What causes an object to accelerate?

Hint :The rate at which an object's velocity changes with respect to time is called acceleration. Acceleration is a vector quantity since it has both a magnitude and a direction. The orientation of the net force acting on an object determines the orientation of its acceleration.

Complete Step By Step Answer:
Newton’s second law states that the force on an object is equal to its mass times its acceleration. If a force is applied to an object, it accelerates at a rate that can be measured using Newton's second Law of Dynamic which is given below:
 $ \overrightarrow {\text{a}} \,{\text{ = }}\,\dfrac{{\overrightarrow {\text{F}} }}{{\text{m}}} $
If a point or an object moves in a straight line faster or slower, it is said to be accelerated. The direction of motion on a circle is constantly changing, because even though the speed is constant, the motion is accelerated. Both effects lead to acceleration of all other types of motion.

Additional Information:
The main difference between velocity and acceleration is that the rate of displacement of a moving object over time is called velocity. The rate of change in velocity over time is called acceleration. Velocity is measured in metres per second whereas acceleration is measured in metres per second per second. Acceleration and Velocity are vector quantities.

Note :
An object can accelerate in one of three ways: by changing velocity, changing direction, or changing both velocity and direction. The limit of the ratio of the change in velocity during a given time interval to the time interval as the time interval approaches zero gives instantaneous acceleration (at a specific moment and location).