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. 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.
tug of war balanced
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.
tug of war unbalanced 1
Unbalanced forces are not equal and opposite. Suppose that one of the teams in tug of war pulls harder than the other team. The forces would no longer be equal. One team would be able to pull the other team in the direction of the larger force.
Force and Motion
More than one force can act on an object at the same time. If you hold a paper clip near a magnet, you, the magnet and gravity all exert forces on the paper clip. The combination of all the forces acting on an object is the net force. When more than one force is acting on an object, the net force determines the motion of the object. In this example, the paper clip is not moving, so the net force is zero.
How do forces combine to form the net force? If the forces are in the same direction, they add together to form the net force. Suppose you and a friend are asked to move a piano for the music teacher. To do this, you pull on one end of the piano, and your friend pushes on the other end. Together, your forces add up to enough force to move the piano. This is because your forces are in the same direction. Because the forces are in the same direction, they can be added together to determine the net force. In this case, the net force is 45 N, which is plenty to move a piano – if it is on wheels, that is!
net force piano
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!
tug of war dogs
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.
A kick on the ball creates a new direction and speed on the ball.
image of foot kicking soccer ball
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:
Two forces are canceling out, so the door does not move.
image of two people pushing on door in opposite directions
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.
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