When an object is subjected to unbalanced forces, its motion changes. The net force, which is the vector sum of all the forces acting on the object, causes the object to accelerate. If the bigger force is in the opposite direction to the object's movement, the object will decelerate. This means that its speed will decrease.

For example, if a car is moving forward and the driver applies the brakes, the car will decelerate. The force of friction between the wheels and the road opposes the car's forward motion, and the net force acting on the car is in the opposite direction to its movement. As a result, the car slows down.

The rate at which an object decelerates depends on the magnitude of the unbalanced force and the mass of the object. The greater the unbalanced force, the greater the deceleration. The greater the mass of the object, the smaller the deceleration.

In the case of the car, the deceleration depends on the force of friction between the wheels and the road, and the mass of the car. If the road is slippery, the force of friction will be smaller, and the car will decelerate more slowly. If the car is heavier, it will decelerate more slowly.