In this lesson, you will learn what is meant in physics by 'balanced forces', including how that applies to Newton's 1st Law, as well as some examples of balanced forces in real life. A short quiz will follow.
What are Balanced Forces?
A force is a push or pull on an object, measured in newtons (N). And if the forces are balanced, it means that in every direction, any pushes and pulls are balanced by another force in the opposite direction. So, for example, you could have an object like this one:
This box has balanced forces because it has two newtons pushing up balanced by two newtons pushing down, and five newtons left balanced by five newtons right. The forces in the x-direction are balanced, and the forces in the y-direction are balanced. If we did this in 3D, the forces in the z-direction would also have to be balanced.
But what do balanced forces mean? What's the difference between the motion of an object with balanced forces and an object with unbalanced forces?
Newton's 1st Law
Newton's 1st Law of Motion tells us what this all means. Newton's 1st Law says that an object in motion stays in motion at a constant velocity, and an object at rest stays at rest unless acted upon by an unbalanced force.
This means that if the forces on an object are balanced, its velocity will stay constant. If it has a velocity of zero, it will stay zero. If it's moving at 10 miles per hour north, it will keep moving at 10 miles per hour north. The only way to change its velocity is to apply an unbalanced force.
The Role of Friction
But hold on a minute. That doesn't seem to fit with everyday life. If you give a shopping cart a good push and let go, it doesn't keep going forever at a constant velocity. After a while it stops, and you're no longer touching it to apply an unbalanced force. So how does this work?
The problem with applying Newton's 1st Law on Earth is that there are a lot of forces people don't think about. If you give the shopping cart a push and let go, there is still an unbalanced force even after it leaves your hand. The ground has friction, and that friction applies a force to the shopping cart to slow it down to a stop. So the shopping cart isn't really an example of balanced forces.
Low Friction Examples
In space, there is less friction and air resistance to worry about. If an astronaut throws a ball in space, it will keep going, pretty much forever! Once the ball has left the astronaut's hand, there are no forces on the ball, so it will keep going at a constant velocity.
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Or think about an ice-skating rink; one little push can make you keep going all the way to the wall of the rink. That's because there isn't much friction, so the forces are more balanced than they would normally be.
An Example of Balanced Forces
Let's take a look at one more example: an airplane. An airplane has several forces acting on it: the force of gravity (weight) acting down, the lift created by the wings acting up, the air resistance (drag) trying to slow it down, and the forwards thrust force created by the engine. If you adjust your controls so that you're flying the plane at a constant velocity, then these forces must be balanced. The drag force is equal to the engine force, and the lift force is equal to the weight force.
Balanced forces are when each force applied to an object is cancelled out by another force pointing in the opposite direction. Newton's 1st Law of Motion tells us what this means. Newton's 1st Law says that an object in motion stays in motion at a constant velocity, and an object at rest stays at rest unless acted upon by an unbalanced force. So an object with balanced forces will move at a constant velocity (though that velocity could just be zero).
This concept might be hard to picture because we're used to objects seemingly stopping on their own, like when we push a shopping cart and let go, and it eventually stops. But the only reason it stops is because friction applies an unbalanced force. In space, it's easier to see that objects with balanced forces can keep moving at a constant velocity.
This lesson on balanced forces could potentially prepare you to:
Understand balanced forces in physics
Highlight the role of Newton's 1st Law of Motion in balanced forces
Recognize the way in which friction affects balanced forces
As we have learned in this lesson, balanced forces result in an object moving at a constant velocity or with no velocity at all. In either case the object's acceleration is zero. The only way this can happen is if all of the forces on the object are balanced, meaning that the forces in each direction cancel each other out. This situation is like putting $100 into the bank as a deposit and then withdrawing $100 from an ATM at the same time. The net change in value of your bank account is zero.
Let's work on some more problems dealing with balanced forces because with physics, practice makes perfect!
A dog that weighs 100 N is sitting in his bed. What is the normal force (upward force from the floor) on the dog if the dog's acceleration is zero?
A train experiences a net frictional force of 10,000 newtons. If it is moving at a constant 10 m/s, what force does the motor provide to the train?
Imagine your three friends are in a tug-of-war match after school. Two of your friends pull in the same direction with 25 N and 50 N, respectively. What must your other friend on the opposite side of the rope pull with to make sure the acceleration of all of them is zero?
You are sitting on a 40 N box. You weigh 150 N. What must the floor push up with (normal force) to ensure that your acceleration is exactly zero?
You are now in a tug-of-war match with your three friends, but you each have your own rope that is tied to a metal ring. You first friend pulls with 100 N eastward. Your second friend pulls with 100 N westward. Your third friend pulls with 75 N southward. With what force (amount of force and direction) do you have to pull with to make sure the metal ring stays stationary?
There are three basic forces acting on a rocket as it is launched: 1) its weight acting downward, 2) the friction forces acting downward and 3) the thrust of the rocket upward. If the rocket's acceleration upward is greater than zero, what can you say about these three forces?
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