-A force that opposes motion -Acts parallel to the surfaces in contact.

Friction Force vs. Weight W FfFf Cause of Friction: the microscopic roughness between surfaces…like two gears locking together. Cause of Friction

walking tires brakes sand on icy streets

If it is not moving, then there is no friction until an applied force acts on the object! What if it is moving? Friction can also cause heat! FFfFf

1. Sliding Friction – occurs when two solid objects slide over each other.

1. Static Friction – not moving 2. Kinetic Friction - moving Static > Kinetic So… it takes more force to start moving an object

Force vs. Time t F Maximum Static Friction Constant Kinetic Friction

 Factors that affect Sliding Friction: a. The normal Force acting on an object. F N (F g only equals F N in a special circumstance, ie. lab) b. The coefficient of Friction ( µ k < µ s, unitless) The texture of the surfaces. The stickiness between surfaces.

General Formula for Friction: friction force = coefficient of friction x normal force or F f = µF N Static - µ s Kinetic - µ k

Which object would have the greatest normal force? So which would experience the greatest friction force? (assume all blocks are moving)

Which would have greater friction force… sand paper or a waxed floor? Why?

What about surface area? Which would have a greater friction force? Friction force does not depend on the area of contact. Same F N and µ!

2. Rolling Friction – (< sliding) wheels and ball bearings

3. Fluid Friction – (< sliding) occurs when a solid object moves through a gas or a liquid substance Liquids

Gases HammerFeather

Now for the problems… just like before… draw your FBD Then: ΣF x = maΣF y = maF f = µF N but now we need:

Blockhawk - Stephen Hawking is moving a 100 kg block from one side of his laboratory to the other. To accomplish this he ties a rope to the block and then ties it to his wheelchair such that the rope is pulled horizontally to the floor. If the coefficient of sliding friction is 0.35, what force must Mr. Hawking apply to the rope to move at constant speed? Ans: 343 N

Warm Up 1. A 30 N block has a coefficient of friction of What force is required to slide it along a level surface at constant velocity? 2. A 35 kg crate requires a force of 110 N to slide at an acceleration of 2 m/s 2. What is the coefficient of friction?

In the figure below the coefficient of friction is Find the tension in the rope then find the acceleration of the 18 N block.

A crate having a mass of 60 kg falls horizontally off the back of a truck which is traveling at 30 m/s. Compute the coefficient of kinetic friction between the road and the crate if the crate slides 45 m on the ground with no tumbling along the road before coming to rest. Assume that the initial speed of the crate along the road is 30m/s. 30 m/s Ans. 1.02

Radio Control Car A 35 kg radio control car runs out of gas and coasts to a stop in 3 seconds. Knowing the coefficient of friction between the ground and the car is 0.4, find the initial velocity and the distance the car rolled.

Atwood Again… A cord running over a pulley connects two objects. The coefficient of friction between the object and the table is Find the acceleration of the system and the tension of the rope. 7 kg 5 kg

Angle Block Problem: A 40 kg lawnmower is being pushed at a constant speed along the ground. The angle at which the force is being applied is 35º below the horizontal. What force will keep the lawnmower moving at a constant velocity if the coefficient of kinetic friction is.3?

How does pulling a block at an upward angle change the normal force? …friction force? FgFg FNFN F y x

How does pulling a block at a downward angle change the normal force? …friction force? FgFg FNFN F y x

If all the boxes are the same, which requires a greater force to pull? Why? F F F

Warm-up A 925 N crate is being pulled across a level floor by a force of 325 N at an angle 25° above the horizon. The coefficient of kinetic friction between the crate and the floor is Find the acceleration of the crate. 925 N 25° 325 N Answer: m/s 2

What if we’re on an incline? If you were sledding down a hill would mass affect your acceleration?

There are always at least two forces acting upon any object that is positioned on an inclined plane - the force of gravity and the normal force. The force of gravity (also known as weight) acts in a downward direction; yet the normal force acts in a direction perpendicular to the surface.

FNFN FgFg F FfFf F g-y F g-x 30º - Break F g into x and y components - Angle between F g and y is equal to the angle of the incline - Rotate the x & y axis

Inclined to be Friction Old Grandma Jones is trying to load a wooden crate full of apples into her truck. To accomplish this, she plans to slide the crate up along a board. If the crate has a mass of 50 kg and the board is at a 30º incline, what is the maximum force she must apply parallel to the board to move the crate at constant velocity into her truck if the coefficient of friction is.25?

That Darn Cat! David is speeding down a hill (incline) when he notices a cat in the road. He immediately switches from the gas pedal to the brakes in order to stop the car. The slope of the incline is 20º and the car has a mass of 1500 kg. a. Draw a force diagram showing all the forces acting on the car. b. Determine the acceleration of the car if the coefficient of friction between the road and the tires is c. If the cat is 25 m away, and David is going 10 m/s when he slams on the brakes, does he hit the cat? That DEAD Cat!

Quiz part 2 1. The block given an initial velocity of 10 m/s is sliding down the hill and will come to a stop due to friction (µ k =.7). Hint (The only force acting on the block down the hill is F gx ) a. Draw a free body diagram of the block. 10 kg d. How far will it travel before it stops? b. Write the Newton’s 2 nd law equations. c. Find the acceleration of the block

Atwood Again… A cord running over a pulley connects two objects. The coefficient of friction between the object and the table is Find the acceleration of the system and the tension of the rope. 7 kg 5 kg Quiz: Part 3

The brakes are slammed on a speeding truck and it skids to a stop. If the truck were heavily loaded so it had twice the total mass the skidding distance would be: a) the same b) 1 ½ times as far c) twice as far d) four times as far

Answer: a) the same Twice the mass means the skidding tires will bear against the road with twice the force, which results in twice the friction. Twice as much friction acting on twice as much mass produces the same deceleration and hence the same stopping distance.