# Centripetal Force

#### GaryM

##### Member
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I've always had problems trying to grasp this and maybe someone could help me out.

Centripetal forces always point toward the centre of a circle, yet an object following a circular path tends to want get further away from the centre. (Like for example, a ball on a string would fly off unless the tension in the string held it in)
Why would it want to fly off if both the centripetal force and the force due to the tension of the string are acting toward the centre???

I'm thinking that the tension in the string is the force that is causing the centripetal force, but is it another force that makes it want to fly off, or am I just viewing this the wrong way???

Any help would be greatly appreciated

#### Parscope

##### Junior Member
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GaryM said:
I've always had problems trying to grasp this and maybe someone could help me out.

Centripetal forces always point toward the centre of a circle, yet an object following a circular path tends to want get further away from the centre. (Like for example, a ball on a string would fly off unless the tension in the string held it in)
Why would it want to fly off if both the centripetal force and the force due to the tension of the string are acting toward the centre???

I'm thinking that the tension in the string is the force that is causing the centripetal force, but is it another force that makes it want to fly off, or am I just viewing this the wrong way???

Any help would be greatly appreciated

Think of it this way:

There isn't really a centripetal force in the way you are thinking. The ball at the end of the string isn't putting force outwards, it wants to keep going straight but is being accelerated towards the center of the circle (Centripetal Acceleration) by the string. The tension in the string is the force that is being applied to the ball to accelerate it around the circle. The ball may have a constant speed, but since its direction is constantly changing, it is being accelerated. If the string were to break, the ball would fly off at a tangent to the circle since there is no longer a force applied to it.

Does that help at all?

#### Daichi Katase

##### Senior Member
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5+ Year Member
you are absolutely right. The tension in the rope is actually causing the centripital force. The force opposing the centripital force is the centrifugal force.

#### MoosePilot

##### Y Bombardier
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7+ Year Member
GaryM said:
I've always had problems trying to grasp this and maybe someone could help me out.

Centripetal forces always point toward the centre of a circle, yet an object following a circular path tends to want get further away from the centre. (Like for example, a ball on a string would fly off unless the tension in the string held it in)
Why would it want to fly off if both the centripetal force and the force due to the tension of the string are acting toward the centre???

I'm thinking that the tension in the string is the force that is causing the centripetal force, but is it another force that makes it want to fly off, or am I just viewing this the wrong way???

Any help would be greatly appreciated

Why does it want to fly off? Newton's laws or to put it another way, inertia. It flying away is the natural state, it rotating is an oddity caused by an outside force (tension).

#### GaryM

##### Member
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7+ Year Member
Thanks for all the replies

It now makes perfect sense to me.

Can't wait until all this is over with!!!

#### Turkeyman

##### Trickster Poultry
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Also, think of it this way:

If you're swinging a ball on a rope, the tension is EQUAL to mv^2/r, the centripetal force. The mv^2/r is your total NET force that keeps the object in the cirle, and all other forces sum up to it.

Another example --> When a car is circling in a parking lot, what's keeping it in? The force that is keeping it in the circle is friction, and so friction ( u * mg) is EQUAL to the net force, mv^2 / r

Lastly, the moon circling the earth is held in by a gravitational force, and this(Gmm/r^2) is EQUAL to the net force, mv^2/r

In fact, go find the mass of the earth, moon, the distance between their centers, and the value of G, and plug it all in and solve for the velocity of the moon yourself. It works =)

Hope that maybe helped

Btw...centrifugal force is a non-real force and doesn't exist --> the only force keeping something in a circle is whatever is pulling something inward

If something were pushing it outward then...it wouldn't be a circle

#### jigglyboo

##### Member
10+ Year Member
When i think of centripetal force i think of those clips of skateboarders or motorcyclist going through loops without falling. the centripetal force helps them go through a complete circle. however if they don't have enough initial velocity, they fall when they get to the top of the loop. Is it centripetal force only that helps?

Also, i'm getting confused with Mass and acceleration. Usually it's easy for me to think that they're inversely proportional, right? Well there are some questions where acceleration is independent of mass (ie. 100kg mass falling vs 50 kg, it's all the same 10g even on different planets). I'm assuming this is b/c mass is a scalar and acceleration is a vector.

