(1) A boy weighing 350 N runs up a flight of 30 steps, each 20cm high in 1 minute. Calculate: (a) the work done and (b) the power spent
(2) A truck weighing 1000 kgf changes its speed from 36 km/hr to 72 km/hr in 2 minutes. Calculate: (a) the work done by the engine and (b) its power (g=10m/s²)
(3) How much energy is gained by a box of mass 20 kg when a man
(a) carrying the box waits for 5 minutes for a bus
(b) runs carrying the box with a speed of 3 metre per second to catch the bus?

2

Answers

2015-11-13T20:29:21+05:30
1: a: given,
force = 350 N
1 step = 20 cm therefore 30 such steps will be of = 30 * 20 = 600 cm =6 m
time = 1 min = 60 sec
work done = Fs
= 350 * 6
=2100 J
therefore work done = 2100 J
___________________
b: now,
power = energy / time
power = 2100/60 = 210/6 = 70/2 
power = 35 W
therefore power spend = 35 W
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2: given , 
mass = 1000 kg
initial velocity = 36 km/hr = 10 m/s
final velocity = 72 km/hr = 20 m/s
time = 2 min = 2 * 60 = 120 sec
acceleration = 10 m/s
²
work = F*s
therefore s ( distance ) ,
2as = v² - u²

2 ( 10 ) ( s ) = 20² - 10²
20 s = 400 - 100
s = 300 / 20
s = 15 m
therefore , work  = Fs
= 1000 * 10 * 15
= 150000 J = 1.5 * 10⁵ J
therefore work done = 1.5 * 10⁵ J
____________________
b: now ,
power = energy / time
power = 150000/ 120
power = 1250 W.
------------------------------------------------------------------------------------------------------------ 
3: a: no work has been done since the man is waiting he should make some displacement for doing any work therefore the answer is no energy had been transferred by the man .
 __________
b: mass = 20 kg
time  = 5 min = 5 * 60 = 300 sec
velocity = 3 m/s 
therefore energy ,
= 1/2 mv
²
= 1/2 * 20 * 3²
= 10 * 9 
= 90 J
therefore the energy gained by the box = 90 J
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3 5 3
2015-11-13T22:17:08+05:30
1: a: F = 350 N
S = 30 * 20 = 600 cm =6 m
work done = Fs
= 350 * 6
=2100 J
WORK = 2100 J

b:   power = energy / time
power = 2100/60 = 210/6 = 70/2 
power = 35 W

2:  
mass = 1000 kg
initial velocity = 36 km/hr = 10 m/s
final velocity = 72 km/hr = 20 m/s
time = 2 min = 2 * 60 = 120 sec
acceleration = 10 m/s
²
work = F*s
2as = v² - u²

2 ( 10 ) ( s ) = 20² - 10²
20 s = 400 - 100
s = 300 / 20
s = 15 m
 work  = Fs
= 1000 * 10 * 15
= 150000 J = 1.5 * 10⁵ J
b: 
power = energy / time
power = 150000/ 120
power = 1250 W.
3: a: no work has been done since the man is waiting he should make some displacement for doing any work therefore the answer is no energy had been transferred by the man .
 
b: mass = 20 kg
time  = 5 min = 5 * 60 = 300 sec
velocity = 3 m/s 
 energy ,
= 1/2 mv
²
= 1/2 * 20 * 3²
= 10 * 9 
= 90 J

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