Transcript Newton`s 2nd Law

Newton’s 2nd Law
Part II
Friction &Pressure
5.4-5.5
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
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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
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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
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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.