Download and ShareWhen an object moves in a straight line, it’s moving in the same direction. So, any time an object turns, or follows a curved path, it is changing direction. Since velocity is a vector, any change in direction, such as traveling in a circular path, means a change in velocity. A change in velocity is an acceleration, and, based on Fnet=ma (Newton’s Second Law), if there’s acceleration, there must be an imbalance of forces, or a net force. The name we give Fnet when the imbalance of force results in circular motion is “centripetal force,” meaning “center-seeking, ” since it’s always directed toward the center of the circular path. This force imbalance is responsible for systems like orbiting planets, curving roller coasters, and vehicles rounding turns.
When a car goes around a curve, the friction of its tires provides the centripetal force that keeps the car on a curved path. Passengers and objects in the car will tend to continue in the direction they were traveling, so they will feel themselves slide across the seat until they are pressing against the door. That apparent force, making them seem to slide outwards, is not a real Newtonian force. It is just that the passengers are traveling in a straight line due to the absence of a net force, and the path of the car is curving, pushing the doors toward them. If there were no friction and no doors, the passengers would slide out of the car as it turns. The force from the doors and the friction from the seat provide the force to curve the path of the passengers. You’ve probably experienced this effect.
The name “centrifugal” force is given to the non-Newtonian “pseudo” force of being pushed toward the door when going around a curve and of being forced to the outside when going in a circle. This apparent force is quite real, but it’s not Newtonian since Newton’s laws were written for inertial reference frames - reference frames that are not accelerating. The road is an inertial reference frame; a car on a curving path is not. So, Newton’s laws do not specifically apply within a turning car. Pseudo forces like centrifugal forces are used to explain the behavior of objects within an accelerating reference frame. Seen from the road, though, there is no need for these pseudo forces. In that case, centripetal force is the force that makes an object path’s curve.
You’ve probably experienced the same effect when you feel “heavier” at the bottom of a hill and “lighter” when you’re rounding the top of a hill.
Without centripetal force, an object would move along a tangent in a straight line rather than continue along a curved path. An example would be swinging a mass on a string. As long as there’s tension on the string, the mass can swing along a curved pathway, but if the string breaks, the mass will fly off in a straight line.
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Classroom Demonstration: Bucket Swing
The teacher will need:
Note: This demonstration requires practice and experimentation. For the first several attempts, it’s best to be someplace where it’s actually ok if the water splashes out of the bucket!
For the classroom demonstration: Fill the bucket halfway with water. Hold the bucket by the handle and then swing overhead several times to demonstrate that the water doesn’t splash down even when it’s “upside-down.”
Discussion questions:
Jeff Williams, Astronaut, NASA: Hello, I’m Jeff Williams onboard the International Space Station, Expedition 22, and today I would like to talk to you about centripetal force, a big word, but I know you use it and take advantage of it every day. We’ve heard about acceleration, linear acceleration and angular acceleration. With angular acceleration there’s a force that’s produced. A force is required to produce an acceleration. That’s true in linear acceleration and angular acceleration.
Let’s say we were to take this object here. This is a special tool we use onboard the space station, actually during space walks. But it’s a heavy metal object and that’s what I really wanted to use. You can see I have it tied here with a string onto this bungee right here, and it’s just floating here in weightlessness. And you can see the string here is rather loose and it’s just kind of floating randomly. If I were to rotate this thing around the string, around this bungee, and let me do this, you can see that the string pulls taut and stays tight and the tool continues to rotate around the bungee. In fact, if you look really closely at the bungee, you can see that the bungee bends at the point that it’s rotating, and it bends towards the tool. And that is caused by the centripetal force due to the angular acceleration of the tool as it rotates around, keeping this string tight and keeping the rotation in a circular motion. It’s the same kind of force that applies to the rotation of planets around the Sun, or the Moon around the Earth, or the Space Station around the Earth.
Well here in space, in weightlessness, I have this bag of tea, and you can see it has bubbles in it, but they don’t rise to the top. In fact I’ll shake them up here, and you can see the bubbles spread out throughout the tea. Of course, we’re in weightlessness, we have the absence of gravity, and we know that bubbles rise for what reason? In the presence of gravity, the air is lighter than the liquid, so the air floats to the top. Gravity causes the liquid to go down and it actually pushes the bubbles up. There’s the bubbles spread throughout the tea, and if I rotate it, the bubbles coalesce to the center of the tea and eventually form a circle. Why do they do that? Because of the centripetal force spread throughout the tea, the liquid tea goes to the outside of the bag and it forces the air into the center because the air is less dense than the liquid, so the air goes to the center. I’ll try it one more time. See the air coalesce to the center and form a circle due to the centripetal force applied to this. Because it’s turning, it’s rotating, the liquid goes to the ends and the air ends up in the center of the liquid.
Okay, that’s enough, I’m just going to pull away from there. I’m going to take my piece of dental floss and I’m just going to use my dental floss to guide the water bubble. So let’s see here. What I’m going to do is I’m going to try and rotate the water bubble and we’ll see what happens. Actually, I’m going to rotate the water bubble this way because if I give you this angle, you’ll be able to see it change shape, and actually break into several parts. And you can see they’re no longer spheres. This one is rotating like this, and that’s due to the centripetal force in the water bubble.
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