Rao NaveedHussain Rearsch in Spacetime Physics
Wednesday, September 26, 2018
Wednesday, September 12, 2018
Monday, July 16, 2018
First Year Physics Notes chap 4, Work and Energy
4.7:
Conservation of Energy
Energy can neither be created nor destroyed but can
transform from one to another such that toatal energy remains constant
Total
Energy = PE + KE
SHORT
QUESTIONS AND ANSWERS FROM EXERCISE
4.1:
A person holds a bag of groceries, while standing still, talking to a friend. A
car is stationary with its engine running. From stand point of work, how are these two situation similar?
Ans:
In both cases no displacement is present so work done is zero in both case
4.2:
Calculate the work done in kilojoules in lifting a mass of 10 kg at a steady
velocity through vertical height of 10 m.
Ans:
mass m = 10 kg, h = 10 m, g = 9.8ms-2, w = ?
W = mgh
W = 10 ´ 9.8 ´10
J
W
= 980 J = 0.98 kJ
4.4:
In which case more work is done? (i)
When a 50 kg bag of books is lifted through 50 cm or (ii) when a 50 kg crate is pushed through 2m
across the floor with a force of 50N?
Ans:
(i) w = mgh
W = 50 ´ 9.8 ´
½ J
W = 25 ´ 98/10 J = 5 ´49J
W
= 245 J
(ii) W = F´d
W = 50 ´2 J
W = 100 J
In case (i), more work is done
4.5:
An object has 1 J of Potential energy. Explain what does it mean?
Ans:
We
know that
PE = mgh
The object has weight
of IN at a height of 1 m or possesses 1
J potential energy
4.6:
A
ball of mass m is held at a height h1 above the table. The table top
is at a height h2 from the floor. One student says that the ball has
PE = mgh1 but another says it is mg(h1 +h2).
Who is correct?
Ans:
Both
students are correct according to their observation
First student is
considering the PE from the top of the table while the other is looking from
the floor to the top
4.7:
When a rocket re-enters the atmosphere, its nose cone becomes very hot. Where
does this heat come from?
Ans:
When
the rocket re-enters in the atmosphere, its velocity increases rapidly so due
to high kinetic enrgy and high friction, its nose cone becomes very hot.
4.8:
What sort of energy is in the following?
(i) Compressed
spring (ii) Water in a high Dam (ii) A moving car
Ans:
(i)
Elastic PE (ii) Gravitational PE (ii)
Kinetic Energy
4.9:
A
girl drops a cup from a certain height, which breakes into pieces. What energy
changes are involved?
Ans:
Potential
energy of the cup is converted into KE, heat and sound
4.10:
A boy uses a catapult to throw a stone which accidently smashes a green house
window. List the possible energy changes?
Ans:
The
elastic potential energy of the catapult is converted into kinetic energy of
stone, heat and sound produced
NUMERICAL
PROBLEMS OF EXERCISE
4.1:
A
man pushes a lawn mover with a 40 N force directed at an angle of 20 downward from horizontal . Find the work done
by the man as he cuts a strip of grass
20 m long?
Data:
F = 40 N,
d = 20 m, q
= 20, w = ?
Solution:
W = Fdcosq
W = 40 ´
20´cos20
W
= 750 J
4.2:
A rain drop of mass 3.5 10-5
kg falls vertically at a constant speed under the influence of force of gravity
and friction. In falling through 100 m,
how much work is done by (i) gravity
(ii) friction
Solution:
W = mgh
W = 3.5
10-5
´
9.8 ´
100 J
W
= 0.0328 J
W
= -
0.0328 J
4.3:
Ten
blocks, each 6cm thick and mass 1.5 kg, lie flat on a table. How much work is
required to stack them one on the top of another?
Data:
Mass m = 1.5 kg, height h = 6cm = 0.06 m, work done = ?
Solution:
W = mgh
Work done to place 2nd
brick on ist, 3rd on 2nd and finally 10th on 9th
W2®1
= mgh1 = 1.5´9.8 ´1( 0.06) J
W3®2
= mgh2 = 1.5´9.8 ´2( 0.06) J
W4®3
= mgh3 = 1.5´9.8 ´3( 0.06) J
.
.
.
W10®9
= mgh9 = 1.5´9.8 ´9( 0.06) J
Wtotal = mgh
= 1.5´9.8
´[1
+ 2 + 3 + 4 + … + 9]( 0.06) J
Wtotal = mgh
= 1.5´9.8
´45(
0.06) J
Wtotal
= 40 J
4.4:
A car of 800 kg travalling at 54 kmh-1
is brought to rest in 60 m. Find the average retarding force on the car. What
has happened to original kinetic energy?
