# Tag Info

62

Summary Centrifugal force and Coriolis force exist only within a rotating frame of reference and their purpose is to "make Newtonian mechanics work" in such a reference. So your teacher is correct; according to Newtonian mechanics, centrifugal force truly doesn't exist. There is a reason why you can still define and use it, though. For this reason, your ...

60

Actually, this is rather insightful. The normal force from the ground does not quite cancel out the effect of gravity. The difference between them is precisely the centripetal force that keeps you rotating around with the Earth's surface. Of course, you won't notice this because the centripetal force is so small compared to the gravitational force on you. ...

58

In the lab frame of reference, you need to reverse the question - don't ask yourself what pulls the particles apart but what keeps them together. By Newton's laws, everything on which no force acts keeps travelling in a straight line. So what requires explanation is not that a collection of moving particles - such as a rotating flywheel - flies apart but ...

56

The water does fall. It just doesn't fall faster than the bucket. By pulling on the bucket, you keep it around the (also falling) water.

55

It is a common misconception that objects have to move in the direction of the force. This is false; the acceleration points in the direction of the force. This means the change in velocity points in the direction of the force. It is not the velocity that points in the direction of the force. At the top of the circle the water is definitely pushed down by ...

47

Put a stationary astronaut in a small room inside a large spinning cylinder. After an instant walls of that room will hit him, and suddenly he will have the same velocity as the room. Due to angular motion, the room accelerates towards the axis of the cylinder. Subsequently, through the support force from the floor (the floor is at the surface of the ...

46

The trick is, centrifugal force is a fictitious force. Centrifugal force exists! To everyone denying it, do this to them: xkcd.com/123. However it is a fictitious force. To quote wikipedia: A fictitious force is an apparent force that acts on all masses whose motion is described using a non-inertial frame of reference, such as a rotating reference frame. ...

41

This is a common misinterpretation of Newton's third law, often stated as "to every action, there's an equal and opposite reaction." As you surmise, "action" and "reaction" refer to forces. However, they refer to forces acting on different things. Otherwise, nothing could accelerate, ever: if every force were always canceled out by an equal and opposite ...

37

You are correct in that if the astronaut is undergoing no translational or rotational motion relative to the centre of rotation of the space station the astronaut will feel weightless as in diagram $A$ and will not touch the space station. This is equivalent to jumping onto a rotating turntable with no friction acting. That feeling of weightlessness is due ...

34

If the Sun were to magically disappear then the Earth would fly off at a tangent to its orbit. The trajectory would look like this: The green dot shows the position of the Earth at the instant the Sun disappears. The distance from the Sun, $d$, is the Earth's orbital distance and the velocity $v$ is the Earth's orbital velocity. When the Sun disappears the ...

33

Suppose you are at a red light in your car. You apply Newton's second law on the street light. $$F=ma$$ $$F=0N, a=0ms^{-2}$$$$0N=0N$$ It works!! Now the light turns green and you start accelerating. Suppose your acceleration is $1ms^{-2}$. According to you, you are at rest. Do you see your nose moving? Apparently not. It means your body is at rest wrt you. ...

32

We have the ball orbiting at a distance $R$ from the centre of rotation and the string inclined at angle $\theta$ with respect to the horizontal. Two main forces act on the ball: gravity $mg$ ($m$ is the mass of the ball, $g$ the Earth's gravitational acceleration) and $F_c$, the centripetal force needed to keep the ball spinning at constant rate. $F_c$ is ...

32

The statement: The Gravitational Pull is counterbalanced by the Centripetal Force is rubbish. The satellite undergoes a centripetal acceleration because it is acted on by the gravitation force. Some people call the force which causes a centripetal acceleration the centripetal force. So in such a case the gravitational force and the centripetal force are ...

23

I'm not fond of calling centrifugal force an "imaginary" force. I prefer to explain it in terms of reference frames: In an inertial reference frame, Netwon's laws are sufficient to model (non-relativistic) motion. In a non-inertial reference frame, additional forces (like centrifugal, Coriolis, etc.) must be added to the model. What makes centrifugal ...

22

Try to imagine, instead of a big stone, a big plate. On top, you fill the plate with sand. Now you start spinning the plate. What's going to happen to the sand? The sand is going to leave the plate very fast, and spill in all directions. This is the basic, natural state of things, and it's from here that you should start questioning. How do we keep the ...

