# Friction directions

#### ascky

Just a quick question that I can't figure out the answer to...

I know that if I have a car accelerating forwards, the friction force will be forwards to oppose the relative motion between the wheel and road. Now if I put, say, a cylinder on a slope, the friction force will be uphill (I think?)... why is it uphill?

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#### OlderDan

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ascky said:
Just a quick question that I can't figure out the answer to...

I know that if I have a car accelerating forwards, the friction force will be forwards to oppose the relative motion between the wheel and road. Now if I put, say, a cylinder on a slope, the friction force will be uphill (I think?)... why is it uphill?
Friction always acts in a direction that tends to oppose motion. For a cylinder on a slope gravity tends to pull the cylinder down the slope, so friction acts in the opposite direction IF you start the cylinder from rest. Friction could be directed up the slope under other conditions, such as if you were rolling the cylinder up the slope.

#### whozum

Simplify your car tire to a cylinder rolling down a slope :)

#### ascky

OlderDan said:
Friction always acts in a direction that tends to oppose motion. For a cylinder on a slope gravity tends to pull the cylinder down the slope, so friction acts in the opposite direction IF you start the cylinder from rest.
I still don't understand what it means to 'act in a direction to oppose motion'. To oppose the motion of what exactly?

#### ascky

whozum said:
Simplify your car tire to a cylinder rolling down a slope :)
Good point. So friction acts differently when things start stationary and when they're moving...?

#### whozum

ascky said:
Good point. So friction acts differently when things start stationary and when they're moving...?
In a sense, yeah, but this wasnt Dan's point. Dan was saying how if you initially threw the cylinder up the ramp, then as it is rolling up, the friction force will point down the ramp (friction always acts opposite the direction of motion). Once the cylinder stopped rolling and started rolling back down, the friction force will start pointing up the ramp.

#### ascky

Mmm, ok. Thanks for the replies.

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