# Mechanics Question Help

Hey guys, could you please help with question 9 b)? I got the value for acceleration. However, why does the air resistance (F) and friction force act upwards. Should they not resist the acceleration up the slope and thus act downwards?
(edited 8 months ago)
Friction will oppose motion, so will act in the opposite direction to velocity, not acceleration.
Original post by mqb2766
Friction will oppose motion, so will act in the opposite direction to velocity, not acceleration.

Thank you! That has really cleared it up. So because it continues to travel down while decelerating, the two forces act in the opposing direction? (Just wanted to confirm)
Original post by Nithu05
Thank you! That has really cleared it up. So because it continues to travel down while decelerating, the two forces act in the opposing direction? (Just wanted to confirm)

Yes, but the acceleration/deceleration is irrelevant. If its moving down the slope (velocity downwards), friciton will act up the slope.

A simple thought experiment. Just think about a skydiver at terminal velocity (zero acceleration). Moving downards, air resistance/friction acts upwards and at terminal velocty, the air resistance force must be opposite and equal to the gravitational force, so the net force/acceleration is zero.
Original post by mqb2766
Yes, but the acceleration/deceleration is irrelevant. If its moving down the slope (velocity downwards), friciton will act up the slope.

A simple thought experiment. Just think about a skydiver at terminal velocity (zero acceleration). Moving downards, air resistance/friction acts upwards and at terminal velocty, the air resistance force must be opposite and equal to the gravitational force, so the net force/acceleration is zero.

Thanks a lot! The example has definitely enhanced my understanding.