Newton’s 2nd LawNewton’s 2nd LawPart IIPart II

Friction &PressureFriction &Pressure5.4-5.55.4-5.5

Review

• Q: If a car can accelerate at 2 m/s2, what acceleration can it attain if it is towing another car of equal mass?

• A: The same force on twice the mass produces half the acceleration, or 1 m/s2.

• Q: What kind of motion does a constant force produce on an object of fixed mass?

• A: a constant force produces motion at a constant acceleration. Force doesn’t change --> acceleration doesn’t change

Objectives

1. Describe the effect of friction on stationary and moving objects

2. Distinguish between force and pressure

Friction• Rub your hands on the

desk. • The carpet. • Your pants. • Rub your hands

together.• These are all different

types of friction.

Friction

• Friction: – Acts on materials in

contact with each other– Acts in direction to

oppose motion• Depends on kinds of

material in contact• Concrete vs. steel road

dividers; which one is more effective in slowing down a car?

Friction is Everywhere• Friction is in fluids• What’s a fluid?• Fluid:

– Anything that flows– Liquids and gasses

• Friction occurs as objects push fluid aside – Improving your bat speed– skydiving

• Air resistance:– Friction acting on something

moving through air

Friction• Remember, when friction is present and an object moves

at constant velocity, what does that tell us about net force?

• Net force is zero!

Friction Demo

• Let’s go into the lab for a demo/lesson on friction.

• Bring your notebooks.

Force vs. Pressure• A book in different positions in your hand.• Is the force of the book on your hand going to change?• No.• Will the pressure change? • Feel the difference.• The area of contact changes• Pressure:

– The amount of force per unit area• pressure = force / area of application

– P = F/A

Force vs. Pressure

• Everybody stand up on two legs• Lift one leg• Try to stand on your toes.• Force stays the same, but pressure increases• There’s a decreased area of contact• Pressure is measured in newtons per square meter,

or pascals (Pa)• What are some other examples of pressure

differences?

Balloon and Nails Demo

• Why won’t the balloon pop on the bed of nails?

Pressure• Why isn’t the guy that

is getting sandwhiched between two beds of nails harmed?

• Force is distributed evenly between the hundreds of nails.

Chris Angel: It’s not magic, its physics!

Check Questions• Q: In attempting to do the demonstration in the previous

slide, would it be wise to begin with a few nails and work upward to more nails?

• A: NO! There would be one less physics teacher if the demo were performed w/ fewer nails. Too much pressure!

• Q: Imagine the same demo, but placing a cement block on the top bed and smashing it with a sledge hammer. Which would provide more safety, a less massive or more massive cement block?

• A: The greater the mass of the block, the smaller the acceleration of the block (a=f/m) and bed of nails to the guy sandwhiched in between. More intertia as well, less likely to move.