Question

Question 6.5 Answer the following:
(a) The casing of a rocket in flight burns up due to friction. At whose expense is the heat energy required for burning
obtained? The rocket or atmosphere?
(b) Comets move around the sun in highly elliptical orbits.The gravitational force on the comet due to the sun is not
normal to the comet's velocity in general. Yet the work done by the gravitational force over every complete orbit of the
comet is zero. Why?
(c) An artificial satellite orbiting the Earth in very thin atmosphere loses its energy gradually due to dissipation against
atmoshpheric resistance, however small. Why then does its speed increase progressively as it comes closer and
closer to Earth?
(d) In Fig. 6.13(i) the man walks 2m carrying a mass of 15 kg on his hands. In Fig. 6.13(ii), he walks the same distance
pulling the rope behind him. The rope goes over a pulley and a mass of 15 kg hangs at its other end. In which case is
the work done greater?

Chapter Work, Energy And Power Page 135

Answer 1

Hi--------------
case1
Rocket
because rocket reduce it mass by burn fuel.
____________________

b)
here workdone is zero
because gravitational force is conserved force
hence workdone in close path is zero.
___________________

c)
energy is conserved ,so if potential energy increase ,then kinetic energy is decrease ,
because total energy is some of potential energy and kinetic energy .
____________________

d)
see in image
case1)

Mass, m = 15 kg
Displacement, s = 2 m
Work done, W=fscosQ
here Q=90
cos90=0
hence
w=0

case2)

Mass, m = 15 kg

Displacement, s = 2 m

Here, the direction of the force applied on the rope and the direction of the displacement of the rope are same.

so Q=0
cos0=1
here f=mg
g=9.8
m/s^2

w=fscosQ
w=15**9.8*2cos0
w=294j

so we need more work in case2)

hope it help u

Answer 2

(a) HEAT energy required for burning of the casing of a rocket in flight is obtained from the rocket itself not from the ATMOSPHERE. after burning , mass of it decreases so, total energy of the rocket decreases.
_______________________________________________

(b) actually , Gravitational force is a conservative force means work done by Gravitational force over a closed path { doesn't matter which type of shape of close path } is always ZERO .

hence, work done by Gravitational force over every complete orbit of the comet is zero .
_____________________________________________

(c) if Artificial satellite comes closer to the earth its gravitational potential decreases but according to law of conservation of energy .
Total energy = constant
K.E + P.E = constant
hence, kinetic energy increases . but kinetic energy is directly proportional to square of speed of particle . hence, speed also increases .
______________________________________________

(d) see the figure ,
in first case :
work done by man = Force applied by man in object × displacement of object along force
but displacement of object along force = 0 { because man moves the object perpendicular direction of force .}
hence,
work done by man = F.0 = 0

in 2nd case :
The man applied force in horizontal direction , and displacement of object is also in horizontal direction.
so,
F = F.S cos0°
= Mg × S
= 15 × 9.8 × 2 Joule
= 294 Joule .