CHAPTER 15 WORK, ENERGY AND POWEREXERCISE 78, Page 173

1. Determine the work done when a force of 50 N pushes an object 1.5 km in the same direction as the force. Work done = force distance moved in the direction of the force = 50 N 1500 m = 75000 J (since 1 J = 1 Nm)i.e. work done = 75 kJ

2. Calculate the work done when a mass of weight 200 N is lifted vertically by a crane to a height of 100 m.

When work is done in lifting then: work done = (weight of the body) (vertical distance moved)Weight is the downward force due to the mass of an object. Hence work done = 200 N 100 m = 20000 J = 20 kJ 3. A motor supplies a constant force of 2 kN to move a load 10 m. The force is then changed to a constant 1.5 kN and the load is moved a further 20 m. Draw the force/distance graph for the complete operation, and, from the graph, determine the total work done by the motor. The force/distance graph is shown below.

Total work done = area under the force/distance graph = (2000 10) + (1500 20) = 20000 + 30000 = 50000 J = 50 kJ

4. A spring, initially relaxed, is extended 80 mm. Draw a work diagram and hence determine the work done if the spring requires a force of 0.5 N/mm of stretch. Force = 0.5 N/mm 80 mm = 40 N

The work diagram is shown below.

Total work done = area under the diagram

= base height = 80 mm 40 N

= (80 m) 40 N = 1.6 J 5. A spring requires a force of 50 N to cause an extension of 100 mm. Determine the work done in extending the spring (a) from 0 to 100 mm, and (b) from 40 mm to 100 mm.

The work diagram is shown below.

(a) The work done in extending the spring from 0 to 100 mm

= base height = (100 m) 50 N = 2.5 J(b) The work done in extending the spring from 40 mm to 100 mm = area ABCE = area ABCD + area ADE

= (60 m)(20 N) + (60 m)(30 N) = 1.2 + 0.9 = 2.1 J

6. The resistance to a cutting tool varies during the cutting stroke of 800 mm as follows: (i) the resistance increases uniformly from an initial 5000 N to 10000 N as the tool moves 500 mm, and (ii) the resistance falls uniformly from 10000 N to 6000 N as the tool moves 300 mm. Draw the work diagram and calculate the work done in one cutting stroke.

The work diagram is shown below.

Work done in one cutting stroke = area under the diagram = A + B + C + D

= (500 5) + (500 5) + (300 4)

+ (300 6) = 2.5 kJ + 1.25 kJ + 0.6 kJ + 1.8 kJ = 6.15 kJEXERCISE 79, Page 175

1. A machine lifts a mass of weight 490.5 N through a height of 12 m when 7.85 kJ of energy is supplied to it. Determine the efficiency of the machine.

Work done in lifting mass = force distance moved = weight of body distance moved = 490.5 N 12 m = 5886 J = useful energy outputEnergy input = 7.85 kJ = 7850 J

Efficiency, = = = 0.75 or 75%

2. Determine the output energy of an electric motor which is 60% efficient if it uses 2 kJ of electrical energy.

Efficiency, = thus =

from which, output energy = 2000 = 1200 J = 1.2 kJ

3. A machine that is used for lifting a particular mass is supplied with 5 kJ of energy. If the machine has an efficiency of 65% and exerts a force of 812.5 N to what height will it lift the mass?

Efficiency, = i.e. =

from which, output energy = 5000 = 3250 J Work done = force distance moved hence 3250 J = 812.5 N height

from which, height = = 4 m

4. A load is hoisted 42 m and requires a force of 100 N. The efficiency of the hoist gear is 60% and that of the motor is 70%. Determine the input energy to the hoist.

Output energy = work done = force distance = 100 N 42 m = 4200 J

For the gearing, efficiency = i.e. =

from which, the input energy to the gears = 4200 = 7000 JThe input energy to the gears is the same as the output energy of the motor. Thus, for the motor,

efficiency= i.e. =

Hence, input energy to the hoist = 7000 = 10000 J = 10 kJ

EXERCISE 80, Page 177

1. The output power of a motor is 10 kW. How much work does it do in 1 minute?

Power = from which, work done = power time = 10000 W 60 s = 600000 J = 600 kJ

2. Determine the power required to lift a load through a height of 20 m in 12.5 s if the force required is 2.5 kN.

Work done = force distance moved = 2500 N 20 m = 50000 J

Power = = = 4000 W or 4 kW

3. 25 kJ of work is done by a force in moving an object uniformly through 50 m in 40 s. Calculate (a) the value of the force, and (b) the power.

