What causes an object to slow down or speed up?

Unbalanced forces

Explanation:

Unbalanced forces acting on an object can change the object's speed, causing it to speed up or slow down.

An object can slow down or speed up depending on the external force that is applied. But it must be unbalanced and not equal in order to move the object, doesn't concern if it has to decrease or increase in speed.

In order to slow down an object, friction often works here. Friction is a force that occurs when two surfaces slide past  one another.  The force of friction opposes the motion of an object,  causing moving objects to lose energy and slow down.  When objects move through a fluid, such as air or water, the  fluid exerts a frictional force on the moving object. For example: tires skidding to a stop on a road,  sandpaper rubbing against wood, air pushing against the  nose of an airplane.

When objects fall to the ground, gravity causes them to accelerate. Acceleration is a change in velocity, and velocity, in turn, is a measure of the speed and direction of motion. Gravity causes an object to fall toward the ground at a faster and faster velocity the longer the object falls. This works to speed up when something falls. When moving in horizontal direction, forces make things speed up/accelerate. When a force pushes or pulls the object, the object will move in the direction of the force. The bigger the force, and the lighter the object, the greater the acceleration.

And sorry for making it too long! But I hope I could make it clear for you! ^^

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