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jsmith613
29th March 2012 - 09:41 PM
When a car moves in a circle the frictional force provides the centripetal force
(as per this image: http://bit.ly/HfC1Qr)

WHY DOES frictional force act perpendicular to the direction of motion (normally it acts to oppose the direction of motion)?

Robittybob1
29th March 2012 - 10:02 PM
QUOTE (jsmith613+Mar 29 2012, 09:41 PM)
When a car moves in a circle the frictional force provides the centripetal force
(as per this image: http://bit.ly/HfC1Qr)

WHY DOES frictional force act perpendicular to the direction of motion (normally it acts to oppose the direction of motion)?

Sliding would be one of it's motions without the required traction allowing cornering.

Once it has finished accelerating in the forward direction it doesn't need any more traction (friction) in that direction.
jsmith613
29th March 2012 - 10:07 PM
QUOTE (Robittybob1+Mar 29 2012, 10:02 PM)
Sliding would be one of it's motions without the required traction allowing cornering.

Once it has finished accelerating in the forward direction it doesn't need any more traction (friction) in that direction.

would this be a good answer (I just thought of it)

I am moving in a circle so MUST feel a force in a horizontal plane
as I am NOT accelerating in any other direction (i.e: my speed is constant) there can be no NET force in any other direction

If I were to resolve the friction vector (i.e: I took the direction of wheels as / and hence direction of friction force as /)
I would have a force both forward and to the centre of the circle (i.e: I would feel an an acceleration in BOTH directions)

A change in speed is caused by acceleration but as my speed is constant THIS CANNOT BE POSSIBLE (i.e: the resolving method)

THUS the ONLY POSSIBLE DIRECTION in which friction can be acting is towards the centre of the circle (otherwise I COULD NOT move in a circle)

is this correct?
Robittybob1
29th March 2012 - 10:21 PM
QUOTE (jsmith613+Mar 29 2012, 10:07 PM)
would this be a good answer (I just thought of it)

I am moving in a circle so MUST feel a force in a horizontal plane
as I am NOT accelerating in any other direction (i.e: my speed is constant) there can be no NET force in any other direction

If I were to resolve the friction vector (i.e: I took the direction of wheels as / and hence direction of friction force as /)
I would have a force both forward and to the centre of the circle (i.e: I would feel an an acceleration in BOTH directions)

A change in speed is caused by acceleration but as my speed is constant THIS CANNOT BE POSSIBLE (i.e: the resolving method)

THUS the ONLY POSSIBLE DIRECTION in which friction can be acting is towards the centre of the circle (otherwise I COULD NOT move in a circle)

is this correct?

I think that was about 65% correct.
"I am moving in a circle so MUST feel a force in a horizontal plane"

R: wrong as not always true. Orbital motion you won't feel this force.

"as I am NOT accelerating in any other direction (i.e: my speed is constant) there can be no NET force in any other direction"

R: Would be OK if you defined the direction first so we know what the "other direction" were.

"If I were to resolve the friction vector (i.e: I took the direction of wheels as / and hence direction of friction force as /)
I would have a force both forward and to the centre of the circle (i.e: I would feel an an acceleration in BOTH directions)" Possible?

"A change in speed is caused by acceleration but as my speed is constant THIS CANNOT BE POSSIBLE (i.e: the resolving method)"
R: They say acceleration causes a change in velocity not speed.
But your velocity is not constant for you are changing direction.

"THUS the ONLY POSSIBLE DIRECTION in which friction can be acting is towards the centre of the circle (otherwise I COULD NOT move in a circle)" R:OK


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