(a) Work done = force distance hence 25000 J = force 50 m

from which, force = = 500 N

(b) Power = = = 625 W

4. A car towing another at 54 km/h exerts a steady pull of 800 N. Determine (a) the work done in

hr, and (b) the power required.

(a) Work done = force distance moved.

The distance moved in 15 min, i.e. h, at 54 km/h = = 13.5 km. Hence, work done = 800 N 13500 m = 10800 kJ or 10.8 MJ

(b) Power required = = = 12000 W or 12 kW

5. To what height will a mass of weight 500 N be raised in 20 s by a motor using 4 kW of power?

Work done = force distance. Hence, work done = 500 N height.

Power =, from which, work done = power time taken = 4000 W 20 s = 80000 J

Hence, 80000 = 500 N height, from which, height = = 160 m

6. The output power of a motor is 10 kW. Determine (a) the work done by the motor in 2 hours, and (b) the energy used by the motor if it is 72% efficient.

(a) Work done = power time taken = 10 kW 2 h = 20 kWh = 20000 60 60 Ws = 72 MJ

(b) Efficiency = i.e. =

Hence, energy used by the motor = 72 = 100 MJ

7. A car is travelling at a constant speed of 81 km/h. The frictional resistance to motion is 0.60 kN. Determine the power required to keep the car moving at this speed.

Power = force velocity

= 0.60 kN 81 km/h = 600 N = 13500 N m/s = 13500 J/s = 13.5 kW 8. A constant force of 2.0 kN is required to move the table of a shaping machine when a cut is being made. Determine the power required if the stroke of 1.2 m is completed in 5.0 s.

Work done in each cutting stroke = force distance = 2000 N 1.2 m = 2400 J

Power required = = 480 W

9. The variation of force with distance for a vehicle that is decelerating is as follows: Distance (m)6005004003002001000 Force (kN) 24 20 16 12 8 40 If the vehicle covers the 600 m in 1.2 minutes, find the power needed to bring the vehicle to rest.

The force/distance graph is shown below.

Work done = area under the force/distance graph

= base height = 600 m 24 kN

= 7200 kJ

Power needed to bring the vehicle to rest = = 100 kW

10. A cylindrical bar of steel is turned in a lathe. The tangential cutting force on the tool is 0.5 kN and the cutting speed is 180 mm/s. Determine the power absorbed in cutting the steel.

Power absorbed in cutting the steel = force velocity = 0.5 kN 180 mm/s = 500 N 0.180 m/s = 90 J/s = 90 W

EXERCISE 81, Page 180

1. An object of mass 400 g is thrown vertically upwards and its maximum increase in potential energy is 32.6 J. Determine the maximum height reached, neglecting air resistance. Potential energy = mgh

i.e. 32.6 = (0.4 kg)(9.81 m/s)(h)

from which, maximum height, h = = 8.31 m

2. A ball bearing of mass 100 g rolls down from the top of a chute of length 400 m inclined at an angle of 30 to the horizontal. Determine the decrease in potential energy of the ball bearing as it reaches the bottom of the chute.

With reference the above diagram, sin 30 = from which, h = 400 sin 30 = 200 m Hence, increase in potential energy = mgh

= 0.1 kg 9.81 m/s 200 m = 196.2 J 3. A vehicle of mass 800 kg is travelling at 54 km/h when its brakes are applied. Find the kinetic energy lost when the car comes to rest.

Kinetic energy = mv= (800 kg)i.e. kinetic energy lost = 90000 J or 90 kJ

4. A body of mass 15 kg has its speed reduced from 30 km/h to 18 km/h in 4.0 s. Calculate the power required to effect this change of speed.

Change in kinetic energy of body = m v - m v

where mass, m = 15 kg, v= initial velocity = 30 km/h = m/s = 8.3333 m/s, and

v= final velocity = 18 km/h = m/s = 5 m/s.

Hence, change in kinetic energy = m(v- v) = (15)(8.3333- 5) = 333.329 J

Braking power = = 83.33 W

5. Supplies of mass 300 kg are dropped from a helicopter flying at an altitude of 60 m. Determine the potential energy of the supplies relative to the ground at the instant of release, and its kinetic energy as it strikes the ground.

