Normal Forces & Friction Applied Science III Mr. Finau

What’s Newton’s 1st Law of Motion? An object in motion (or at rest) will remain in motion (or at rest) unless acted upon by a Net Force

What’s Newton’s 2nd Law of Motion? The acceleration of an object is: Directly Proportional to the force acting on it Inversely Proportional to the mass of the object

What’s Newton’s 3rd Law of Motion? For every action (or force), there is an equal and opposite reaction (or force) Examples: sitting on a chair - the chair pushes back on you so that you don’t fall through the chair Karate chop on bricks – chop attempts to break bricks; bricks hurt hand

If I kick a football w/a force of 50 N, how much force does the football apply to my foot? According to Newton’s 3rd law, there should be an equal and opposite amount of force on my foot, so 50 N.

Then why doesn’t my foot stop instantly when I kick the ball? Although the forces are equal and opposite, which has more mass…the ball or your foot? According to Newton’s 2nd Law, F = ma So, if the force is the same and the foot has a much higher mass than the ball, which has more acceleration? The ball – which goes flying off

When I stand on the desk, how much force do I apply to the desk? I apply a force equal to my weight Gravity pulls me downward, thus my weight is the amount force caused by the gravitational force.

How much force does the desk apply to me? In what direction? According to Newton’s 3rd Law, the desk will apply an equal and opposite force to my weight. Because it’s opposite, the force by the desk will be upward, opposite of my weight. This force is known as the normal force

What is Normal Force? Force applied perpendicular to the contact surface that pushes to the surfaces together

What does perpendicular mean? Perpendicular means “meeting a given line or surface at right angles.” For a level table, perpendicular is straight up What if I push a book against the wall? What’s perpendicular to the wall? What if I push a book against the ceiling? What’s perpendicular to the ceiling?

Draw pictures for these 4 situations and arrows showing Weight, Force Applied, and Normal Force: An elephant sitting on a table A book being pushed against the wall A person pushed against the ceiling A couch being pushed up a ramp LABELS: W = Weight FN = Normal Force FA = Applied Force

Draw 3 books stacked on top of each other and label A, B, C from top to bottom. Each book weighs 20 N. How much force is acting on B by A? How much force is B pushing up on A? How much force is acting on C? How much force does C apply to B?   How much force is applied by C on A?

What is friction? A slowing force that opposes the direction of motion What exactly does friction do? Slows down an object; causes a deceleration (negative acceleration)

What does friction depend on? What’s the difference between what happens when you walk on concrete and when you walk on ice? Interaction between materials coefficient of friction = μ If you are pushing a couch across the floor and your buddy jumps on top of the couch, is it easier or harder to move? Depends on Normal Force = FN

What does the interaction between materials look like up close?

How does Normal Force affect friction? The greater the normal force, the more friction Directly Proportional…as one increases, the other increases

Bill Nye on Friction

Your car breaks down close to a gas station. You try to push it there Your car breaks down close to a gas station. You try to push it there. What’s harder to do…start the car into motion or keep the car in motion? Why? Start the car into motion There must be 2 types of friction…

What are the 2 types of friction? Static Friction Force needed to overcome in order to START a car in motion Kinetic Friction Force needed to overcome in order to KEEP a car in motion

Which type of friction is greater? How do we know? Static Friction is larger It’s much harder to start moving an object than to keep it in motion

What does friction depend on again What does friction depend on again? How do I arrange them in an equation to find friction? Depends on 1) interaction between materials & 2) normal force Equation: Ff = (μ)(FN)

If the coefficient of kinetic friction between the carpet and my couch is .3 and my couch weighs 500 N, how much force do I need to push with to keep moving the couch? Ff = μ FN Ff = .3(500) = 150 N

If the coefficient of kinetic friction between the carpet and my couch is .2 and my couch weighs 500 N, how much force do I need to push with to keep moving the couch? Ff = μ FN Ff = .2(500) = 100 N

If I have to pull my dog that weighs 100 N with 10 N of force in order to keep him sliding across the floor, what is the coefficient of kinetic friction? Ff = μ FN μ = Ff/FN μ = 10/100 = .1