Data;
M = 800 kg, vi = 54km/h = 15 m/s, vf = 0, s= 60 m,
F = ?
Solution:
Using work energy principle
Fs = ½ m(vf2 - vi2)
Fs = ½ m(0 - vi2)
F = -½ mvi2/s
F = -½ ´ 800 ´225/60
N
F
= -1500
N
4.5:
A 1000 kg automobile atb the top of an incline
10 m high 100 m long is released and rolls down the hill. What is its speed at
the bottom of the incline if the average retarding force due to friction is 480
N?
Data:
M = 1000 kg, vi = 0, vf = ?, s = 100 m,
h = 10 m, F = 480 N
Solution:
W = DKE
Fs = ½ m(vf2
- vi2)
Fs = ½ m(vf2
- 0)
vf2 = 2Fs/m
vf2 = 2´480´100/1000
vf
= 9.7 ms-1
4.6:
100
m3 water is pumped from a reservoir into a tank, 10 m higher than
the reservoir, in 20 minutes. If density of water is 1000 kgm-3. Find
(a) Increase in PE (b) power delivered
Data:
Volume v = 100 m3, height h = 10 m, time t = 20 min = 1200 s, PE = ? P = ?
Solution:
PE = mgh m = r´
V = 1000´100
Kg
PE = 1000000´
9.8 J
PE
= 9.8 ´106
J
Power = mgh/t
Power = 1000000´
9.8/1200
Power
= 8.2 ´103
watts
4.7:
A
force of 400 N is required to overcome road friction and air resistance in
propelling an automobile at 80 kmh-1.
What power (kw) must be engine develop?
Data:
F = 400 N,
v = 80 kmh-1
= 22.22ms-1, Power P = ?
Solution:
P = F.v
P = 400 ´
22.22 watts
P = 8900 watts
P
= 8.9 kw
4.8:
How
large a force is required to accelerate an electron of mass 9.1´10-31
kg from rest to a speed 2´107 ms-1
through a distance of 5 cm?
Data:
m = 9.1´10-31
kg, vi = 0, vf = 2´107
ms-1, d = 5 cm = 0.05m, F= ?
Solution:
W = DKE
Fs = ½ m(vf2
- vi2)
Fs = ½ m(vf2
- 0)
F = ½ mvf2
/s
F = ½ ´9.1´10-31
´2´107
´2´107 /0.05 N
F
= 3.6´10-15 N
4.9:
A diver weighing 750 N dives from a board of 10 m above the surface of the pool
of water. Use the conservation of mechanical energy to find his speed at a
point 5 m above the water surface, neglecting the air resistance/
Data:
W = mg = 750 N, h1 = 5m, h2 = 10 m, v1 = 0, v2 = ?
Solution:
Gain in KE = loss in PE
½ m(v22
- v12) = mg(h2
- h1)
½ m(v22
- 0) = mg(h2 - h1)
v22 = 2 g(h2 - h1)
v22 = 2 ´9.8(10 - 5)
v22 = 2 ´9.8 ´
5 = 9.8 ´
10 = 98
v2 =
Ö98
v2 =
9.9 ms-1
4.10:
A
child starts from rest at the top of a slide of height 4 m (a) What is the
speed at the bottom if the slide is frictionless (b) If he reaches the bottom,
with the speed of 6 ms-1.
What percentage of this total energy at
the top of the slide is lost as a result of friction?
Data:
h1 =
0m, h2 = 4 m, v1 = 0, v2 = v = ?
Solution:
Gain in KE = loss in PE
½ m(v22
- v12) = mg(h2
- h1)
½ m(v22
- 0) = mg(h2 - 0)
v22 = 8g
v2 = Ö8´9.8
v2 = 8.85 ms-1
Special
Questions
Q: How can we get relation
between energy and work in a single step?
Ans:
If we multiply 3rd equation of motion by mass m then we get work
energy relation in a single step
2as = vf2 -
vi2
2mas = mvf2 -
mvi2
Fs = ½ mvf2 -
½ mvi2
W
= D(
½ mvf)
Q: Is there any formula for
work other than W = F.d?
Ans:
Yes, it exists
W = F.d
Writing F = P/t
W = P/t.d
W = P.d/t but d/t = v
W = P.v
Q: Can we get relation for
absolute gravitational potential energy from Newton’s law of gravitation?
Ans:
We know that
F = G mM//R2
Multyplying by R on both sides
FR = - GmM/R
Ug = - GmM/R FR = Ug
Negative sign is taken
due to work done against gravity
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