Usually, Students struggle with the concept of Newton’s Third Law of Motion. This is not as simple as it looks to be. One must have a command on the very concept before one dive in the questions of past papers regarding Newtonian physics (dynamics).
(Sir Muhammad Abdullah Shah)
Generally, the 3rd law of newton is defined as to every action there is an equal and opposite reaction, or every action is an equal and opposite reaction. Now, this is often wrongly interpreted.
The entire concept of Newtons 3rd law revolves around 4 important points:
In Newtonian physics, forces always exist in a pair: action and reaction. They are usually termed as third law pair forces.
The pair of forces are always in opposite direction and of equal magnitude. They are always in the same lie of action.
This very pair of force acts on two different bodies.
And, the action and reaction pair of forces are always of the same type of force.
Let us now comprehend each of them separately.
What do action and reaction mean?
First of all, this action and reaction mean nothing but forces. This same law can also be written in a different way and in a more understandable meaning as this is a better explanation of Newton's 3rd law of motion. ‘’When a mass exerts a force on the other mass, the other mass exerts a force on the first mass which is equal in magnitude but opposite in direction.’’ For example. If a student exerts some force on wall, the student also feel some pain. Now what actually happens here is a system of two bodies. The wall and the student’s hand. So, when student’s hand hits the wall, it exerts a force on the wall; simultaneously, the wall exerts the same force on the student’s hand. This is why the student also feels pain. So, if the student hits the wall harder, the student would also experience a harder force. Remember, we also call the action and reaction forces as the Third Law Pair. One more thing, these pair of forces exist simultaneously, not in a way that one force causes the other. It would be unfair to say that force exerted by the student’s force existed first and then wall exerted the force. Action and reaction forces always exist simultaneously or at the same time.
The action and reaction forces of equal magnitudea must act in opposite directions
It is simple that the relationship of action and reaction is of equal magnitude but in opposite directions. They can’t be in the same direction.
The action and reaction forces acts on two different bodies, never on a same body.
The third law pair always act on two different objects. Have a look at the figure below :……. In figure a there are two forces acting on it; they are equal in magnitude and opposite in direction. For sure, they will cancel out each other. But, now consider figure b the two forces act on two separate masses. On one mass there is a force of 100 Newton acting in upward direction and on the other mass there is a force of 100 Newton acting in downward direction. There is no point that they will cancel out each other because these two forces are not on the same object. So action reaction can never cancel out each other.
The third law pair forces are always of the same type
Remember, the third pair forces (action and reaction) are always of the same type. If one is gravitational force, the other has to be a gravitational force as well. It can never be like one is a gravitational force and the other force is electrostatic. And this is where the tricky questions are mostly asked in the question papers.
Exam Question # 01: [AQA GCSE/Physics Specimen/2018/Q9(i)]
Figure 15 shows the forces acting on a child who is balancing on a pogo stick.
The child and pogo stick are not moving.
The downward force of the child on the spring is equal to the upward force of the spring on the child. This is an example of which one of Newton’s Laws of motion? [1 mark]
Tick one box.
First Law
Second Law
Third Law
Solution
Now, the question itself tells us about the forces. It says the one force acts by the child on the spring, which is in the downward direction, and the other force acts by the spring on the child in the upward direction. Let us check those 4 points one by one:
Is there a pair of forces? | Yes, the one force is exerted by the child on the spring and the other force is exerted by the spring on the child. |
Are the two forces equal in magnitude and opposite in directions? | Yes, both are equal in magnitude and the direction of the force exerted by the child on the spring is downwards whereas the force exerted by the spring on the child is in upward direction. |
Are these two forces acting two different masses? | Yes, the one is exerted on spring and the other on the child. a |
Re they of the same type? | Yes, they are the same type. They both are contact forces. |
So, now its is evident that the two forces are the third pair forces, and therefore this is a 3rd law of newton example. So, the answer is
Third Law
Exam Question # 02: [Edexcel IAL]
Complete the table below by giving one example of each type of force.
Type of force | Example |
Gravitational |
|
Electromagnetic |
|
Nuclear |
|
(3)
The diagram shows forces acting on the Sun and the Earth. These forces form a Newton’s third law pair.
State three properties of these forces which are necessary for them to be a Newton’s third law pair.
(3)
Solution
For its first part, one must have knowledge of Types of Forces.
