Question

The tension of a string is 18 N. What is the mass of the hanging object?

Answer 1

According to the third Newton's law, the tension in the string is equal to the weight of the object. If the object is in equilibrium, the tension of a string is equal to the gravity force that acts on the object:

$$T = mg$$

where $m$ is the mass of the object, and $g = 9.8 N/kg$ is the gravitational acceleration. Since $T= 18N$, obtain:

$$m =\dfrac{T}{g} = \dfrac{18N}{9.8N/kg}\approx 1.8kg$$

Answer. 1.8 kg.