Download Supplementary Exercise for Newton`s laws of motion

Supplementary Exercise for Newton’s laws of motion
1. A girl uses the following set-up to test the performance of her
fan. She uses a thread to hang a ping-pong ball from the
ceiling and directly in front of the fan.
When switch '1' is pressed, the angle between the thread and
the vertical is 28.
(a) When switch '2' is pressed, the angle between the thread
and the vertical changes from 28 to 56. Should the girl
conclude that the blowing force becomes double?
Explain your answer.
(b) How does the increase of the following factors affect ?
(i) Weight of the ball.
(ii) Area of the ball, on which the blowing force acts.
Multiple Choice Questions
1. At time t = 0, a table-tennis ball is released from a point above the ground
and bounces several times on the ground. The graph shows the variation of
the velocity v of the ball with time t. (Note: Velocity pointing upwards is
taken to be positive.) Which of the following represents the time of impact
with the ground?
A. P R
B. P Q
C. Q R
D. R S
2. Which of the following diagrams correctly shows the forces (represented by arrows) acting on the body
described in the respective situations?
(1) A simple pendulum
swinging in a vertical plane
A. (1) and (2) only
(2) A block is sliding down a
smooth inclined plane
B. (1) and (3) only
(3) A sphere falling freely in a
vacuum
C. (2) and (3) only
D. (1), (2) and (3)
3. Which of the following graphs show(s) that a constant resultant force is acting on the object?
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
3.1
D. (1), (2) and (3)
4. A block of mass 9 kg is pushed up an incline plane of slope 30o at a constant speed of 1 m s-1. The frictional
force acting on the block is 4 N. The distance traveled by the block on the incline is 10 m. What is the force
required to push up the block in this process?
A. 4 N
B. 40 N
C. 45 N
D. 49 N
5. A uniform cube of weight 600 N is held in equilibrium in the air by four identical cables
as shown. If each cable makes an equal angle of 20o with the vertical, find the tension in
each cable.
A.
B.
C.
D.
150 N
160 N
412 N
439 N
6. Two blocks, of masses m1 and m2 (m1 > m2), are connected by a light inextensible string and placed on a
smooth horizontal surface. Let T1 and T2 be the tensions in the string when a horizontal force of magnitude
F is applied to the blocks as shown in Figures (a) and (b) respectively. Which of the following relationships
are correct?
(1) T1 > T2
(2) T1 / m1 = T2 / m2
(3) T1 + T2 = F
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
7. An experiment is carried out by an astronaut on the surface of the Moon. A coin and a feather are dropped
at the same instant from the same height. Which statement is correct?
A. They fall together, taking a longer time than the coin would in falling from the same height on
Earth.
B. They fall together, taking a shorter time than the coin would in falling from the same height on
Earth.
C. The coin falls faster than the feather, but both take a longer time than if they were falling from the
same height on Earth.
D. The coin falls faster than the feather, but both take a shorter time than if they were falling from the
same height on Earth.
8. The figure shows a game in an amusement park. A passenger is fastened to a seat which is then raised to the
top of a vertical pole. At time t = 0, the seat is released from rest and falls freely. After 3 s, the seat is
brought to rest at a point P near the ground. Which of the following graphs best shows the relation between
the gravitational force acting on the passenger W and t?
3.2
Answers for supplementary exercise for Newton’s laws of motion
1.
(a)
Marks
Let
B be the blowing force,
T be tension of the tread,
W be the weight of the ball and
 be the angle between the tread and the vertical.
From the force diagram below,
B = T sin ……………………………………………...….(1)
W = T cos ………………………..……………………...(2)
(1)
 B = W tan 
(2)
When switch ‘1’ is pressed, blowing force B1 = W tan 28
When switch ‘2’ is pressed, blowing force B2 = W tan 56
(b)
MC
B2 W tan 56 

 2.79  2
B1 W tan 28 
Therefore, the girl should not conclude that the blowing force
becomes double.
(i) A heavier ball makes  smaller.
(ii) A larger area makes  bigger.
1-5 A B C D B
1A
1A
1A
1A
1A
1A
6-8 D A A
1. P corresponds to the moment the ball reaches the ground.
R corresponds to the moment the ball leaves the ground.
2. For (2), there are only weight of block and normal reaction from the inclined plane, there is no force
parallel to the incline.
3. Constant force means constant acceleration. So s-t graph should be a parabola, v-t graph should be a
straight line, a-t graph should be a horizontal line.
4. Since the block is traveling with constant speed,
Required force = mg sin  + friction = 9 x 10 sin 30o + 4 = 45 + 4 = 49 N
5. Vertical direction is in equilibrium, net force is zero.
4 T cos 20o = 600
T = 160 N
6. Accelerations in both cases are the same, with magnitude a given by
F = (m1 + m2) a
(the two blocks can be regarded as a whole system)
Note that T1 and T2 are internal forces.
So T1 = m1a ------(1)
T2 = m2 a. ------(2)
Since m1 > m2, so T1 > T2.
T1 + T2 = ( m1 + m2 ) a = F. Hence statement 3 is correct.
8. The gravitational force, i.e. the weight of the passenger remains unchanged during the fall of the machine.