Type of force | Example |
Gravitational | Weight or force of attraction between two masses |
Electromagnetic | Electrostatic force between the two stationary point masses |
Nuclear | The force between the protons which make them reside in the nucleus though they have alike charges |
In the second part, the examiner asks us about the three conditions necessary for the forces to be a Newton third law of pair. So, they are 4 in total, student may write any three of them.
The forces must be of equal magnitudes.
They must be in opposite directions.
The forces should act on two different or separate masses like in this case one force acts on the sun and the ither acts on the earth.
They both must be of the same type as in this case both forces are gravitational forces.
Exam Question # 03: [Edexcel IAL]
(a) Complete the following statement of Newton’s third law of motion.
“If body A exerts a force on body B, then body B … ‘’
(2)
(b) A man checks the weight of a bag of potatoes with a newton meter. Two of the forces acting are shown in the diagram
The table below gives these forces. For each force there is a corresponding force, the ‘Newton’s third law pair force’. In each case state
• the body that the Newton’s third law pair force acts upon
• the type of force (one has been done for you)
• the direction of the Newton’s third law pair force
Force | Body the Newton’s third law pair force acts upon | Type of force | Direction of the Newton’s third law pair force |
Weight of potatoes |
|
|
|
Push of ground on man |
| Normal contact force |
|
Solution
If body A exerts a force on body B, then body B would exert equal and opposite force on body A. (This is what third law of newton tells us.)
Force | Body the Newton’s third law pair force acts upon | Type of force | Direction of the Newton’s third law pair force |
Weight of potatoes | Earth (When potatoes’ weight acts on the bag of potatoes, so its reaction force – the other force of a pair – would act on earth.) | Gravitational force (Weight is example of gravitational force.) | Upwards (As we know that the pair of forces are always in opposite direction, so when weight acts downwards, the direction of reaction force should be upwards.) |
Push of ground on man | Ground (The reaction force would be the push of man on ground.) | Normal contact force | Downwards (Since both the forces in the pair are opposite in direction.) |
Exam Question # 04: [Edexcel IAL/U1/Jan 2019/Q5]
According to Newton’s third law, when two objects interact, they exert forces on
each other.
Which of the following statements is not a correct description of these forces?
A) The forces act at the same time.
B) The forces act in the same direction.
C) The forces act on different objects.
D) The forces have the same magnitude
Solution
The 4 assumptions that are considered while discussing Newton 3rd law of motions are as follow
The forces must be of equal magnitudes.
They must be in opposite directions.
The forces should act on two different or separate masses
They both must be of the same type
So, the option B is not a correct description of these forces. The forces must act in opposite directions.
Exam Question # 05: [Cambridge IAL 9709/P1/Jan 2016/Q12]
A box is shown resting on the ground. Newton’s third law implies that four forces of equal magnitude are involved. These forces are labelled P, Q, R and S.
Forces P and Q act on the box. Forces R and S act on the Earth.
For clarity, the forces are shown slightly separated
Which statement about the forces is correct?
A) P is the equal and opposite force to Q and both are forces of contact.
B) Q is the equal and opposite force to P and both are gravitational forces.
C) R is the equal and opposite force to S and both are forces of contact.
D) S is the equal and opposite force to Q and both are gravitational forces.
Solution
Let us first understand which force does each arrow represents.
P is the Normal Contact Force (the force exerted by the ground on the box). It is an example of contact force. The easiest way to know is the force a contact force or not is that this force only occurs at the point of contact and the very arrow of P is also been starting from the point of contact – the ground.
R is the force exerted by the ground on the box in reaction. So here P & Q are equal in magnitude, opposite in directions, and of the same type, meaning thereby they both are contact forces.
Q is the weight of the box, and weight is gravitational force. The arrow of weight always generates from the centre of gravity.
S is the force exerted by the earth on the box. It is also gravitational force. So, the forces Q & S are equal in magnitude and opposite in direction.
To sum up, P & R and Q & S are the third pair forces.
So, the correct option is D: S is the equal and opposite force to Q and both are gravitational forces.
The correct answer is B because:
Newton's third law of motion states that when two bodies interact, the forces they exert on each other are
Equal in size
Opposite in direction
The same type of force
The person exerts force Q on the trolley
The trolley exerts force R on the person
Therefore, Q and R are the third law pairs
This is answer B
Remember that for third law pairs, you are looking for a pair of objects exerting a force on one another that is equal in magnitude but opposite in direction, and of the same type
Force P is the normal contact force of the floor on the trolley
Force S is the total drag force of friction and air resistance on the trolley
Both of these forces are acting on the trolley so they cannot be third law pairs
