓
- Investigate how conserved energy
works in pulley systems
- Study the force of friction on a wooden
block