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Forces and Motion Review

Forces and Motion Review. 8 th Grade TAG Physical Science. Force. A push or pull Unit of force = kg*m/s 2 = 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

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Forces and Motion Review

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  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! • 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)

  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 = 0 Is there acceleration? No (since no change in motion) Direction of Motion Fn Fpull Ffr Fg

  13. Ex: Coaster Car after it just comes off the ramp. • 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

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