# Direction of frictional force [closed]

How will I find the direction of frictional force for any kind of motion of a moving object?

• What have you read? Friction opposes the sliding of one surface across another. Have you searched for other questions on this subject? – Bill N Aug 22 '15 at 22:05
• I'm voting to close this question as off-topic because the author did do sufficient research first. – ja72 Dec 12 '15 at 0:39

Kinetic friction is in the direction opposite the moving object's velocity relative to whatever surface it is sliding on.

Static friction opposes a stationary object's tendency to slide relative to the surface it is resting on. In other words, it is opposite the direction in which the object would move—relative to the surface—if there were no friction.

See Bill N's comment for a good example.

• Your first sentence is correct only because you include the relative statement. Your static friction statement is wrong. Static friction opposes the tendency to slide across a surface. Consider a block resting on a piece of paper. Pull the paper in the +x directionsuch that the block moves with the paper. The block moves in the +x direction and the static friction on the block is in the +x direction. – Bill N Aug 22 '15 at 22:16
• You said that kinetic friction is in the direction opposite the moving object's velocity..but in circular motion the direction of frictional force is radially inwards which is perpendicular to the direction of velocity(which is tangential) ...why? – Subhadip Pal Aug 23 '15 at 4:35
• Please send me a generalised method for finding the direction of frictional force for any kind of motion.. – Subhadip Pal Aug 23 '15 at 4:37
• @SubhadipPal You are probably thinking of a car turning in a circle, right? If so, the friction force in question is actually static, as the wheels are rolling without slipping, meaning the contact point between the wheels and the road has $v = 0$. If the turn is banked, the car will want to slip up or down the bank depending on its speed. Friction will resist this tendency. – Rations Aug 23 '15 at 12:36

Friction is always tangential to the object's surface. If it's propulsive (causing the motion of the object) then it acts in the direction of motion. If it's resisting, then it acts opposite to the direction of motion.

• No. Friction opposes sliding of surfaces across each other. Friction can actually produce motion. – Bill N Aug 22 '15 at 22:03
• I changed it. Check this one. – jjack Aug 22 '15 at 23:00
• That's not wrong, but it's incomplete. There are an infinite number of directions which in the plane tangential to the surface. – Bill N Aug 22 '15 at 23:05
• Added another sentence. – jjack Aug 23 '15 at 9:44