Forces and Motion Review

8th Grade TAG Physical Science

Force

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

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.

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

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

Friction (cont.)

Friction acts in a direction opposite to motion

Force of Friction=Ffr

Speed vs. Velocity-very similar!

Speed 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)

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):

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)

Roll down the ramp

Is car speeding up, slowing down, no change?

Speeding up (so accelerating)

Fnet Fnet

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

Falling Objects

What happens to the air resistance as the speed of a falling object increases?

Air resistance increases

FairFair

Fg Fg

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

Ex: Toy Car

Drag toy car across the floor at a constant speed

Ffr

Fg

Fn

Fpull

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?YEScar is speeding up, so POSITIVE Acceleration

Direction of Motion

Ex: Car just after you release it (but still moving)

Direction of motion

Fnet

Force Diagram

Ffr

Fn

Fg

Note: Fn & Fg equal in length but opposite direction

Are the forces Balanced or Unbalanced?

Unbalanced

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

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)

Ffr

Fnet

Fn

Fg