Transcript Forces and Motion Review

Forces and Motion
Review
8th Grade TAG Physical Science
Force
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A push or pull
Unit of force = kg*m/s2 = N (newton)
How do forces affect the
motion of an object?
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When forces are balanced, net force = 0, so
no change in an objects motion
When an unbalanced force acts on an object,
the object accelerates.
Can on object be moving and still
accelerate?
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Yes!
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Ex: Driving on GA400 at a constant velocity
(56 mi/hr), not speeding up, slowing down, or
changing direction.
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Car is moving, but 0 acceleration, Fnet=0
Friction
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A force resulting from 2 objects moving past
each other
3 types: Sliding, Rolling, Fluid
What is friction depended on? (hint: think of the
class demo)
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We double the mass and the resulting force was
doubled
We changed the surface (table, carpet, sandpaper,
and the force increased)
Coefficient of friction, 0 to 1
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If close to 0, very low friction
If close to 1, high friction
Friction (cont.)
Friction acts in a direction opposite to motion
Force of Friction=Ffr
Speed vs. Velocity-very
similar!
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Speed
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How fast something is going
Units of mi/hr or m/s
Ex: driving on GA400 at a speed of 65mi/hr
Can NOT draw a vector for speed
Velocity
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The speed of an object in a particular direction
Units of mi/hr north or m/s north
Ex: driving on GA400 at a velocity of 65mi/hr north
(included direction)
Can draw a vector for velocity (since vector shows
magnitude and direction)
Acceleration
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The rate at which velocity changes over time
Units of m/s2
If an object is speeding up, slowing down,
turning…there is acceleration
Net force diagram of coaster car (fnet):
Fnet
Roll down the ramp
Fnet
rolling across the floor
Is car speeding up,
slowing down, no
change?
Is car speeding up, slowing
down, no change?
Speeding up (so
accelerating)
Slowing down, so accelerating
(negative or deceleration)
Car stopped
What is the Net Force?
Fnet = 0
Is car speeding up, slowing
down, no change?
No change, so no acceleration
Acceleration (cont)
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All objects accelerate toward the earth at the
same rate, regardless of their mass
All objects accelerate toward Earth at a rate
of 9.8 m/s2
Air resistance (fluid friction) affects some
objects more than others (depends on size
andFair
shape of object)
Fair
The force of
gravity is the
same for both
objects
Falling Objects
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What happens to the air resistance as the
speed of a falling object increases?
Air resistance increases
Fair
Fair
Fg
Fg
Falling Objects (cont)
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What happens when air resistance of a falling
object becomes equal to the downward force
of gravity?
Fg = Fair, so Fnet =0 (object stops
Fair
accelerating…but still moving)
The object is falling at a constant velocity
TERMINAL VELOCITY
Fg
Ex: Toy Car
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Drag toy car across the floor at a constant
speed
Fn
Direction of
Motion
Force of the pull continues to act on
the car (hand still touching car)
Which direction is friction acting?
Opposite to motion of car
Is object moving?
Fpull
Yes
Ffr
Fg
What is Fnet?
Fnet =  (subtract Ffr from Fpull)
Is there acceleration?
YES
car is speeding up, so POSITIVE
Acceleration
Ex: Car just after you release it
(but still moving)
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Direction of motion
Fn
Are the forces Balanced or
Unbalanced?
Note: Fn &
Fg equal in
length but
opposite
direction
Ffr
Fg
Force Diagram
Unbalanced
Fnet
Is the car speeding up,
slowing down or speed not
changing?
Slowing Down, so still has
acceleration.
Ex: Soccer Ball, rolling on the
ground, just after the kick
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Direction of motion
Remember, there is nothing pushing the ball
forward after the kick, so no more forward force.
Is there an Fnet?
Fn
Yes!
How is the ball’s
motion changing?
Ffr
Slowing down
Fnet
Fg
So..is there
acceleration?
Yes! (negative
acceleration)