1. Forces and Motion Review
8th Grade TAG Physical Science

2. Force
A push or pull
Unit of force = kg*m/s2 = N (newton)

3. How do forces affect the motion of an object?
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.

4. Can on object be moving and still accelerate?
Yes!
Ex: Driving on GA400 at a constant velocity (56 mi/hr), not speeding up, slowing down, or changing direction.
Car is moving, but 0 acceleration, Fnet=0

5. Friction 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)
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
If close to 0, very low friction
If close to 1, high friction

6. Friction (cont.) Friction acts in a direction opposite to motion
Force of Friction=Ffr

7. Speed vs. Velocity-very similar!
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
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)

8. Acceleration 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
Fnet
Roll down the ramp
Is car speeding up, slowing down, no change?
Speeding up (so accelerating)
Car stopped
What is the Net Force?
Fnet = 0
Is car speeding up, slowing down, no change?
No change, so no acceleration
rolling across the floor
Is car speeding up, slowing down, no change?
Slowing down, so accelerating (negative or deceleration)

9. Acceleration (cont)
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 and shape of object)
Fair
Fair
The force of gravity is the same for both objects

10. Falling Objects
What happens to the air resistance as the speed of a falling object increases?
Air resistance increases
Fair
Fair Fg Fg

11. Falling Objects (cont)
What happens when air resistance of a falling object becomes equal to the downward force of gravity?
Fg = Fair, so Fnet =0 (object stops accelerating…but still moving)
The object is falling at a constant velocity TERMINAL VELOCITY
Fair Fg

12. Ex: Toy Car Drag toy car across the floor at a constant speed
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?
Yes
What is Fnet?
Fnet =  (subtract Ffr from Fpull)
Is there acceleration?
YES
car is speeding up, so POSITIVE Acceleration
Direction of Motion Fn
Fpull
Ffr Fg

13. Ex: Car just after you release it (but still moving)
Direction of motion Fn
Are the forces Balanced or Unbalanced?
Unbalanced
Is the car speeding up, slowing down or speed not changing?
Slowing Down, so still has acceleration.
Note: Fn & Fg equal in length but opposite direction
Ffr
Fnet Fg
Force Diagram

14. Ex: Soccer Ball, rolling on the ground, just after the kick
Direction of motion
Remember, there is nothing pushing the ball forward after the kick, so no more forward force.
Is there an Fnet?
Yes!
How is the ball’s motion changing?
Slowing down
So..is there acceleration?
Yes! (negative acceleration) Fn
Ffr
Fnet Fg