Newton’s 2nd Law Part II Friction &Pressure

Review Q: If a car can accelerate at 2 m/s 2, 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/s 2. 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.