Here's where the trickiness comes in with inertia: mass is the measure of inertia, if M changes, I changes. If Inertia increases, acceleration decreases. ... soo doesn't that equate mass is inversely proportional to Accel rather than independent? (EK 1001 physics, #154)

What am i missing? Bah, i hate physics.

#### Turkeyman

##### Trickster Poultry
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jigglyboo said:
When i think of centripetal force i think of those clips of skateboarders or motorcyclist going through loops without falling. the centripetal force helps them go through a complete circle. however if they don't have enough initial velocity, they fall when they get to the top of the loop. Is it centripetal force only that helps?

Yeap, the normal force of the loop (pushing perpendicular to their tangential velocity) IS the centripetal force

jigglyboo said:
Also, i'm getting confused with Mass and acceleration. Usually it's easy for me to think that they're inversely proportional, right? Well there are some questions where acceleration is independent of mass (ie. 100kg mass falling vs 50 kg, it's all the same 10g even on different planets). I'm assuming this is b/c mass is a scalar and acceleration is a vector.

They're not really inversely proportional...beacuse usually for a set problem, mass of an object will be constant. It's more like....the more force on an object, the more acceleration. Acceleration is directly proportional to the force on the object. Mass for an object will be the same throughout a problem.

Falling for objects is the same because of Newton's law of gravitation, F = GMm/R^2 --> where g = GM/r^2, and m = m

The bigger an object is, the more force it'll have on it...and that's why everything falls at the same rate.

jigglyboo said:
Here's where the trickiness comes in with inertia: mass is the measure of inertia, if M changes, I changes. If Inertia increases, acceleration decreases. ... soo doesn't that equate mass is inversely proportional to Accel rather than independent? (EK 1001 physics, #154)

What am i missing? Bah, i hate physics.

I think you might not be grasping the fact that inertia(or mass) is simply a measure of an object's tendency to resist motion, or remain in its present state of motion. But you must also remember that with free-falls, bigger objects have bigger forces on them, and smaller objects have smaller forces on them, so everything is "equal" in a sense --> everything falls at the same rate.

Now if you apply the SAME force to two different sized objects, that's where they WONT accelerate at the same rate, because its a fixed force for two different objects....obviously the more massive one will accelerate slower.

#### MoosePilot

##### Y Bombardier
10+ Year Member
7+ Year Member
jigglyboo said:
When i think of centripetal force i think of those clips of skateboarders or motorcyclist going through loops without falling. the centripetal force helps them go through a complete circle. however if they don't have enough initial velocity, they fall when they get to the top of the loop. Is it centripetal force only that helps?

Also, i'm getting confused with Mass and acceleration. Usually it's easy for me to think that they're inversely proportional, right? Well there are some questions where acceleration is independent of mass (ie. 100kg mass falling vs 50 kg, it's all the same 10g even on different planets). I'm assuming this is b/c mass is a scalar and acceleration is a vector.

Here's where the trickiness comes in with inertia: mass is the measure of inertia, if M changes, I changes. If Inertia increases, acceleration decreases. ... soo doesn't that equate mass is inversely proportional to Accel rather than independent? (EK 1001 physics, #154)

What am i missing? Bah, i hate physics.

With the skater or cyclist trying to go around the loop, inertia would make him go straight, but there's a solid loop in his way. So normal force is the centripetal force.

Mass and acceleration are only inversely proportional when you're thinking of a force acting on an object, because F=ma. If the force is constant, then as m increases, a has to decrease to make the equation work. Gravity is like that because the mass terms cancel, I think.

#### MattD

##### Curmudgeon
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Just to clarify, centripetal force is not what keeps the guy from falling off the loop. The centripetal force (normal force from the ramp) is actually pushing him TOWARD the center of the loop, which is what makes him curve in a circle in the first place. But that means that at the top of the loop centripetal force is actually trying to MAKE him fall... inertia is what prevents that from happening, as it resists the acceleration due to that force.

#### drechie

question: i thought centripetal force always pointed towards a circle. in a TBR example, where a ball on a string is going around a vertical circle, tension is pointing toward the circle (string), and the centripetal force is in the direction opposite of tension, i.e.: outwards of the circle.

I thought Centripetal force was always toward the circle?

This direction of tension and centripetal force in this image is representative of the TBR problem:

http://image.slidesharecdn.com/cent...95/centripetal-force-17-728.jpg?cb=1326223134

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