Potential energy of supplies at release = mgh

= (300 kg)(9.81 m/s)(60 m) = 176580 J = 176.6 kJBy the principle of conservation of energy, kinetic energy as the supplies strikes the ground = potential energy at release = 176.6 kJ 6. A shell of mass 10 kg is fired vertically upwards with an initial velocity of 200 m/s. Determine its initial kinetic energy and the maximum height reached, correct to the nearest metre, neglecting air resistance.

Initial kinetic energy = mv= (10 kg)(200 m/s)= 200 kJAt the maximum height, the velocity of the canister is zero and all the kinetic energy has been converted into potential energy. Hence, potential energy = initial kinetic energy = 200000 J Then, 200000 = mgh = (10)(9.81)(h)

from which, height h = = 2039 mi.e. the maximum height reached is 2039 m or 2.039 km

7. The potential energy of a mass is increased by 20.0 kJ when it is lifted vertically through a height of 25.0 m. It is now released and allowed to fall freely. Neglecting air resistance, find its kinetic energy and its velocity after it has fallen 10.0 m.

Potential energy of mass = 20.0 kJ = mgh

from which, mass, m = = 81.55 kgPotential energy after falling 10.0 m = mgh = (81.55)(9.81)(10.0) = 8000 JKinetic energy = potential energy = 8000 J = 8 kJ

Kinetic energy = mv i.e. 8000 = (81.55)v

from which,

and velocity after falling 10.0 m, v = = 14.0 m/s

8. A pile-driver of mass 400 kg falls freely through a height of 1.2 m on to a pile of mass 150 kg. Determine the velocity with which the driver hits the pile. If, at impact, 2.5 kJ of energy are lost due to heat and sound, the remaining energy being possessed by the pile and driver as they are driven together into the ground a distance of 150 mm, determine (a) the common velocity after impact, (b) the average resistance of the ground.

The potential energy of the pile-driver is converted into kinetic energy.

Thus, potential energy = kinetic energy, i.e. mgh = mv

from which, velocity v = = = 4.85 m/s.Hence, the pile-driver hits the pile at a velocity of 4.85 m/s

(a) Before impact, kinetic energy of pile driver = mv= (400)(4.85) = 4704.5 J = 4.705 kJ Kinetic energy after impact = 4.705 2.5 = 2.205 kJ Thus the pile-driver and pile together have a mass of 400 + 150 = 550 kg and possess kinetic energy of 2.205 kJ

Hence, 2205 = mv= (550)v

from which, velocity v = = 2.83 m/s Thus, the common velocity after impact is 2.83 m/s(b) The kinetic energy after impact is absorbed in overcoming the resistance of the ground, in a distance of 150 mm. Kinetic energy = work done = resistance distance i.e. 2205 = resistance 0.150

from which, resistance = = 14700 N Hence, the average resistance of the ground is 14.70 kN

EXERCISE 82, Page 181

1. Calculate the kinetic energy of a solid flat disc of diameter 0.6 m and of uniform thickness of

0.1 m rotating about its centre at 50 rpm. Take the density of the disc material as 7860 kg/m.

Angular velocity, = 2 50 = 5.236 rad/s

From Table 14.1, page 168, I = R t

= 7860 0.3m 0.1 m

i.e. I = 10 kg m

Hence, kinetic energy = = = 137.1 J

2. If the disc of Problem 1 had a hole in its centre of 0.2 m diameter, what would be its kinetic energy?

From Table 14.1, page 168, I = t

= 7860 m 0.1 m

i.e. I = 9.877 kg m

Hence, kinetic energy = = 135.4 J

3. If an annulus of external diameter 0.4 m and internal diameter 0.2 m were rotated about its centre at 100 rpm, what would be its kinetic energy? Assume the uniform thickness of the annulus is

0.08 m and the density of the material is 7860 kg/m.

From Table 14.1, page 168, I = t

= 7860 m 0.08 m

i.e. I = 1.482 kg m

Angular velocity, = 2 100 = 10.47 rad/s

Hence, kinetic energy = = 81.2 J

EXERCISE 83, Page 181Answers found from within the text of the chapter, pages 170 to 181.

EXERCISE 84, Page 1821. (b) 2. (c)3. (c) 4. (a)5. (d) 6. (c) 7. (a) 8. (d)9. (c) 10. (b) 11. (b) 12. (a) 13. (d) 14. (a) 15. (d) 16. (c)

215 John Bird & Carl Ross Published by Taylor and Francis