Work, Energy & Power

1. Work, Energy & Power

2. Quick Review We've discussed FORCES Magnitude – How hard is the “push” Direction – Which way does it act upon the object Applying a FORCE causes an object to accelerate (F=ma) m F F = m x a (Units) lbs = lb-sec2/ft x ft/sec2 lb-sec2/ft = slug

3. Quick Review We've discussed TORQUE A FORCE that serves to “spin” an object around a given point Torque = Force x Distance T F D T = F x D (Units) ft-lbs = lbs x ft

4. Quick Review We discussed gaining “mechanical advantage” Linear forces - Lever Mechanisms Rotary force (torque) - Gears, sheaves/belts, sprockets/chain Take the Next Logical Step!

5. Work Work is the application of a force over a distance Lifting a weight from the ground and putting it on a shelf is a good example of work W = F x D Wt D (Units) ft-lb = lbs x ft Wt

6. Energy Capacity for doing Work Two types - Potential Energy (stored energy) Battery Stretched rubber band Elevated weight Kinetic Energy (energy of motion) Car speeding down the road Many times both are present

7. Energy Kinetic Energy For an object of mass m, moving with velocity of magnitude V, this energy can be calculated from the formula E = ½ m x V2 (Units) ft-lbs = lb-sec2/ft x ft2/sec2

8. POWER Power is the work done in a unit of time Power is a measure of how quickly work can be done POWER (P) is the rate of energy generation (or absorption) over time: P = E/t The unit of power is the Watt 746 Watts = 1 Horsepower

9. Work & Power What can we say about the two examples shown below? What can you say about how much work is done for each? How about power requirements? (watts) Wt Wt Lift in 4 Seconds Lift in 2 Seconds 10 ft Wt Wt

10. Work & Power Work = F x D Force and Distance is independent of time Work done is identical Power = E/t Energy (E) = ½ m x V2 Time (t) = halved So E goes by V2 and t is halved means Power required is doubled Wt Wt Lift in 4 Seconds Lift in 2 Seconds 10 ft Wt Wt

11. 3 Ways We Deliver Power Mechanical Stored Energy Bungy, rubber band, spring / trigger required Use lever principles to obtain “mechanical advantage” Pneumatics Stored compressed air acts on cylinder Use lever as above for “mechanical advantage” Motors Variety of 12 VDC motors allowed Use sprockets, sheaves and gears to gain advantage

12. MOTOR POWER 1HP = 746 watts HP = Torque x Speed Constant So let's look at 2 different motors...

13. Typical Motor Curve

14. Typical Motor Curve

15. Work-Energy-PowerSummary Work – application of force over a distance W = F x D Energy - capacity for doing Work E = ½ M x V2 Power – How quickly work can be done P = E/t t = time Horsepower = T x N Constant

16. ? 1. An example of Kinetic Energy would be: a) a moving car b) a stretched rubber band that was just released c) a charge particle in an electric field d) all of the above

17. An example of Potential Energy would be: a) a moving car b) a battery c) a book resting on a table d) both b and c

18. An example of a system having both kinetic and potential energy would be: a) a book resting on a table b) a piece of sugar c) an object in free fall d) a stretched rubber band

19. Which of the following statements is not correct a) energy is the capacity to do work b) Work can be express as Force x Distance c) power is the amount of work done in a unit of time d) the unit of power is the ft-lb