Work, Power, and Energy

Work W = F * d Work = Force x distance Work: the result of a force applied to an object which causes the object to experience a displacement. unit: Joule Work = Force x distance W = F * d (Joule) = (Newton) x (meter) 1 meter A B C 1 N Only the Force component in the same direction as the motion can count as the “work”! 1 meter

What if I double the distance? What if I double the force? Since …… Work = Fd, Then ……… (2F) x d = 2Fd And that equals twice the work! 2 W = (2)Fd What if I double the distance? Since …… Work = Fd, Then ……… F x (2d) = 2Fd And that equals twice the work! 2 W = F (2)d

Work Practice Susan used 7.5 Newtons of force to lift an object 18.4 meters. How much work did Susan do? W = 140 J You did 170 joules of work lifting a 140 N backpack. How high did you lift the backpack? d = 1.2m If Sam performs 4,000 joules of work while dragging a tarp 10 meters, how much force did he use? F = 400 N

Power P = t W Power: the rate at which work is done unit: watt Power = Work / time (watt) = (joule) / (seconds)

If I double the power, How much time will it take? Remember P = W / t 2P is the same work in half as much time or 2P is the same amount of time but doing twice as much work

Power Practice What is the power of a man that lifts 356J in 42.0 seconds? P = 8.48 watts You can lift your own weight (500 N) up a staircase that is 5 meters high in 30 seconds. How much power do you use? P = 83 watts How does this compare to a 100 watt light bulb? less than a light bulb

Work and Power Practice (10 min) Human Power Stair Lab (30 min) Assignments: Work and Power Practice (10 min) Human Power Stair Lab (30 min)

Energy Energy: ability to do work stored work Potential Energy create a force over some distance stored work measured in joules E = W Potential Energy Kinetic Energy PE = mgh KE = ½ mv2 J = (kg)(m/s2)(m) J = (kg)(m/s)

Potential Energy potential energy: stored energy that comes from position “possible” energy due to gravity PE = mgh J = (kg)(m/s2)(m) You need to put a 1 kg mass that is on the floor way on a shelf that is 3 meters high. How much energy will it have while sitting on the shelf? PE = 29.4 joules

Kinetic Energy kinetic energy: energy of motion (has a velocity) KE increases with increasing speed KE increases with increasing mass …… takes more F to move the object KE = ½ mv2 J = (kg)(m/s)2

Energy Practice (10min) Phet Skate Park (30 min) Assignments: Energy Practice (10min) Phet Skate Park (30 min)

Conservation of Energy law of conservation of energy: energy is neither created nor destroyed … just transformed from one form into another some energy may be “lost” to heat from friction, but not destroyed MEtotal = PE + KE PE KE 50 J PE KE 15 J 35 J mgh = ½ mv2 PEtop = KEbottom PE KE 50 J

Conservation of Energy v = 6.3 m/s

Conservation of Energy 2. A ball slides down a frictionless track shown below. The ball has no velocity at position A.   ___________ a. To what point does the ball rise on the opposite incline? ___________ b. At what point(s) does the ball have a maximum speed? ___________ c. At what point(s) is the kinetic energy of the ball a maximum? ___________ d. At what point(s) is the potential energy of the ball a maximum? ___________ e. At what point(s) is the potential energy of the ball a minimum? C A B D E F E C C A , E C

Conservation of Energy 2. KE= 66 J

Conservation of Energy 3 m 8m 12 m C B A D 4. At point A the 68.0 kg skier is moving at 6.3 m/s. Remember TME is “total mechanical energy”. At point A, find : KE = ___________ PE = _____________ TME = ______________   At Point B, find: KE = ___________ PE = _____________ TME = ______________ 1349.46 J 7996.8 J 9346.26 J 4015.06 J 5331.2 J 9346.26 J

Conservation of Energy 3 m 8m 12 m C B A D 4.   At Point C, find: KE = ___________ PE = _____________ TME = ______________ At point C, find the skier’s velocity. At point D, what is the TME? 9346.26 J 0.0 J 9346.26 J v = 16.57… m/s 9346.26 J

Roller Coaster of Doom (20 min) Quiz Tomorrow (W, P, E) Assignments: Roller Coaster of Doom (20 min) Quiz Tomorrow (W, P, E)

Conservation of Energy law of conservation of energy: energy is neither created nor destroyed … just transformed from one form into another some energy may be “lost” to heat from friction, but not destroyed MEtotal = PE + KE PE KE 50 J PE KE 15 J 35 J mgh = ½ mv2 PEtop = KEbottom PE KE 50 J

A Mexican jumping bean jumps with the aid of a small worm that lives inside the bean. The bean jumps 0.015 meters from your hand into the air. What is the velocity of the bean as it lands back on your palm? v = 0.54 m/s

Energy Energy: ability to do work stored work Potential Energy create a force over some distance stored work measured in joules E = W Potential Energy Kinetic Energy PE = mgh KE = ½ mv2 J = (kg)(m/s2)(m) J = (kg)(m/s)

Work – Energy Theorem The work-energy theorem says… the work done equals the change in energy. PE = mgh PE = (2)(9.8)(8) PE = 156.8 J PE = 160 J Work was done to move the box to the new position because a force was applied to move the box some distance. W = Fd W = Fgd W = (2*9.8)(8) W = 156.8 J W = ΔPE W = PEf – PEi W = 156.8 – 0 W = 156.8J PE = 0 J

Work – Energy Theorem The work-energy theorem says… the work done equals the change in energy. KE = ½ mv2 KE = ½ (0.8)(5)2 KE = (.4)(25) KE = 10 J KE = 10 J Work was done to give the ball a new KE when a Force was used to accelerate the ball, giving it a new velocity. W = ΔKE W = KEf – KEi W = 10 – 0 W = 10 J W = Fd W = (7.8)(1.3) W = 10 J vi = 0 m/s KE = 0 J

Work – Energy Theorem (30 min) Assignments: Work – Energy Theorem (30 min)