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! 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): 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)

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 Fair Fair 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 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

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

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