9702_s22_qp_12 Question 8

A child of mass 20 kg stands on the rough surface of a sledge of mass 40 kg. The sledge can slide on a horizontal frictionless surface. 

One end of a rope is attached to the sledge. The rope passes around a fixed frictionless pole, and the other end of the rope is held by the child, as shown. 

The rope is horizontal. The child pulls on the rope with a horizontal force of 12 N. This causes the child and the sledge to move with equal acceleration towards the pole. 

What is the frictional force between the child and the sledge? 

A 4.0 N 

B 6.0 N 

C 8.0 N 

D 12 N  

ANSWER: A

The Physics Behind

• From the statement/given information "This causes the child and the sledge to move with equal acceleration towards the pole.", the following should be satisfied:
• For the sledge

a = F_s / m_T

where  F_s = 12 N + 12 N and m_T = 60 kg

F_s is the resultant force on sledge

m_T is the total mass

• For the child

a = F_c  / m_c

where F_c = 12 - f and m_c = 20 kg

F_c  is the resultant force on the child

m_c is the mass of the child

f is the frictional force

• The accelerations are equal so

0.40 N kg-1 = (12 N - f) / 20 kg

8.0 N = 12 N - f

f = 8.0 N - 12 N

f = 4.0 N

9702_s22_qp_12 Question 7

Which statement is not a requirement of a pair of forces that obey Newton’s third law of motion? 

A The forces act in opposite directions. 

B The forces act on different objects. 

C The forces act on objects in contact. 

D The forces are of equal magnitude.  

ANSWER: C

The Physics Behind

• The law applies in all cases where there interaction is present, may it be by contact or not.
• Non-contact interaction are those we see with masses, with charges, and magnets/moving charges.

9702_s22_qp_11 Question 12

When must an object be in equilibrium? 

A when no resultant force acts on the object 

B when no resultant force and no resultant torque act on the object 

C when no resultant torque acts on the object 

D when the upward force on the object is equal and opposite to its weight  

ANSWER: B

The Physics Behind

• Equilibrium is meet only after 2 conditions are satisfied.
• The sum of all forces on the object is zero.
• The sum of clockwise moments equals the sum of the anti-clockwise moments.
• Option B has statement / phrases equivalent to the above.

9702_s22_qp_11 Question 7

An object is moving along the ground in a straight line at a constant speed. 

Which statement about the resultant force acting on the object is correct? 

A The resultant force acting on the object is equal to its weight. 

B The resultant force acting on the object is equal to the product of its mass and its velocity. 

C The resultant force acting on the object is equal to the resistive force. 

D The resultant force acting on the object is equal to zero.

ANSWER: D

The Physics Behind 

• Newton's 2nd Law of Motion [or the Law of Acceleration] states that an object's acceleration is directly proportional to the resultant force on it when mass is unchanged.
• A movement along a straight line at constant speed means that the acceleration is zero.
• Therefore, by virtue of the 2nd Law, resultant force on the object must also be zero.