What Causes Friction? Friction is the force that opposes the motion between two surfaces that touch (parallel to the surface). The surface of any object is rough. Even an object that feels smooth is covered with tiny hills and valleys. The contact between the hills of valleys of two surfaces causes them to stick, resulting in friction. What is friction? What affects friction? How does friction affect motion? What Causes Friction? The amount of friction depends on: Roughness of the surfaces (µ) Force pushing the surfaces together There are two types of friction: Ø Static Friction the frictional force present just before motion begins. Starting Friction is when Static Friction is at its maximum. Ø Kinetic Friction the frictional force present with motion v Note: Static Friction is usually higher than Kinetic Friction 1
Kinetic friction occurs when force is applied to an object and the object moves. Examples: Sliding Friction: pushing an object across a surface Fluid Friction: opposes the motion of objects traveling through a fluid (air or water) Affecting Friction To reduce the amount of friction, apply a lubricant between two surfaces. Motor oil, wax, and grease are examples. Friction can also be reduced by rolling, rather than pushing, an object. Static friction occurs when force applied to an object does not cause the object to move. Examples: Rolling Friction: between wheels and a surface Stationary object Affecting Friction Friction increases as surfaces are made rougher. Friction increases when the force between two objects is increased. 2
Friction forces are independent of area. 4 N 4 N Friction forces are independent of temperature, provided no chemical or structural variations occur. 4 N 4 N If the total mass pulled is constant, the same force (4 N) is required to overcome friction even with twice the area of contact. For this to be true, it is essential that ALL other variables be rigidly controlled. Heat can sometimes cause surfaces to become deformed or sticky. In such cases, temperature can be a factor. Friction forces are independent of speed. 5 m/s 20 m/s 2 N 2 N The force of kinetic friction is the same at 5 m/s as it is for 20 m/s. Again, we must assume that there are no chemical or mechanical changes due to speed. The Coefficient of Friction Frictional Force, both static and kinetic, is directly proportional to the force pressing the two surfaces together. The more weight, the more friction. Ffriction F normal The missing link to the above equation is the Coefficient of Friction, or µ: f = µf N µ s = f s F N µ k = f k F N 3
Problem #1 A 75 kg crate is to be pushed up an incline plane 5 m long that makes an angle of 20 with the horizontal. If the coefficient of static friction between the crate and the inclined plane is 0.20, how much force must be given to get it started up the incline? If the coefficient of kinetic friction is 0.15, how much applied force is needed to keep it going at a constant speed up the incline? A 75 kg crate is to be pushed up an incline plane 5 m long that makes an angle of 20 with the horizontal. If the coefficient of static friction between the crate and the inclined plane is 0.20, how much force must be given to get it started up the incline? If the coefficient of kinetic friction is 0.15, how much applied force is needed to keep it going at a constant speed up the incline? Problem #1 A 75 kg crate is to be pushed up an incline plane 5 m long that makes an angle of 20 with the horizontal. If the coefficient of static friction between the crate and the inclined plane is 0.20, how much force must be given to get it started up the incline? If the coefficient of kinetic friction is 0.15, how much applied force is needed to keep it going at a constant speed up the incline? Problem #1 Problem #2 A 300 kg sled is pulled at constant speed over a level, horizontal, snow covered surface. The rope that is used to pull the sled makes a 30 angle with the horizontal. If the coefficient of friction is 0.10, find the force required. 4
A 300 kg sled is pulled at constant speed over a level, horizontal, snow covered surface. The rope that is used to pull the sled makes a 30 angle with the horizontal. If the coefficient of friction is 0.10, find the force required. Problem #2 5