Work, Power, and Energy: Explaining the causes of motion without Newton. KIN335 Spring 2005

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1 Work, Power, and Energy: Explaining the causes of motion without Newton KIN335 Spring 2005

2 What you should know Definition of work and its characteristics Definition of energy (including kinetic energy and potential energy) Work-energy relationship Conservation of mechanical energy Definition of power Other expression of power and its meaning

3 Work Mechanical definition The product of force and the amount of in the direction of that (it is the means by which energy is transferred from one object or system to another (Webster s New World Dictionary, p. 1538) W = F d Unit : joule ( ) = 1N 1m English Unit : ft lb

4 Work Three things to know for determining the amount of work done on an object The average exerted on the objects The of this force The of the object along the line of action of the force during the time the force acts on the object. Positive or Negative Work? Examples

5 Energy Definition Different form of energy Chemical E, Thermal E, Mechanical E Two forms of mechanical energy energy due to the motion (v) energy due to the position (h) Mechanical E = Kinetic E (KE) + Potential E (PE)

6 Kinetic Energy A moving object has the capacity to do work due to its motion Quantification : KE = ½ m v 2 Unit : kg (m/s) 2 = kg (m/s 2 ) m = N m = J Q1. How much kinetic energy does a 2-kg discus have if its velocity is 20 m/s?

7 Potential Energy The energy (capacity to do work) that an object has due to its Two types of potential energy Gravitational potential energy position relative to the earth PE = W h =mg h Strain energy due to the deformation of an object SE = ½ k x 2 Examples The greater the deformation of the object, the greater the strain energy

8 Strain Energy Energy due to the of an object Ex) Fiberglass vaulting pole bent SE Related with the object stiffness, its material properties, and its deformation SE = ½ k x 2 Muscle mechanics and Tissue mechanics

9 Mechanical Energy At the top of a giant swing on the high bar, a 50-kg gymnast s velocity is 1 m/s and he is 3.5 m high. What is the gymnast s total mechanical energy at this instant?

10 Work-Energy Relationship Work = Energy? (Y/N) Energy = capacity to work Unit? When you see units for both Work = F d = (kg m/s 2 ) (m)=kg m 2 /s 2 Kinetic energy = ½ m v 2 = kg (m/s) 2 = kg m 2 /s 2 Potential energy = m g h = (kg) (m/s 2 ) (m) = kg m 2 /s 2 Work done= E = E f E i = KE + PE

11 Why is the relationship between work and energy so important? Change of velocity Work-energy relationship indicate how kinetic energy can be changed by doing work Energy = capacity to work Work done = A large change in kinetic energy a force applied over a long distance cf) Impulse-Momentum (ΣF) t = m (V f V i )

12 Doing work to decrease energy Catching a ball (negative work, absorbing energy) Landing from a jump or fall Absorbing energy These actions increase distance over which the force acts, thus decreasing the average value of the force. Follow-throw in pitching Safety and protective equipments

13 Conservation of Mechanical Energy Mechanical E = Kinetic E (KE) + Potential E (PE) When no external force acts other than No work done because of no external forces act W = E = 0 0 = KE + PE E = E f E i =0 E f = E i The total mechanical energy of an object is constant if no external forces other than act on the object. Q. When we drop a baseball (m = 0.5 kg) at 10.41m at Computer Common, what would it be a calculated drop velocity on the ground?

14 Definition Power Ability to do a given work in a short time Rate of doing work How much work done in a specific amount of time How quickly or slowly work is done P = Unit : Watt ( ) = 1 J/ 1 s Q8. In vertical jump-and-reach test, a 60-kg student jumps 60 cm, whereas a 90-kg student jumps 45 cm. Assuming both jumps took the same time, which jumper was more powerful?

15 Another expression of Power P = W/ t = (F d)/ t = F (d/ t) = Power is the product of and along the line of action of that force Examples Moving a stack of books Pedaling a bicycle Running kinematics (SR X SL) Muscle contraction

16 Q7. An Olympic weight lifter snatches 100 kg. In a snatch, the barbell is moved from a stationary position on the floor to a stationary position over the athlete s head. Only 0.5 s elapsed from the first movement of the barbell until it was overhead, and the barbell moved through a vertical displacement of 2.0 m. What was the weight lifter s average power output during the lift?

17 Q5. A baseball strikers the catcher s glove with a horizontal velocity of 40 m/s. The mass of the baseball is 0.15 kg. The displacement of the baseball due to the deformation of the catcher s glove and the movement of the catcher s hand is 8 cm from the instant if first makes contact with the glove until it stops. a. How much kinetic energy dose the baseball possess just before it strikes the glove? b. How much work does the catcher do to the baseball during the catch? c. Is the work done positive or negative? d. What is the average impact force exerted by the glove on the baseball?

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