Which of the following statements is true? a) an object that is accelerating is always changing direction. b) an object at rest has an instantaneous acceleration of zero. c) instantaneous acceleration is always changing. d) an object that is accelerating is always changing speed.

An object at rest has an instantaneous acceleration of zero is true.

Option B

Explanation:

As we all know, acceleration is given by a, and formula is F/m, where F is force applied and m is mass of an object. Considering Instaneneous acceleration, it means the force per unit mass body is feeling during infinitesimally small time. Let us say 1 microsecond, acceleration of the body during that span of time is known as instanteneous acceleration. Considering option A, it might not be always true, as consider a vehicle going in straight direction, speed is changing and not direction. Option D is not again true, as consider earth rotating around sun, direction is changing but not speed. However, as explained above, instantaneous acceleration is always constant, and hence, body at rest have instantaneous acceleration zero.

we need to know the time interval during which the force acts. impulse of force = momentum change