Acceleration is caused by a change in velocity over time, as you correctly stated. But to understand what causes acceleration, we need to look at the factors that can influence changes in velocity.
Acceleration can occur due to several reasons:
Force: According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. When an unbalanced force is applied to an object, it can cause acceleration. For example, when you push a car, the car accelerates due to the force you exert on it.
Gravity: Gravity is a force that attracts objects toward each other. When an object is in the gravitational field of another massive object, such as the Earth, it experiences a force due to gravity. This force can cause acceleration. For example, when you drop an object, it accelerates downward due to the gravitational force pulling it.
Changes in direction: Acceleration can also occur when there is a change in the direction of motion, even if the speed remains constant. This is because acceleration is a vector quantity that takes into account both the magnitude and direction of velocity. So, if an object changes its direction, it experiences acceleration. For example, when you drive a car around a curve, even if your speed remains constant, you experience acceleration towards the center of the curve.
It's important to note that acceleration can be positive (when an object speeds up) or negative (when an object slows down or changes direction against its motion). In the case of negative acceleration (also called deceleration), it indicates a change in velocity in the opposite direction of the initial motion.