Question

If the mass is tripled to 15kg*3=45kg, what is the new change of motion

Answer 1

I assume that the particle with the mass $m_1 = 15kg$ is moving under the constant force $F$. Then, according to the second Newton's law, the acceleration of the particle is:

$$a_1 = \dfrac{F}{m_1}$$

If we triple the mass $m_2 = 3m_1 = 45kg$ and remain force the same, then the new acceleration will be:

$$a_2 = \dfrac{F}{m_2} = \dfrac{F}{3m_1} = \dfrac{a_1}{3}$$

Answer. The change of motion is the following: acceleration decreases by 3.