DPP 15-17 PDF
DPP 15-17 PDF
DPP 15-17 PDF
= ) j
5 i
5 i
t 2 ( s
2
=
. Work ork
done by this force in t =0 to 2 sec is: [3]
(A) 23 J (B) 32 J (C) zero (D) cant be obtained.
Q.3 A constant force produces maximumvelocity V on the block connected to the
spring of force constant K as shown in the fig. When the force constant of spring
becomes 4K, the maximumvelocity of the block is [3]
(A)
V
4
(B) 2V (C)
V
2
(D) V
Paragraph for question nos. 4 to 6
The kinetic energy of any body depends on the frame of reference of the observer. The kinetic energy
is given by 1/2 mv
2
rel
. Similarly the displacement of the object fromdifferent frames of reference will
be different. But the forces acting on the body remain unchanged. So work done by the forces as
seen from different frames will be different. But work energy theorem will still be hold in every inertial
reference frame.
For example, if a block of mass 2 kg is moving with velocity of 1 m/s towards east on a rough surface,
its K.E. =
2
1
2 1
2
=1J
If it comes to rest, its K.E. =0.
Work done by friction =K
f
K
i
= 1 J
If we observe it from a frame 2 moving with 1 m/s toward east, its initial velocity will appear to be
11=0.
Initial K.E. =
2
1
2 0
2
=0
Final velocity =0 1 =1
Final K.E. =
2
1
2 1
2
=1J
Work done by friction =1 0 =1 J
Q.4 According to passage. [3]
(A) In 2
nd
frame, force of friction was opposite to displacement.
(B) In 2
nd
frame, force of friction was in same direction as displacement.
(C) In ground frame, force of friction is in same direction as the displacement.
(D) None of these
Q.5 What should be the velocity of an observer so that he will report the work done by friction on the
block to be 0. [3]
(A)
2
1
m/s W (B)
2
1
m/s E (C) 1 m/s W (D) 1 m/s E
Q.6 Choose correct statement. [3]
(A) In ground frame, work done by friction on ground is positive.
(B) In ground frame, work done by friction on ground is negative.
(C) In frame 2, work done by friction on ground is negative.
(D) In frame 2, work done by friction on ground is positive.
Q.7 Column-I shows certain situations and column-2 shows information about forces. [6]
Column - I Column - II
(A)
Situation
Front view of a car
rounding a curve with
constant speed.
F
1
F
3
F
2
(P)
3 2 1
F F F
+ + is centripetal force.
(B)
F
1
F
2
F
3
is static friction.
(C)
F
1
F
2
F
3
Particle kept on rough surface
of a bowl, no relative motion
of particle in bowl, bowl has
constant angular velocity
e
(R)
1
F
\
|
+ M
2
m
gh (C)
|
.
|
\
| +
2
M m
gh (D) None of the above
Q.4 Choose the correct alternative(s): [4]
(A) When a conservative force does positive work, the potential energy of the systemdecreases.
(B) Work done by friction on a body always results in a loss of its kinetic energy.
(C) Work done in the motion of a body over a closed loop is zero for every force in nature.
(D) Work done by friction on a body may increase its kinetic energy.
Q.5 A spring having a relaxed length L
o
=25cm has a block attached to one
end, while the other end is attached to a ceiling as shown. A person slowly
lowers the block a distance d until the block just hangs without moving, as
shown in the second diagram. Which statements below
are true? (Mark ALL that apply. Use k =2N/cm and m =600g.)
k, L
0
d
m
(A) The distance d is 6cm. [4]
(B) The potential energy stored in the spring when the block is simply hanging fromit is 0.09J.
(C) The change in gravitational potential energy of the Earth-block systemis 0.18J.
(D) The decrease in gravitational potential energy is equal (in magnitude) to the increase in potential
energy stored in the spring.
Q.6 A block suspended froma spring is released from rest when spring is unstretched. x represents stretch
in spring. Select the appropriate graph taking quantities in column-I as y-axis. [6]
Column-I Column-II
4
1
2
3
x
x
max
(A) The KE of block (P) 1
(B) The work done on the block by gravity (Q) 2
(C) The magnitude of work done on the block by spring (R) 3
(D) The total mechanical energy of block-earth-spring system (S) 4
Q.7 A block of mass m rests on an incline which makes an angle u with the horizontal plane (see figure).
There is friction between the block and the surface. The static friction coefficient
s
is larger than the
kinetic friction coefficient
k
. The block is attached to a "massless" spring of spring constant k. In the
absence of any forces on the spring, its (relaxed) length would be l. [6]
(a) We pull on the block and extend the spring till its length is l +x. What is the
maximumextension, x
max
, of the spring for which the block will remain stationary
when released?
In the following (b) & (c) parts, use the symbol x
max
(b) In this position the block is then gently touched at time t =0. It starts moving. For what value of x will the
block reach its maximumspeed?
(c) As the block moves, the spring will get shorter. At some point in time, t
1
, the extension is x. How
much work was done by (i) gravity, (ii) the spring force and (iii) by friction between t =0 and t
1
.
(d) As the block moves uphill, the spring gets shorter. What is a necessary requirement for the spring to
become at least as short as its relaxed length l?
PAGE # 3
PHYSI CS
Dai l y Practi ce Probl ems
Target IIT JEE 2011
Class:XIII(X) Discussion: 16-17/08/2010 Time: 35 Min. M.M.: 30 DPP. NO.-17
Q.1 In figure, a block of mass mis released fromrest when spring was in its
natural length. The pulley also has mass mbut it is frictionless. Suppose
the value of mis such that finally it is just able to lift the block M up after
releasing it. [4]
m
M
Rod
String
(A) The weight of mrequired to just lift M is
g
2
M
(B) The tension in the rod, when mis in has zero acceleration
g
2
M
(C) The normal force acting on M when m has zero acceleration
g
2
M
(D) The tension in the string when displacement of mis maximumpossible is Mg
Q.2 A conservative force has the potential energy function U(x) as shown by the graph. A particle moving in
one dimension under the influence of this force has kinetic energy 1.0J when it is at position x
1
. Which of
the following is/are correct statement(s) about the motion of the particle ?
(A) It oscillates
(B) It moves to the right of x
3
and never returns [4]
(C) It comes to rest at either x
0
or x
2
(D) It cannot reach either x
0
or x
2
Q.3 Choose the correct option(s) [4]
(A) If only conservative forces act on a particle, the kinetic energy remains constant.
(B) If the net force acting on an object is zero, then the object is at rest
(C) If net mechanical work is done on a body, the body must accelerate
(D) If net mechanical work is done on a body, the speed of body must change
Q.4 The spring is compressed by distance a and the systemis released. The block again comes to rest when
the spring is elongated by a distance b. During this process [3]
(A) work done by the spring on the block =
2
) b a ( k
2
1