Question

The strength of gravity on Mars is only 40% of that of earth. If a child has a mass of 60kg on earth, what would the child’s weight be on Mars?
A. 24N
B. 60N
C. 240N
D. 600N
E. None of the above is correct within 10%

Answer 1

Hint: Weight and mass are quite different. Mass is the property of the body, meanwhile, weight is the force by which another body attracts a body. As gravity changes the force by which it attracts the body also changes.

Formula used:
\[W=m\cdot g\]

Complete step by step answer:
In our daily lives the terms Weight and Mass are often used interchangeably but in terms of physics, both these quantities are quite different. Mass is the property of the body and it does not depend upon the external factor. Meanwhile, weight is the force by which another body attracts a body. It is given by
\[W=m\cdot g\]
Where W is the weight of the body, m is the mass and g is the acceleration due to gravity of the planet.

Given: the mass of the child is 60kg.
So, \[m=60kg\]
It is also given that the gravity of mars is 40% of that of earth. So,
 \[\begin{align}
  & g=10\dfrac{m}{{{s}^{2}}} \\
 & \Rightarrow {{g}_{m}}=0.4g \\
 & \Rightarrow {{g}_{m}}=0.4\times 10\dfrac{m}{{{s}^{2}}} = 4 m/s^2\\
\end{align}\]
Where g is the acceleration due to gravity of the earth and \[{{g}_{m}}\] is the acceleration due to gravity of mars.
We already know that
\[W=m\cdot g\]
So, \[{{W}_{m}}=m\cdot {{g}_{m}}\] where \[{{W}_{m}}\] is the weight of the child on mars
Plugging in all the calculated variables
\[\begin{align}
 & \Rightarrow {{W}_{m}}=m\cdot {{g}_{m}} \\
 & \Rightarrow {{W}_{m}}=60\cdot 4 \\
 & \Rightarrow {{W}_{m}}=240N \\
\end{align}\]
Thus, the weight of the child on mars would be 240N.

So, the correct answer is “Option C”.

Note:
The confusion between mass and weight is exploited in questions. Statements are made purposely vague. It is important to identify and apply the terms correctly. The units can be an adequate tool for the purpose. Unit of mass is kg while weight is expressed in N and thus these quantities can easily be differentiated.