Centripetal force basic questions

[scilearner]

Hello guys,

 

I wasn't interested in these stuff those days and I never got it.

 

 

Now when you do a car turn the friction from the tires provide the centripetal force right. I have few quick questions. How does the friction from tires give a force in the centripetal direction, how come it is not in the direction that is opposite to velocity direction? If the centripetal force is towards the centre why is car going in a circle, wouldn't it be sucked into the centre, Am I missing some resulting forces? Thanks guys

[swansont]

The car doesn't get sucked into the center for the same reason the earth doesn't fall into the sun — it's moving with sufficient speed to miss, and move in a circular path instead.

 

The force on the tire is inward because you have turned the wheel; you have to look at the different components of the force to see this. The car wants to move in a straight line, so there is a force opposing that from friction, and there is also the frictional force along the direction of the tire, which is no longer lines up with the car's body.

[scilearner]

Oh I get it the car wants to travel in a straight line and it is the centripetal force that keeps it in a circle. Thanks swansoft

 

But where is the centrifugal force of earth coming from to stop it from getting sucked into the sun?

 

For example let's say sun just shut down the earth would travel in a straight line, why would it do that, what made it move in the first place, was it once stationary?

[Sisyphus]

 

But where is the centrifugal force of earth coming from to stop it from getting sucked into the sun?

 

For example let's say sun just shut down the earth would travel in a straight line, why would it do that, what made it move in the first place, was it once stationary?

 

Forces cause things to accelerate, and acceleration is a change in the velocity (speed and/or direction). Things which do not have forces acting on them do not experience acceleration. i.e., they have a constant velocity, whatever that velocity might be.

 

This is Newton's first law. Answering "why" it is is kind of tricky, because it's just a fundamental law of how things work.

 

However, maybe it would help to think of it this way. Does it at least make intuitive sense that something which is motionless won't spontaneously start moving unless some force disturbs it? Well, Newton's first law is basically only saying that. The difference between zero velocity and some nonzero constant velocity is only one of perspective. If you are moving relative to the Earth, it is just as valid to say that the Earth is moving relative to you, and you are motionless. All the physics works exactly the same.

 

(So, to more directly answer the question of "why would the earth keep moving in a straight line if there was no gravity," it's because in order to not move in a straight line, it would have to accelerate, and acceleration requires force.)

Edited March 29, 2010 by Sisyphus

[swansont]

Oh I get it the car wants to travel in a straight line and it is the centripetal force that keeps it in a circle. Thanks swansoft

 

Oh, my. And here I thought "swansnot" was a bad typo.

 

But where is the centrifugal force of earth coming from to stop it from getting sucked into the sun?

 

The earth is moving at just the right speed so it continually misses — it's basically in perpetual freefall. There is no centrifugal force exerted on the earth.

[Mr Skeptic]

But where is the centrifugal force of earth coming from to stop it from getting sucked into the sun?

 

The sun exerts a centripetal force (gravity) on the earth, accelerating the earth toward the sun. However, this acceleration is perpendicular to the direction of movement, so it causes the earth to deviate from a straight line. Were the earth stationary, both its movement and its acceleration would be toward the sun and it would fall in. But since it is moving, it just continuously turns toward the sun and ends up going in a circle.

 

The equal and opposite force is Earth's inertia (centrifugal force, the earth pulls the sun just as hard). This is a force on the sun, not on the earth.

 

For example let's say sun just shut down the earth would travel in a straight line, why would it do that, what made it move in the first place, was it once stationary?

 

Inertia. Things tend to resist a change in velocity. That is, whatever is moving will continue moving in that direction (a straight line) without any change in speed. If it is stationary, it will remain stationary.

 

To overcome inertia (that is, change the velocity, also called acceleration), you need to apply a force.

 

For more, see Newton's Laws of Motion

[viiovix]

why am i blocked from commeting, this is the first i could find

 

things tend to resist indeed, science is cool

 

thats why we have inertial dampners on the enterprise

[physicist danny]

i have always believed diagrams are the best way to explain things. this isn't exactly a scientifically scaled drawing but i hope it makes things a little clearer. also i would avoid using the solar system as it is very complex

 

the first picture is someone throwing a ball on the earth the trajectory will change such that the ball begins falling towards the earth.

 

similarly, the second is scaled up in velocity and distance from the centre of the earth. so the distance moved by the projectile now compensates exactly for how much the object has "fallen." The force of gravity in the second picture IS the centripetal force. It is the only force involved.

 

Centripetal force is the resultant of all the other forces in a system.

 

(the first picture would look the same but in this case the curvature of the earth is too small, or the speed of the projectile is too low, or the object isn't far away enough).

 

centrifugal forces don't really exist until you look in a rotating reference frame - i would definitely avoid this. like the plague. just view the solar system with the sun stationary and ignore the wobble due to planetary gravity and your life will be much more bearable.