POWER AND EFFICIENCY

1. POWER AND EFFICIENCY

2. APPLICATIONS Engines and motors are often rated in terms of their power output. The power output of the motor lifting this elevator is related to the vertical force F acting on the elevator, causing it to move upwards. Given a desired lift velocity for the elevator (with a known maximum load), how can we determine the power requirement of the motor?

3. APPLICATIONS (continued) The speed at which a truck can climb a hill depends in part on the power output of the engine and the angle of inclination of the hill. For a given angle, how can we determine the speed of this truck, knowing the power transmitted by the engine to the wheels? Can we find the speed, if we know the power? If we know the engine power output and speed of the truck, can we determine the maximum angle of climb of this truck ?

4. POWER AND EFFICIENCY Power is defined as the amount of work performed per unit of time. If a machine or engine performs a certain amount of work, dU, within a given time interval, dt, the power generated can be calculated as P = dU/dt Since the work can be expressed as dU = F• dr, the power can be written (from yesterday’s lecture) P = dU/dt = (F• dr)/dt = F• (dr/dt) = F•v Thus, power is a scalar defined as the product of the force and velocity components acting in the same direction.

5. POWER Using scalar notation, power can be written P = F• v = F v cos q where q is the angle between the force and velocity vectors. Work = F * s cosq So if the velocity of a body acted on by a force F is known, the power can be determined by calculating the dot product or by multiplying force and velocity components. The unit of power in the SI system is the Watt (W) where 1 W = 1 J/s = 1 (N · m)/s . In the FPS system, power is usually expressed in units of horsepower (hp) where 1 hp = 550 (ft · lb)/s = 746 W .

6. EFFICIENCY The mechanical efficiency of a machine is the ratio of the useful power produced (output power) to the power supplied to the machine (input power) or e = (power output) /(power input) If energy input and removal occur at the same time, efficiency may also be expressed in terms of the ratio of output energy to input energy or e = (energy output) /(energy input) Machines will always have frictional forces. Since frictional forces dissipate energy, additional power will be required to overcome these forces. Consequently, the efficiency of a machine is always less than 1.

7. PROCEDURE FOR ANALYSIS • Find the resultant externalforce acting on the body causing its motion. It may be necessary to draw a free-body diagram. • Determine the velocity of the point on the body at which the force is applied. Energy methods or the equation of motion and appropriate kinematic relations, may be necessary. • Multiply the force magnitude by the component of velocity acting in the direction of F to determine the power supplied to the body (P = F v cos q ). • In some cases, power may be found by calculating the work done per unit of time (P = dU/dt). • If the mechanical efficiency of a machine is known, either the power input or output can be determined.

9. Force in the direction of displacement

10. EXAMPLE Given: A 50 kg block (A) is hoisted by the pulley system and motor M. The motor has an efficiency of 0.8. At this instant, point P on the cable has a velocity of 12 m/s which is increasing at a rate of 6 m/s2. Neglect the mass of the pulleys and cable. Find: The power supplied to the motor at this instant. Plan: 1) Relate the cable and block velocities by defining position coordinates. Draw a FBD of the block. 2) Use the equation of motion to determine the cable tension. 3) Calculate the power supplied by the motor and then to the motor.

11. Here sP is defined to a point on the cable. Also sA is defined only to the lower pulley, since the block moves with the pulley. From kinematics, • sP + 2 sA = l  aP + 2 aA = 0 • aA = − aP / 2 = −3 m/s2 (↑) sm sB 2T mA aA A A WA EXAMPLE (continued) Solution: 1) Define position coordinates to relate velocities. Datum SP SA Draw the FBD and kinetic diagram of the block: =

12. EXAMPLE (continued) 2) The tension of the cable can be obtained by applying the equation of motion to the block. +↑ Fy = mA aA 2T − 490.5 = 50 (3)  T = 320.3 N 3) The power supplied by the motor is the product of the force applied to the cable and the velocity of the cable. Po = F •v = (320.3)(12) = 3844 W The power supplied to the motor is determined using the motor’s efficiency and the basic efficiency equation. Pi = Po/e = 3844/0.8 = 4804 W = 4.8 kW

13. TUTORIAL ON POWER AND EFFICIENCY (Don’t submit) F14-7 (pg 196); F14-12 (pg 196); 14-61 pg 199) OWN READING ON CENTROID AND CENTRE OF MASS

14. CONSERVATIVE FORCES, POTENTIAL ENERGY AND CONSERVATION OF ENERGY

15. APPLICATIONS The weight of the sacks resting on this platform causes potential energy to be stored in the supporting springs. As each sack is removed, the platform will rise slightly since some of the potential energy within the springs will be transformed into an increase in gravitational potential energy of the remaining sacks. If the sacks weigh 100 lb and the equivalent spring constant is k = 500 lb/ft, what is the energy stored in the springs?

16. APPLICATIONS (continued) The boy pulls the water balloon launcher back, stretching each of the four elastic cords. If we know the unstretched length and stiffness of each cord, can we estimate the maximum height and the maximum range of the water balloon when it is released from the current position ?

17. APPLICATIONS (continued) The roller coaster is released from rest at the top of the hill. As the coaster moves down the hill, potential energy is transformed into kinetic energy. What is the velocity of the coaster when it is at B and C? Also, how can we determine the minimum height of the hill so that the car travels around both inside loops without leaving the track?

