Spring scale reads 20N as it pulls a 4kgs object across a table. What is the magnitude of the force exerted by an object on the spring scale?
A. 40N
B. 20N
C. 4N
D. 5N

Hint: We know that the force applied is given by the product of mass of the object and acceleration. There are many daily life examples which relate the laws of motion. Here, the third law of motion will help us to find the answer to the above question.

Complete step by step answer:
As we know, there are three equations of motion. According to Newton's third law of motion every action has its equal and opposite reaction.
So, from the third law of motion we can say that the object exerts the same amount of force on the spring scale as force exerted by the spring scale on the object i.e., 20N.

So, the correct answer is “Option B”.

Additional Information: Force is simply any push or pull. The S.I unit of force is Newton represented by N. The acceleration is defined as the increase in the velocity of an object. The acceleration is measured in $m/s^2$.
There are three laws of motions given by Newton. These laws of motion relate an object's motion to the forces acting on it.
First law of motion states that an object continues to be in rest or in motion in a particular direction until and unless any external force is applied on it.
 In the second law of motion, the force on an object is equal to its mass times its acceleration. This law also gives the relation of momentum and force.
In the third law of motion, every action has an equal and opposite reaction.

Note: Force applied on an object changes its motion, speed, direction and also its shape. There is a very small difference between the force applied and the pressure applied on an object. Pressure is always given as the force per unit area.