Question

A boy is wearing a shirt of mass 150 g. How much force is he exerting on the dress? Do not forget to state the direction.

Answer 1

To solve this problem, we need to use the second and third laws of Newton’s law of motion.
Second law of Newton’s law gives us the numerical definition of force i.e. Force = Mass x Acceleration
Third law states that the action and reaction are equal and opposite to each other.

Complete step by step solution:
The weight is a kind of force which acts on the body on Earth due to the gravitational force.
As per Newton’s second law, we have –
Force, $F = ma$
where m = mass and a = acceleration.
Weight, $W = mg$
where g = acceleration due to gravity. The acceleration due to gravity is the acceleration with which the body undergoes free fall towards the Earth’s surface due to the influence of gravity. The value of $g = 9 \cdot 81m{s^{ - 2}}$
Consider the case of a boy wearing a shirt of mass 150g as shown.

The shirt exerts weight in the downward direction due to the gravitational force acting on the shirt.
The third law of Newton’s laws of motion states that, whenever there is action, there is an equal and opposite reaction.
We can see in the figure that a reaction force is exerted by the boy in reaction to the downward direction of weight.
Hence,
$R = W$
$R = mg$
Given,
Mass of the shirt, m = 150g = 0.15 kg
Acceleration due to gravity, $g = 9 \cdot 81m{s^{ - 2}}$
Hence,
Reaction, $R = 0 \cdot 15 \times 9 \cdot 81 = 1 \cdot 47N$

Therefore, the reaction force exerted by the boy to the shirt is equal to 1.47 N and is in the upward direction.

Note: The exact value of acceleration due to gravity, $g = 9 \cdot 80665m{s^{ - 2}}$. Sometimes, it is rounded off to 10, in some smaller level calculations. However, unless mentioned in the problem, the value of g should not be assumed to be 10 since it can cause precision error. It is safe to consider the value of g up to 2 decimal places i.e. 9.81.