newton’s 2nd law part ii friction &pressure 5.4-5.5
TRANSCRIPT
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.