Balanced and Unbalanced Forces

Think Deeply

An object in space can remain in motion with no forces acting upon it. Explain why this is so.

In a game of tug-of-war, one team pulls the rope in one direction and the other team pulls the rope in the opposite direction. What happens if both teams pull with the same force? What happens when one team pulls with a stronger force than the other team?

When both teams use the same amount of force in opposite directions, the forces cancel each other out. They are balanced forces. When the forces acting on an object are balanced, there is no change in motion.

To win a game of tug-of-war, one team must pull with greater force. The forces acting on the rope are not equal. Forces that are not equal and do not cancel each other out are unbalanced forces.

When there are unbalanced forces acting on an object, there is always a change in motion.

The forces are unbalanced and there is a change in motion.

The forces are balanced and there is no change in motion.

Think Deeply

In small groups, describe all of the forces acting on the soccer ball in terms of strength and direction. Is the net force zero? How do the forces affect the motion of the ball?

Balanced Forces

Look at the picture of the boy holding the soccer ball. Why is the ball not in motion? The forces acting on an object have both strength and direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force. When the boy holds the ball and presses it against his side, all of the forces acting on the ball results in zero net force and there is no change in motion. A soccer ball on the ground is being pulled down by the force of gravity. At the same time, the ground is exerting a push force in an equal and opposite direction. The forces are balanced – there is zero net force – and the ball remains at rest.

Unbalanced Forces

Forces that do not sum to zero can cause changes in an object’s speed or direction. When the boy kicks the ball, he applies a force greater than the force of gravity. This creates an unbalanced force and sets the ball in motion.

When there are unbalanced forces acting on an object, a change in motion occurs. Unbalanced forces can cause an object to: • start moving • speed up • slow down • change direction • stop moving

AB Activities 11.14 – 11.15