When does ... ?

• A force acting on an object make its velocity increases without

changing its direction.

When the force is applied along the direction of motion of the object.

Explanation:

When the force is applied in the direction of the velocity vector of the object, then only the magnitude of the velocity changes and the direction of motion remains unchanged. For example is you push an already moving cart in the direction its moving, then you will only changed its speed and not its direction.