21

If there is no atmosphere, and the station is a relatively smooth cylinder, you can indeed float there as the exterior walls spin around you (in the middle, or just above a wall, or anywhere). Now, suppose you start drifting towards a wall (maybe you threw your shoe the other way). You move towards the wall, but do not accelerate due to the rotation of the ...

21

Your confusion lies in the way you configured the problem. Let two charged particles revolve around the center of the system. It's quite clear in that viewpoint that any change in linear momentum of one particle is matched by a corresponding change of linear momentum in the second particle. Thus the linear momentum of the whole system remains constant. ...

20

Great photo! Edit: My language is "sloppy" (I like talking physics in "lay person" terms so anybody can understand) but @dcmkee made really nice comment clarifying my answer for the more advanced people. Thanks @dcmkee! Since the plane is in a loop there is significant g's due to centripetal acceleration. The water was being accelerated upward$^{1}$ with ...

20

If you were in a zero-gravity environment (e.g., in earth orbit or in a coasting trajectory en route to Mars), alone in your space suit, you would feel no gravity. If a big pipe were placed around you in your zero-g environment, you would still feel no gravity. The pipe would have no effect on you at all. If the pipe were spun with you inside it but not ...

18

In the case that you describe, an individual swinging a mass horizontally on the end of a string, the string does not run directly to the centre of rotation. Instead, it runs to your hand, which in turn is moving in a circle about its centre of rotation . Sometimes the arm is involved, sometimes only a rotation at the wrist. ( Mime winding up a sling to ...

17

The centripetal force can be made up of any type of force, whether gravitational, friction or tension. The centripetal force is not a force type, it is just a net force that is always radial. So it is a sum of forces, no matter the type. So yes, it is a tension force. It just acts as a centripetal force.

16

You say If there is no centripetal force But that is not true. When you measure your weight at the North Pole or at the Equator, you get a different answer. The shape of the Earth (a slightly flattened sphere) is part of the reason*); but the rotation of the earth (which incidentally causes the flattening) also plays a role. At 24 hr/revolution, and a ...

16

No, the moon will not eventually fall to earth. Reducing the tangential velocity by a small amount will affect the orbital trajectory of the moon. Since the path followed by the moon is already elliptical ($e=0.00549$), the actual affect depends on where the tangential velocity is reduced. If reduced at the apogee, the orbit will become more elliptical but ...

15

The proper derivation of the centripetal acceleration—without assuming any kinematic variables are constant—requires a solid understanding of both the stationary Cartesian unit vectors $\hat{i}$ and $\hat{j}$ as well as the rotating polar unit vectors $\hat{e}_r$ and $\hat{e}_\theta$. The Cartesian unit vectors $\hat{i}$ and $\hat{j}$ are stationary and ...

14

There are several interesting things going on when a car turns. First - let's take the simple diagram of two front wheels turned by 45°: As you can see, the top tire would like the car to turn around the point $C_1$, but the bottom tire (at the same angle) wants to turn around $C_2$. This means that in reality both tires will experience some lateral slip. ...

14

You don't have to explain this by centrifugal force, or any fictitious force at all. All what centrifugal force is about is inertia. As your stone is spinning, it has some velocity. But since initially there's a centripetal force, this velocity constantly changes towards the center of rotation. When part of the stone breaks off, it's no longer held by ...

13

It doesn't actually have anything to do with the plane being upside down, or even changing from a vertical direction to a horizontal one. It's purely the vertical velocity that's at play here. Imagine water being thrown upward. You know what, imagine a fountain, a really big fountain. As soon as the water leaves the underground pump, it starts falling back ...

13

The force you feel when you round a corner in your car is the friction force of the car seat on your behind, and perhaps the pushing force of the door on your shoulder. These are very real forces that occur when your car tries to turn while your body tries to continue moving in a straight line. But from your point of view in the car, with the windows ...

13

As I disagree with all the answers I am going to try to explain some of the fundamentals of science: Science in its very essence can not explain why things happen the way they do, it simply tries to model reality based on observations in the past to predict events in the future. In other words, defining a centrifugal force is possible as for example your ...

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