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Forces: Balanced and Unbalanced

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  • 0:07 Forces Come in Pairs
  • 1:11 Forces Can Be Balanced
  • 3:41 Forces Can Be Unbalanced
  • 4:35 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Forces are needed to start or stop an object's motion, but can also be involved when an object is at rest or already traveling at constant velocity. In this video lesson, you'll identify the difference between balanced and unbalanced forces, understanding how they affect the movement of objects.

Forces Come in Pairs

In another lesson, we learned about force, which is a push or pull resulting from an interaction between objects. Because a force has both magnitude and direction, it is a vector quantity. The magnitude is the 'how much,' and the direction is the 'which way' of the force.

All things can exert forces, sometimes without even touching the other object it's interacting with! Friction, which is the force that acts on an object in the direction opposite to the motion, is the resistance you feel when you drag something across the floor. Weight is another force, but this one is the force due to gravity. You don't need to be touching the earth to be pulled down to it by gravity!

Regardless of whether or not the objects are touching, if there is an interaction, there will be at least one pair of forces in that interaction. The bag you're dragging across the floor exerts a force opposing the friction. When you push your accelerator pedal in your car, the pedal pushes right back on your foot. Even just standing on the ground, gravity pulls you down, and at the same time, the floor pushes you upward.

Forces Can Be Balanced

Often, the pair of forces are balanced, meaning the forces are equal in size and opposite in direction. When this happens, the object will maintain its state of motion. When you are standing in place on a surface, you are interacting with the ground, but you may not notice it because the forces in the interaction are balanced. Gravity is pulling you downward just as much as the floor pushes up on you.

Think about a large basket hanging from a string. If it's just hanging there without moving, the basket is also involved in an interaction and is experiencing balanced forces. The upward tension on the string has the same magnitude as the weight of the basket pulling down, but the forces are in opposite directions, so they essentially cancel each other out.

When an object remains in its state of motion, we say it is in mechanical equilibrium. This is when there is no change in an object's state of motion. To write this, we use the equation:

Mechanical equilibrium equation
force equation

The 'sigma' symbol means 'vector sum of,' and 'F' is the forces. What we're saying is that the vector sum of the forces is zero - because remember, force is a vector quantity, so it's the sum of the vectors.

Since equilibrium is a state of 'no change,' it's not limited to objects at rest. State of motion includes objects that are traveling in a straight line path, too. And, as long as the forces are balanced, that object will keep traveling in that straight line path.

Take a bowling ball for example. If you send a bowling ball rolling down the lane and it rolls at a constant velocity, it remains in equilibrium until it hits the pins. This is because the vector sum of the forces on the ball is still zero, even though it's moving!

Let's look at another example. An airplane flying at a constant velocity (so, flying at a constant speed and direction) is in equilibrium because the thrust of the propeller pushes it forward at the same magnitude as the opposing air resistance. Because the forces are acting in opposite directions but are equal in magnitude, the forces are balanced, and the moving object is in equilibrium - a state of no change.

To distinguish between the two, we specify equilibrium for stationary objects as static equilibrium, and equilibrium for moving objects as dynamic equilibrium. But remember, they are both mechanical equilibrium.

Forces Can Be Unbalanced

As you have probably experienced, forces are not always balanced. Unbalanced forces are when the forces are not equal in magnitude, which causes a change in the object's state of motion.

If your coffee table is at rest in your living room, you can get it moving by pushing on it, which results in an unbalanced force. That bowling ball from before? When it hits the pins, there's a new interaction with the pins and a resulting unbalanced force that both disrupts the ball from its path and knocks over the pins.

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