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Plan for Today (AP Physics 1). Go over tests Lecture/Notes on Work and Energy. Test Results. Average: 86.6 Without the curve: 82 percent 25 As. Students do the following. Curl a bowling ball 10 times Take a bowling ball to the water fountain and back Push a brick across a table. Vote.
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Plan for Today (AP Physics 1) • Go over tests • Lecture/Notes on Work and Energy
Test Results • Average: 86.6 • Without the curve: 82 percent • 25 As
Students do the following. . . • Curl a bowling ball 10 times • Take a bowling ball to the water fountain and back • Push a brick across a table
Vote • Which student did the most work? • Only one to do work
Equation • Work = Force times distance times cos angle • Work = F x cos 0 • Units: • N*m • Kgm/s^2 *m = kgm^2/s^2 • Joules – energy • This is a scalar quantity • No direction • Note: Physics definition of work • Very narrowly defined
Why do you need cos in the equation? • Force must be in the DIRECTION of motion
Example using Angles • If we pull a brick with 20 N of force at a 25 degree angle above horizontal for three meters, how much work is done?
Example using Angles • If we pull a brick with 20 N of force at a 25 degree angle above horizontal for three meters, how much work is done?
Example using Angles • If we pull a brick with 20 N of force at a 25 degree angle above horizontal for three meters, how much work is done?
Situations • Carry a brick across the room • Was any work done? • No • Force and displacement in different directions • No change in its energy • Velocity and height don’t change
Situations • Carry a brick across the room • Was any work done? • No • Force and displacement in different directions • No change in its energy • Velocity and height don’t change
Situation • What if I pick a brick straight up? • Force of hand is like the normal force • In same direction as the distance traveled • So yes, work is done • Work is positive
Situation • What if I pick a brick straight up? • Force of hand is like the normal force • In same direction as the distance traveled • So yes, work is done • Work is positive
Situation • Pushing a rolling chair and allowing it to roll to a stop • What happened? • I put energy into the chair • Force times distance applied = work • What happens to the energy? • From my work it converts to kinetic energy • What happens to KE? • Does this energy disappear?
Chair situation continued • What happens to the kinetic energy? • We did work, it had kinetic energy, but then it stopped • Work done • Kinetic energy converted to what? • Heat from friction
Chair situation • The work done by friction opposed motion • So the work is negative
Generalization • Negative Work slows down • Positive work speeds up
Example • A 10 kg mass moves 5 m. The coefficient of friction is .2. • What is the net work if a force of 40 N is applied at 30 degrees above horizontal?
What did this mean? • What is ½*m*v^2? • KE = ½*m*v^2 • Are we sure this is correct? • How should we check? • Units! • KE = (kg)(m/s)^2 =kgm^2/s^2 • What is this? • Joules!
Example • A car with a mass of 1000 kg starts at 10 m/s and accelerates to 20 m/s in 50 m. • How much work is done? • What force is required?
150000 • 3000