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WORK AND POWER WHEN OBJECTS MOVE
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Bellwork Pick up notes and activity sheet from the back
Bring your October Sky Book and a calculator to class tomorrow Test is Thursday!
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Changes in Motion Any change in an object’s motion is caused by a FORCE. When we apply a force on an object to cause this motion, we do WORK on the object Work done is a result of a force over a DISTANCE
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Work Work is done only if the force causes an object to move in the SAME direction as the force
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Concept Check State whether work is done in each of the following situations:
A teacher applies a force to a wall and becomes exhausted. A book falls off a table and free falls to the ground. A waiter carries a tray full of meals above his head by one arm straight across the room at a constant speed. (Careful! This is a difficult question.) A rocket accelerates through space.
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Doing Work When a weight lifter raises a heavy barbell, he does work on the barbell. When a weight lifter simply holds a barbell overhead, he does NO work on the barbell.
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Calculating Work
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1 J = 1 Nm The unit for work is the Newton-meter
UNIT of Work The unit for work is the Newton-meter We call the newton-meter the joule (J). One joule of work is done when a force of one Newton is exerted over a distance of one meter. 1 J = 1 Nm
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Example 1 Together, two students push on a car with 825 N of force. If the students push the car 35 m, how much work do the students do on the car?
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Example Problem 2 Bud, a very large man of mass 130 kg, stands on a pogo stick. How much work is done as Bud compresses the spring of the pogo stick 0.50 m?
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Work done at an ANGLE Remember that a force is a vector quantity
The work done is parallel to the direction of the objects motion. This is the horizontal component of the force if the object moves in a horizontal direction or the vertical component if the object moves in a vertical direction
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Calculating Work at an Angle
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Example Problem 3 A force of 50 N acts on the block at the angle shown in the diagram. The block moves a horizontal distance of 3.0 m. How much work is done by the applied force? W = F (cosɵ)d W = (50 N)(cos 30)(3 m) W = Joules
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Power
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Power The quantity that deals with the rate at which work is done is called power Power is the amount of work done in a given amount of time. Compare the work done between two climbers making their way up a mountain- one a rock climber, and one a hiker- which one has more power The hiker has a greater power rating because it takes less time for her to reach the top of the mountain
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Power The SI unit for power is the watt, W.
1 watt is equal to a joule per second.
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Power Equation
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Example 1 A tired squirrel (mass of approximately 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm in order to do a mere 0.50 Joule of work. If the tired squirrel does all this work in 2 seconds, then determine its power.
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Example 2 When doing a chin-up, a physics student lifts her 42.0-kg body a distance of 0.25 meters in 2 seconds. What is the power delivered by the student's biceps?
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