Forces Friction The Lucky Cow The driver of the car applies the brakes to avoid hitting the cow. But how does this cause the car to slow down and stop? The brakes cause the wheels to stop turning and to slide on the road surface. This action produces a force that resists the forward movement of the car. This force is called friction.

Forces Friction

Forces Friction All objects encounter friction of some sort

Forces Friction “How do we calculate the frictional force?”

Forces Friction Types of Frictional Forces: 1.Static 2.Kinetic 3.Rolling 4.Fluid

Forces Friction Kinetic (Dynamic) Friction It’s the friction encountered in motion. Let's say you had a crate and you wanted to move it somewhere. First, you have to overcome the static friction between the floor and the crate to get it moving. Once you have it moving, you have to keep it moving by overcoming the kinetic (moving) friction - which is always much easier!

Forces Friction Friction depends on: 1.The type of surfaces (u) “mu” - coefficient of friction 2.How hard the surfaces are pressed together (F N ) – Normal force

Forces Friction   Friction always opposes motion.   The applied force must be greater than the static frictional force to move the block.

Forces Friction static frictional force “Mu” coefficient of static friction Normal force maximum force of static friction f s max is the force you must exceed to move the object.

Forces Friction Once object moves.  Use  k  f k is constant  There is no “maximum f k.” Kinetic frictional force “Mu” coefficient of Kinetic friction Normal force

Forces Friction   The coefficient of static friction is where u s = coefficient F N = normal force f = friction force  The coefficient of friction is dimensionless – no units.  The greater the u s the more force needed to move the object.

Forces Friction  The frictional force increases until it reaches a maximum (in this case, 2.5N)  If we push harder still, (say, 2.6N), the block will move.

FAFA On the verge of slipping Maximum Static Friction Applied Force, F A Friction, f Sliding (Kinetic) Friction Sliding f Forces Friction Link

Forces Friction  Static friction, f s  Kinetic friction, f k

Forces Friction The object begin to move if the applied force is larger than the max static friction. Here the kinetic friction remains constant no matter how large is the applied force. Here, f l stands for the max static friction and f k is the kinetic friction. It is slightly less than the max static friction.

Forces Friction Approximate Coefficients of Friction

Forces Friction SUMMARY  Friction force, f, is proportional to normal force, F N.  u s and u k are coefficients of friction  Frictional force is opposite the motion  Values of u s and u k depend on surfaces  u s and u k don’t depend on surface area  u s and u k don’t depend on the velocity  Friction is due to the surfaces interacting with each other on the microscopic level (sliding over bumps, chemical bonds)

Forces Friction

Forces Friction You push a giant barrel o’ monkeys setting on a table with a force of 63 N. If  k =.35 and  s =.58, when will the barrel have moved 15 m? answer: Never, since this force won’t even budge it! 63 < 0.58 (14.7) (9.8)  83.6 N 14.7 kg

Forces Friction Friction as the net force A runner attempts to steal third base. He’s running at a speed v and has a mass m. The coefficient of kinetic friction between his uniform and the base pass is . 1.How far from second base should he begin his slide in order to stop right at the base? Give the answer in terms of the given quantities and any constants. m, , and v.

Forces Friction Measuring u k A hockey puck is given an initial speed of 20.0 m/s. It slides 115 m before coming to rest. (a)Determine the coefficient of kinetic friction between the puck and the ice. (b)Could the coefficient of friction be larger than 1?

Forces Friction Experience might lead you to believe that if M was very large, say 2000 Kg, m would not budge. This is where friction comes in.

Forces Friction.. A crate of mass 20 kg is sliding across a wooden floor.  k between the crate and the floor is 0.3. Determine the strength of the net force acting on the crate. strength of the net force acting on the crate. If the crate is being pulled by a force of 90 N (parallel to the floor), find the acceleration of the crate. FNFN fkfk F FgFg

Forces Friction Problem: Forces are being applied to a box sitting on a surface with friction. Will the box move horizontally (along the surface)? F1=50N, F2=50N, Mass of the block 10kg, and u s =0.4.

Forces Friction Measuring u s A brick is placed on an inclined board as shown in the figure. The angle of incline is increased Until the block starts to move. (a)Determine the static friction coefficient from the critical angle,  c, at which the block starts to move. (b)What is u s if the block starts sliding at  C = 31°

Forces Friction If we slide or attempt to slide a body over a surface, the motion is resisted by a bonding between the body and the surface. The resistance is a force called the frictional force, or simply friction. This force is directed along the surface, opposite the direction of the intended motion.

Forces Friction Block B weighs 711 N. The coefficient of static friction between the table and the block is 0.25: assume the cord between the knot and block B is horizontal. Find the maximum weight of a block A for which the system will be stationary.

Forces Friction Two blocks are connected by a cord over a frictionless pulley. The mass of block A is 10 kg and the coefficient of kinetic friction between and the incline is The angle of the ramp is 30 o. Block A slides down the incline at constant speed. What is the mass of block B?

Forces Friction

Forces Friction

Forces Friction

Forces Friction

Forces Friction