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Quiz 2-7 (Higher tier)
/ 14
(Book 2 Chapter 7)
A Multiple-choice questions (4 marks)
1
Object X collides head on with object Y on a smooth horizontal plane. Which of the
following statements must be correct?
(1) The change in momentum of X has the same magnitude as that of Y in the collision.
(2) The change in velocity of X has the same magnitude as that of Y in the collision.
(3) The force acting on X has the same magnitude as that acting on Y in the collision.
A (1) only
B (1) and (3) only
C (2) and (3) only
D (1), (2) and (3)
2
The figure below shows how the momentum of an object changes with time. Find the
average net force acting on the object during this period.
momentum / kg m s–1
4
0
5
time / s
A 0.4 N
B 0.8 N
C 2.6 N
D 13 N
B
Short questions (10 marks)
3
Two blocks X and Y move together at 4 m s1 towards the left on a smooth horizontal surface
(see the ext page). A spring plunger in X is triggered and the blocks are pushed apart as
shown. Afterwards, Y travels at 2 m s–1 towards the left. The mass of X is 2 kg and the mass
of Y is 1 kg.
New Senior Secondary Physics at Work (Second Edition)
Oxford University Press 2015
1
Chapter Quiz 2-7 (Higher tier)
before triggering
after triggering
4 m s1
X
Y
v=?
2 m s1
X
Y
smooth horizontal
smooth horizontal
surface
surface
(a) Explain whether
the total momentum of the blocks is conserved.
(2 marks)
_______________________________________________________________________
_______________________________________________________________________
(b) Find the elastic potential energy stored in the spring plunger before it is triggered.
Assume that there is no energy loss.
(4 marks)
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
4
A soft plastic ball of mass 0.1 kg is dropped from a height onto a force sensor. The figure
below shows the forcetime graph recorded by the force sensor.
force / N
shaded area = 0.313 N s
6.26
0.15
time / s
(a) Estimate the average force acting on the ball by the force sensor in the impact. (1 mark)
_______________________________________________________________________
(b) Estimate the change in momentum of the ball in the impact.
(3 marks)
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
End of quiz
New Senior Secondary Physics at Work (Second Edition)
Oxford University Press 2015
2
Chapter Quiz 2-7 (Higher tier)
Solutions to Quiz 2-7 (Higher tier)
1
B
(1): By conservation of momentum,
mXuX + mYuY = mXvX + mYvY
(mXvX mXuX) = mYvY mYuY
pX = pY
(2): (mXvX mXuX) = mYvY mYuY
mX vX = mY vY
If mX mY, then vX vy.
(3): During the collision, X exerts a force on Y and vice versa. The two forces form an
action-and-reaction pair.
2
A
Net force =
mv mu 4 2
=
= 0.4 N
t
5
3
(a)
No external net force acts on the blocks
1A
so the total momentum is conserved.
1A
(b) Take the direction towards the left as positive.
(mX + mY)u = mXvX + mYvY
1M
(2 + 1) × 4 = 2v + 1 2
v = 5 m s1
1A
Elastic PE = final total KE initial total KE
=
1
1
1
mXvX2 + mYvY2 (mX + mY) u2
2
2
2
=
1
1
1
2 52 + 1 22 (2 + 1) 42
2
2
2
=3J
1M
1A
4
shaded area 0.313
=
= 2.09 N
0.15
time
(a)
Average force =
(b)
Let R be the average normal reaction acting on the ball by the ground
1A
during the impact. Take the upward direction as positive.
Change in momentum = average net force time of impact
1M
= (R – mg)t
1M
= Rt – mgt
= 0.313 0.1 9.81 0.15
= 0.166 kg m s1
New Senior Secondary Physics at Work (Second Edition)
Oxford University Press 2015
1A
3
Chapter Quiz 2-7 (Higher tier)
0
