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Motion and Force

Motion and Force. “That storm had a lot of force!”. “ Our basketball team is a force to be reckoned with.”. In Science…. Force is anything that changes the state of rest or motion of an object. Net Force. When you throw or catch a ball, you exert a force to change the ball’s velocity.

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Motion and Force

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  1. Motion and Force “That storm had a lot of force!” “ Our basketball team is a force to be reckoned with.”

  2. In Science… Force is anything that changes the state of rest or motion of an object.

  3. Net Force When you throw or catch a ball, you exert a force to change the ball’s velocity. What causes an object to change its velocity, or accelerate? Usually, many forces… the combination of these forces are called the NET FORCE.

  4. The object accelerates in the direction of the net force.

  5. But what if the net force is ZERO? Duh… then it is BALANCED!

  6. Balanced and Unbalanced Forces When the net force is ZERO… it is “balanced”

  7. Unbalanced Forces The object will accelerate in the direction of the greater force…

  8. The Force of FRICTION Imagine a car that is rolling along a flat, evenly paved street. Experience tells you that the car will keep slowing down until it eventually stops. This steady change in the car’s speed gives you a clue that a force must be acting on the car. The unbalanced force that acts against the car’s direction of motion is FRICTION.

  9. Friction occurs because the surface of any object is rough.

  10. Types of Friction Static Friction – the friction between surfaces that are stationary. Kinetic Friction – the friction between moving surfaces. *Static Friction is GREATER than Kinetic Friction

  11. Kinetic Friction • Sliding Friction – when objects slide past each other • Rolling friction – like if a round object rolls over a flat surface • Fluid friction – air sliding past a car

  12. What would we do without friction? Tires would not be able to push against the ground and move the car… Brakes would not stop the car…

  13. You would not even be able to grip the door handle…

  14. Harmful Friction? How to reduce it.. Use lubricants… Ball Bearings help tires move…

  15. Bicyclists wear clothes that fit closely to reduce fluid (air) friction…

  16. Helpful Friction? How to increase it Baseball players wear batting gloves to increase the grip on the bat… Using a scrubber to wash dishes…

  17. Car’s WITHOUT Friction??? Uncontrollable!

  18. WORK ??? I would rather be… NO! NO! NO! FOCUS

  19. What is Work?

  20. Work is done when the force causes a change in the position or the motion of an object in the direction of the applied force.Another words…Work = Force x Distance

  21. WORK Work = Force x Distance W = F x d UNITS: Joules (J)

  22. Is this work? There is a force applied… but is the barbell moving???

  23. POWER Both take the same amount of work… but one takes more POWER!

  24. The rate at which work is done. POWER Power = Work Time Units: Watts (W)

  25. Let’s Practice… A crane uses an average force of 5200 N to lift a container 25 m. How much work does the crane do on the container? W=F/D W= 130,000 J

  26. More Practice… An apple weighing 1 N falls though a distance of 1 m. How much work is done on the apple by the force of gravity? W= 1 J

  27. And ONE More… The brakes on a bicycle apply 125 N of frictional force to the wheels as the bicycle travels 14.0 m. How much work have the brakes done on the bicycle? W= 1, 750 J

  28. Oh… I meant one more on Work… Now we have POWER! While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts? P= W/T P= 66 W

  29. And Another… Using a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output? P = 107 W

  30. NOW… THE LAST ONE! I promise… LOL It takes 1 x 105 J of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? P= 5,000 W

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