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?

Work KE Theorem

Work KE Theorem

Work KE Theorem

Work KE Theorem

Work KE Theorem

Work KE Theorem

Work KE Theorem

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