A book of weight W is at rest on a table. A student attempts to state Newton’s third law of motion by saying that ‘action equals reaction’.

Question 24

A book of weight W is at rest on a table. A student attempts to state Newton’s third law of motion by saying that ‘action equals reaction’.

If the weight of the book is the ‘action’ force, what is the ‘reaction’ force?

A the force W acting downwards on the Earth from the table

B the force W acting upwards on the book from the table

C the force W acting upwards on the Earth from the book

D the force W acting upwards on the table from the floor

Reference: Past Exam Paper – June 2017 Paper 12 Q9

Solution:

Answer: C.

For Newton’s third law, the two forces in each pair must be the same type of force and act on different bodies. (The forces should also be equal in magnitude and opposite in direction.) 

Same type of force: For example, the weight of the book is the gravitational force exerted by the Earth on the book while friction between the book and the table is a type of electric force between the atoms of the book and the table. So, weight and friction are 2 different kinds of forces; they are not of the same type and so cannot constitute the pair of forces referred to in Newton’s third law. 

When an object is resting on a table, the weight of the object is equal and opposite to the contact force from the table but this is also not an action and reaction pair as the forces are of different types. The weight of the object is a gravitational force and the support force is an electrical contact force. [B incorrect]

The equal and opposite reaction force to the weight of the object is the force the object pulls upwards on the Earth. [C is correct]

The other pair of equal and opposite forces is the force the table exerts upwards on the object and the force the object exerts downwards on the table. In each pair, the forces must act on different bodies.

A two-source interference experiment uses the apparatus shown. What is the main purpose of the single slit?

Question 35

A two-source interference experiment uses the apparatus shown.

What is the main purpose of the single slit?

A to make a narrow beam of light

B to make the same amplitude of light incident on each slit

C to provide coherent light

D to provide monochromatic light

Reference: Past Exam Paper – June 2018 Paper 12 Q29

Solution:

Answer: C.

The purpose of the single slit is to provide coherent light.

Coherent light have a constant phase difference.

Light is emitted in all directions (diverging) from the lamp. In the absence of the single slit, light from the lamp (diverging rays) would pass through the double slit, resulting in a variety of different phase differences at the screen.

Two forces, each of 10 N, act at a point P as shown in the diagram. The angle between the directions of the forces is 120°.

Question 16

Two forces, each of 10 N, act at a point P as shown in the diagram. The angle between the directions of the forces is 120°.

What is the magnitude of the resultant force?

A 5N                B 10N              C 17N              D 20N

Reference: Past Exam Paper – June 2003 Paper 1 Q3

Solution:

Answer: B. 

Since the angle on the right of the diagonal vector is 120° with the horizontal, the angle on the LEFT of the diagonal vector is 180° – 120° = 60°with the horizontal.

Consider the resultant vector.

Since the 2 vectors given are of the same magnitude, we can assume that the triangle formed when the resultant is formed will at least be an isosceles triangle – that is, the angle that these 2 vectors make with the resultant vector will have the same value. Let this angle be x.

Sum of angles in a triangle is 180°.

60° + x + x = 180°

Angle x = (180° – 60°) / 2 = 60°

Since all the angles are equal in the triangle, it is actually an equilateral triangle. So, the sides will have the same magnitude.

Thus, resultant vector = 10N

The question can also be tackled using sine or cosine rule (depending on the values available).