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Second Law of
Motion
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Section 3.3
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What does it mean to accelerate?
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What is required to make an objet accelerate?
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Why is it hard to stop when you run downhill?
Review
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5.
Place a quarter and a flat eraser at one end of
a book that is lying flat.
Slowly raise the end of the book, creating an
incline for the objects to slide down.
What object slides first?
Why?
What is this an example of?
Friction Race
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When forces are unbalanced, motion is created.
When two opposing forces are unbalanced, the
combine, or net, force is determined by
subtracting the smaller force from the larger
one.
Suppose you are pushing a couch across the
room, what forces are acting on the couch?
Are the forces balanced or unbalanced?
What is the net force?
Unbalanced Forces
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Force, mass, and acceleration are all related.
Motion of an object changes, or accelerates,
when a force acts on it.
If you know the mass and acceleration of an
object, you can determine the force.
Newton’s Second Law of Motion:
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The net force on an object equals its mass times it
acceleration.
F=ma
Look at the Picture of the two skateboarders on
pg. 64, assuming both girls apply equal force,
why will Maria accelerate faster?
Accelerated Motion
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A direct relationship exists between force (F),
mass (m), and acceleration (a).
Both mass and force can have an effect on
acceleration.
Look at Figure 3.10 on pg. 65:
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The boy applies the same amount of force to both
carts however, their accelerations are different.
Why?
For both carts to have the same acceleration,
what mass should the cart on the left?
What if instead he applied more force to the cart
on the right?
Force, Mass, and
Acceleration
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There are two graphs that can help us
understand this relationship:
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Force vs. Acceleration Graphs
Mass vs. Acceleration Graphs
Looking at the Graph on pg. 65, we see two
lines that represent two sleds being pulled by
sled-dogs.
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Given that there are multiple dogs pulling one
sled, how might each dog affect the acceleration
of the sled?
How does the acceleration of the 200 kg sled
compare to the acceleration of the 100 kg sled?
How does mass affect the acceleration of each
sled?
Graphing Acceleration
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Have you ever had to push a stalled car?
What happens if you pushed it yourself?
What happens if you have others helping you
push it?
What causes the car to move?
Remember:
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Force = mass x acceleration
Acceleration = force/mass
Using Newton’s Second
Law
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Newton’s second law shows the relationship
between what values?
What does Newton’s second law state?
How does the weight of an object affect its
acceleration?
How would increasing the force applied to an
object affect the acceleration?
a.
What if more mass was added to this object?
What would happen to the force?
What have we learned?
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If you had a bowling ball and a marble moving
at the same velocity, which would be easier to
stop?
Both objects contain momentum
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Momentum of a moving object is related to its
mass and velocity.
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Momentum- inertia of motion
momentum = mass x velocity
If an object has a large momentum, it has a
large mass, large velocity, or both.
Momentum
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Momentum is constant unless something
changes.
What would have to change for momentum to
change?
Momentum can transfer from one object to
another.
Transfer of Momentum
Momentum
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Think back to our spinning an eraser tied to a
string…
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What happened?
What was affecting the eraser?
A force is acting on the eraser to keep the
tension moving toward the center
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Centripetal Force- center-directed force that
causes an object to follow a circular path.
Force in Circular Motion
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Look at Figure 3.12 on pg. 68:
How does the centripetal force on sharp
curves change if there is an increase in the
car’s acceleration?
What happens to the centripetal force on
sharp curves if there is a decrease in the car’s
acceleration?
If the tire applies less friction than is
necessary on the curve, how would the car
move?
Why do racing cars have wide tires?
Centripetal Force
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1.
How does a change in the force applied to an
object affect its acceleration?
2.
If an eagle and a bumble bee are both
traveling at 16 km/hr, which has more
momentum? Explain.
3.
A 28-kg meteor hits the surface of the moon at
130 km/s. What is the meteor’s momentum?
Questions?
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