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Unbalanced forces are forces that cause a change in the motion of an object. Learn more about how to identify balanced and unbalanced forces through several examples and test your knowledge with quiz questions.

Unbalanced Forces Defined

Any push or pull is a force.

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To describe a force, you must know two things. You must know the size of the force and the direction of the force. Suppose two teams are playing tug of war. Each team is pulling with equal force, but in opposite directions. Neither team can make the other team move. Forces that are equal in size but opposite in direction are called balanced forces.

Balanced forces do not cause a change in motion. When balanced forces act on an object at rest, the object will not move. If you push against a wall, the wall pushes back with an equal but opposite force. Neither you nor the wall will move. Forces that cause a change in the motion of an object are unbalanced forces.

If two forces are in opposite directions, then the net force is the difference between the two forces, and it is in the direction of the larger force.

Consider two dogs playing tug of war with a short piece of rope. Each is exerting a force, but in opposite directions.Notice below that the dog on the left is pulling with a force of 10 N, and the dog on the right is pulling with a force of 12 N. Which dog do you think will win the tug of war? Because the forces are in opposite directions, the net force is determined by subtracting the smaller force from the larger one. In this case, the net force is 2 N in the direction of the dog on the right. Give that dog a dog biscuit!

Unbalanced Forces in Action

Unbalanced forces can change the motion of an object in two ways.

When unbalanced forces act on an object at rest, the object will move. In the two examples mentioned earlier, the net force on the object is greater than zero. Unbalanced forces produced change in motion (acceleration) and the receivers of the forces – the piano and the rope – moved.

Unbalanced forces are necessary to cause a nonmoving object to start moving.Second, when unbalanced forces act on a moving object, the velocity of the object will change. Remember that a change in velocity means a change in speed, direction or both speed and direction.

For example, consider a soccer game. The soccer ball is already moving when it is passed from one player to another. When the ball reaches the second player, the player exerts an unbalanced force – a kick – on the ball. After the kick, the ball moves in a new direction and with a new speed.

Examples of Unbalanced Forces

A force can act on an object without causing it to accelerate if other forces cancel the push or pull of the force.

Look at this figure:

If you and your friend push on a door with the same force in opposite directions, the door does not move. Because you both exert forces of the same size in opposite directions of the door, the two forces cancel each other.Two or more forces exerted on an object are balanced if their effects cancel each other, and they do not cause a change in the object’s motion.

If the forces on an object are balanced, the net force is zero. If the forces are unbalanced forces, the effects don’t cancel each other. Any time the forces acting on an object are unbalanced, the net force is not zero, and the motion of the object changes.The figure below demonstrates that horizontal force acting on a refrigerator is F. If F is the only force acting on the refrigerator, then it will accelerate horizontally (a). The free-body diagram shows the net horizontal force acting on the refrigerator (b).

This figure shows that when two people apply equal, oppositely directed forces to a refrigerator, these two forces cancel each other, and the net force equals zero. This means there cannot be any horizontal acceleration (a).

If you stand on a skateboard and someone gives you a push, then you and your skateboard will start moving. You began to move when the force was applied. An object at rest – like you on your skateboard – remains at rest unless an unbalanced force acts on it and causes it to move.Because a force had to be applied to make you move when you and your skateboard were at rest, you might think that a force has to be applied continually to keep an object moving. Surprisingly, this is not the case. An object can be moving even if the net force acting on it is zero.

A soccer ball, for example, receives an unbalanced force when it is kicked. However, the ball continues to roll along the ground long after the force of the kick has ended.Sometimes two or more forces can act on one mass at the same time. Such combinations of forces can balance each other. So forces do not always cause motion by pushing or pulling. If the forces are balanced, the object remains at rest.

You can see an example of such balance in this figure. Buoyant and gravitational forces are shown opposing one another. The object in this case remains at rest.

If two forces act on a mass at the same time but in opposite directions, the motion will be in the direction of the larger force if the forces are unequal, as shown in this figure:

Friction is a force that acts in a direction opposite to motion. If there were not force of friction, a baseball player would never stop and would be continuously moving in the direction of motion.Balanced forces do not cause a nonmoving object to start moving. Furthermore, balanced forces will not cause a change in the motion of a moving object. Many objects around you have only balanced forces acting on them.

For example, a light hanging from the ceiling does not move because the force of gravity pulling down on the light is balanced by an elastic force due to tension that pulls the light up. A bird’s nest in a tree and a hat resting on your head are also examples of objects with only balanced forces acting on them. This figure shows another case where the forces on an object are balanced.

Because all the forces are balanced, the house of cards does not move.

Lesson Summary

Forces that are equal in size but opposite in direction are called balanced forces.

Balanced forces do not cause a change in the motion of objects. Forces that cause a change in motion of objects are called unbalanced forces. Unbalanced forces can change the motion of an object in two ways.

Learning Outcomes

Once you’ve completed this lesson, you’ll be able to:

• Differentiate between balanced and unbalanced forces
• Explain how unbalanced forces change the motion of an object
• Describe examples of unbalanced and balanced forces
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