A car is moving forward and braking. Ask ten people which way it is accelerating, and most will say forward, because that is the way it is moving. The car's acceleration points backward.
The equation a = F_net/m relates two vectors, and vectors carry directions. The direction rule is short, and it is not about the direction of motion.
An object's acceleration points in the same direction as the net force on it.
The direction of motion does not enter the rule. The velocity can point one way while the acceleration points another.
The braking car: the motion points forward, but braking friction makes the net force point backward. So the acceleration points backward, and the car slows.
A subway train moving north brakes, and braking friction from the rails is the only horizontal force on it. Which direction is the train's acceleration?
A car speeding up: the net force points forward, the same way as the motion, and the acceleration points forward too.
A ball just hit straight upward: the motion points up, but the ball's weight makes the net force point down. The acceleration points down the whole way up.
A volleyball is rising straight up, just after the hit. Ignoring drag, which direction is its acceleration?
So to find an acceleration's direction, find the net force's direction. The two always match.
Arlo says: an object can be moving forward while its acceleration points backward. Is he correct?
One naming note before you practice. A wind is named by the side it comes from: a wind from the west moves toward the east. Suppose a wind from the west reaches a ball rolling south. The wind's force on the ball points east.
A tour boat moving east cuts its engine, and water resistance is the only horizontal force left on it. Which direction is its acceleration?
The boat moves east, so the water resistance on it points west.
With the engine off, that resistance is the net force: the net force points west.
The acceleration points in the same direction as the net force.
The boat's acceleration points west, opposite its motion: the boat slows.
A puck slides east over frictionless ice. A steady crosswind starts, exerting the only horizontal force on it, perpendicular to its motion and blowing from its south side. Which direction is the puck's acceleration?
Which statement about an object's acceleration is correct?
An elevator moving upward nears its floor, and during these moments the cable's upward tension on it is smaller than its weight. Which direction is the elevator's acceleration?