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### Course: Physics archive>Unit 4

Lesson 2: Centripetal forces

# Centripetal force problem solving

In this video David gives some problem solving strategies for centripetal force problems and explains many common misconceptions people have about centripetal forces. Created by David SantoPietro.

## Want to join the conversation?

• At , how does less Normal force cause you to be airborne? Shouldn't it be the other way round because the centripetal force is more therefore you'd be pulled more towards the centre of the circle?
• Good question. Less normal force does indeed mean that the net centripetal force increases. However, since the speed is increasing you need more centripetal force for the bike to go in the same radius circle. But the smallest the normal force can be is zero, so after that point, the centripetal force is maxed out at mg and any increase in speed will not be able to have a corresponding increase in centripetal force, so the bike will no longer be able to travel in a circle of that same radius (which means it loses contact).
• How many kind of forces are there in physics?
• There are only four fundamental forces: strong nuclear force, weak nuclear force, electromagnetic force, gravitational force (note: it could be considered to be three forces since electromagnetism and the weak nuclear force can be considered two sides of the same more encompassing electro-weak force). But in macroscopic scenarios, we call the various types of "contact forces" which are usually just specific manifestations of the electromagnetic force by different names: Tension, Normal force, elastic/spring force, friction, etc. are all mostly electromagnetic in nature (since bonds between atoms/molecules are due mainly to electromagnetism) but it is useful to classify them by different names.
• At , David says that the force of tension is pointing towards the centre. But in previous videos, David said that the force of tension is only a PULL, not a PUSH. But if the force of tension is pointing towards the centre, wouldn't that be contradictory?
• If the object that is moved around in a circle by a string, suddently lost the string, then the object would keep moving away from the center. This means that the string is pulling the object to stay in the circle. If this makes sense?
• If david said that the centrifugal force doesnt exist then why doesnt the ball plummet into the center if nothing is pushing it outwards?
• The ball plummets to the center because of the centripetal force if no other forces are there. However, if you add a force perpindicular to the centripetal force pulling inwards, the two forces create a circular movement of the ball.
• What I understand about centrifugal force is following -
From an inertial frame of reference we do not need to count the centrifugal force.But from a
non-inertial reference frame (in this case a rotating reference frame) we need to consider the centrifugal force.For example if I was on a merry-go-round and I observe and calculate acting forces on the merry-go-round, I have to consider centrifugal force(along with other real forces).But if I was standing next to the merry-go-round and calculate the acting forces on the merry -go -round ,I do not need to consider centrifugal force.I only need to consider real forces (centripetal force, tension force etc).

But centripetal force( observed from a inertial reference frame) and centrifugal force (observed from a non-inertial rotating reference frame) can be expressed through the same equation which is mv^2/r .

Am I right?
• Centrifugal force is what we call a "pseudo-force" - it doesn't actually exist, but many people believe it does. This misconception stems probably from how certain objects that are not part of (or attached to) the moving body experience an outwards pull.

For example, say you are driving in the passenger seat of a car. While the car is driving straight, you experience no abnormalities. However, when the car turns left or right, you lean in the opposite direction. Is this centrifugal force? The answer is no. What's happening is your body (which is not attached to the car) tries to follow Newton's 1st Law while the car is in circular motion - you are trying to keep moving straight but the car is impeding your efforts, dragging you to the side. There is no outwards "centrifugal" force involved - the car is pulling your body out of its straight path and taking your body (against your will) along the curve.

A situation that disproves centrifugal force would be a catapult. If centrifugal force DOES exist, then the payload from, let's say, a trebuchet* would first fly and curve higher into the sky, thus violating the law of gravity, a some time before beginning to fall. However, since this is not the case and the payload fired from any catapult travels a parabolic path that can be predicted given the initial velocity and angle of release, we must assume that centrifugal force DOES NOT exist.

Hope this helps.

*see http://www.real-world-physics-problems.com/trebuchet-physics.html
• Why do we feel slightly weightless when passing over the hill?If the normal force is less than weight then we should feel more weighted.Can we say that our weight adjusts itself equal to the Normal force?
• Feeling weightless is feeling the gravitational force, hence why the normal force was less than the gravitational. When we feel more weighed that's due to the combination of the forces. The gravitational force accelerates you down, but the normal force counteracts it. We feel the pressure the normal force causes. This is why, when the normal force decreases, we feel less weighed down, because of the decrease in pressure.
• After minute 12, when you're explaining the bike over the hill problem, what would be the centripetal force if you're only halfway up the hill? Would it still be the force of gravity? Would you break the force of gravity up into different vectors, choosing an angle from a perpendicular tangent, and proceed with trig? Thanks
• Yes. You would break gravity into two components: one pointing in the centripetal direction, and one pointing perpendicular to this direction (tangent to the hill). The component in the centripetal direction is your centripetal force.
• The convention we chose makes it feel like centripetal force is helping normal force to counteract gravity. Does that mean that the centripetal force points out from the hill, in the same direction as the normal force? It sounds weird that the "centripetal" force would do that.
• The centripetal force points toward the center of the circle. That's why it's called centripetal. Gravity IS the centripetal force at the top of the hill. Gravity pulls down, normal force pushes up. The normal force is less than the gravitational force because the hill is curving downward away from the bike, and the bike is accelerating downward. Downward is toward the center of the circle, at the moment the bike is atop the hill.
• I don't understand that thing that:

If the puts the gravity as the centripetal force, it means that he deals with the single instant when the bike is on the top of the hill(because only on the top it equals to gravity, on other leverages it should be a part of gravity, because gravity points downwards, not inside the circle)

But, how can then the magnitude of the normal force be NOT equal to the magnitude of the gravitational force, I think it does for a single instant, the instant he is dealing with, it certainly is lower than the gravity when the bike is on lower leverage(because it's not perpendicular to the surface), and it will get lower as the bike would move to that direction, but at that single instant, it MUST be equal to gravity, at the moment when the force of gravity is perpendicular to the surface.