Answer to Question #236450 in Mechanics | Relativity for danj

Question

Can the velocity of an object be negative when its acceleration is positive? What about vice versa? If yes, give examples in each case.

Answer 1

Explanations & Answer

• Both acceleration & velocity are vector quantities that require a direction in addition to its magnitude. Therefore, when describing them a reference direction is needed (Which side/direction is taken to be positive hence the other is negative)

• When an object is thrown vertically upward its velocity is directed upwards & the acceleration it is subjected to is directed downwards.
• If one defined the upward direction to be positive, then the velocity, in this case, is positive & the acceleration is negative. On the other hand where the upward is taken to be negative(not the normal usage in a vertically upward motion) the velocity is negative & the acceleration is positive.

• So the conclusion is velocity & acceleration can be of different directions & the sign of them depends on how the reference is defined.

Examples-

1. During an escalator comes to a stop.
2. Penetration of a coconut in the sand fallen from a nearby tree.
3. During the renounce of a ball.