18. A force F is said to be conservative if the work done is independent of the path followed by the force acting on a particle as it moves from A to B. This also means that the work done by the force F in a closed path (i.e., from A to B and then back to A) is zero. Thus, we say the work is conserved. z B F A ò = F d r 0 · y x CONSERVATIVE FORCE Vertical displacement – along Z direction

19. CONSERVATIVE FORCE The work done by a conservative force depends only on the positions of the particle, and is independent of its velocity or acceleration; eg are the weight (depends only on vertical displacement of weight or spring (elongation or compression). Friction force is non-conservative, depends on path – the longer displacement, the greater the work. Work is dissipated from the body in the form of heat

20. The “conservative” potential energy of a particle/system is typically written using the potential function V. There are two major components to V commonly encountered in mechanical systems, the potential energy from gravity and the potential energy from springs or other elastic elements. Vtotal = Vgravity + Vsprings CONSERVATIVE FORCE A more rigorous definition of a conservative force makes use of a potential function (V) and partial differential calculus, as explained in the text. However, even without the use of the these mathematical relationships, much can be understood and accomplished.

21. POTENTIAL ENERGY Potential energy is a measure of the amount of work a conservative force will do when a body changes position. In general, for any conservative force system, we can define the potential function (V) as a function of position. The work done by conservative forces as the particle moves equals the change in the value of the potential function (e.g., the sum of Vgravity and Vsprings). It is important to become familiar with the two types of potential energy and how to calculate their magnitudes.

22. The potential function (formula) for a gravitational force, e.g., weight (W = mg), is the force multiplied by its elevation from a datum. The datum can be defined at any convenient location. Vg = ± W y POTENTIAL ENERGY DUE TO GRAVITY Vg is positive if y is above the datum and negative if y is below the datum. Remember, YOU get to set the datum.

23. Ve (where ‘e’ denotes an elastic spring) has the distance “s” raised to a power (the result of an integration) or 1 = V k s2 2 e ELASTIC POTENTIAL ENERGY Recall that the force of an elastic spring is F = ks. It is important to realize that the potential energy of a spring, while it looks similar, is a different formula. Elongated position Compressed position Notice that the potential function Ve always yields positive energy.

24. + = + T V T V 1 1 2 2 = Constant CONSERVATION OF ENERGY When a particle is acted upon by a system of conservative forces, the work done by these forces is conserved and the sum of kinetic energy and potential energy remains constant. In other words, as the particle moves, kinetic energy is converted to potential energy and vice versa. This principle is called the principle of conservation of MECHANICAL energy or SIMPLY principle of conservation of energy and is expressed as T1 stands for the kinetic energy at state 1 and V1 is the potential energy function for state 1. T2 and V2 represent these energy states at state 2. Recall, the kinetic energy is defined as T = ½ mv2.

25. EXAMPLE Given: The 2 kg collar is moving down with the velocity of 4 m/s at A. The spring constant is 30 N/m. The unstretched length of the spring is 1 m. Find: The velocity of the collar when s = 1 m. Plan: Apply the conservation of energy equation between A and C. Set the gravitational potential energy datum at point A or point C (in this example, choose point A—why?).

26. Note that the potential energy at C has two parts. VC = (VC)e + (VC)g VC = 0.5 (30) (√5 – 1)2 – 2 (9.81) 1 The kinetic energy at C is TC = 0.5 (2) v2 EXAMPLE (continued) Solution: Similarly, the potential and kinetic energies at A will be VA = 0.5 (30) (2 – 1)2, TA = 0.5 (2) 42 The energy conservation equation becomes TA + VA = TC + VC . [ 0.5(30) (√5 – 1)2 – 2(9.81)1 ] + 0.5 (2) v2 = [0.5 (30) (2 – 1)2 ]+ 0.5 (2) 42  v = 5.26 m/s

27. PROBLEM SOLVING Given: The 800 kg roller coaster starts from A with a speed of 3 m/s. Find:The minimum height, h, of the hill so that the car travels around inside loop at B without leaving the track. Also find the normal reaction on the car when the car is at C for this height of A. Plan: Note that only kinetic energy and potential energy due to gravity are involved. Determine the velocity at B using the equation of equilibrium and then apply the conservation of energy equation to find minimum height h .

28. 2 v å = = F ma m r n n 800 (9.81) = 800 10 PROBLEM SOLVING (continued) Solution: 1) Placing the datum at A: TA + VA = TB + VB  0.5 (800) 32 + 0 = 0.5 (800) (vB)2− 800(9.81) (h − 20) (1) Equation of motion applied at B: 2) Find the required velocity of the coaster at B so it doesn’t leave the track. NB  0 (vB)2 =  vB = 9.905 m/s man mg

29. 2 v å = m F r n NC+800 (9.81) = 800 7 PROBLEM SOLVING (continued) Now using the energy conservation, eq. (1), the minimum h can be determined. 0.5 (800) 32 + 0 = 0.5 (800) (9.905)2− 800(9.81) (h − 20)  h= 24.5 m 3) To find the normal reaction at C, we need vc. TA + VA = TC + VC  0.5 (800) 32 + 0 = 0.5 (800) (vC)2− 800(9.81) (24.5 − 14)  VC = 14.66 m/s NC 14.662 Equation of motion applied at B: =  man mg  NC = 16.8 kN