Transcript Forces Review - Red Hook Central Schools

Force Review
Four Fundamental Forces Nature
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Gravity
Electromagnetic
Strong Nuclear
Weak Nuclear
Common forces
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Gravity – weight, attraction between masses.
Friction - surfaces, air/wind resistance
Elastic/Strain/Tension – deformation of shape.
Electric – attraction / repulsion charges q.
Magnetic - attraction / repulsion magnetic
poles.
• Normal – reaction force perpendicular to
surfaces.
Forces Review
Newton’s 1st Law (Inertia)
• Balanced F = Equilibrium: Fnet = 0 constant v, or v = 0.
• Translational Equilibrium – all F in all directions balanced.
• Vertical equilibrium - upward & downward forces are
balanced.
• Horizontal equilibrium, left & right forces balanced.
• Inertia is a property of matter directly proportional to mass.
Measure of resistance to acceleration.
Fnet ≠ 0 (Newton 2)
• Unbalanced forces cause acceleration in the
direction of Fnet.
• a = Fnet/m
• acceleration rate change in velocity directly
proportional to Fnet, inversely proportional to
mass.
Newton 3: Action Reactions Pairs
• F a,b = - F b,a .
• IB - action/ reaction pairs.
• IB: Normal Force Fn = Reaction Force, R.
Know Newton’s 3 laws of motion
• Inertia
• Acceleration a =Fnet/m
• Action/Reaction –
Forces in pairs.
Fa,b = -F b,a.
Sketch Free Body Diagrams
• Show all forces, (relative or scaled)
magnitudes, directions acting on a
body. Identifies direction/mag of
Fnet.
• No other arrows except forces.
Define
• Translational Equilibrium: linear forces
balanced
• Balanced Forces – no linear acceleration.
Be able to:
• Identify action/reaction pairs.
• Solve problems in net force/ acceleration.
• Remember Fnet = S Forces = ma.
• Can write & state
S Forces = ma = F1 + F2 + F3…
Force Vectors
• 1. The 25-kg sign (Joe’s) is suspended by two cables: A
and B.
• Sketch a free body diagram.
• Find the tension in each cable.
30o.
Ta.
Tb.
Fg = 250 N
• Tb = 144-N
• Ta = 289-N
Using Fnet = S F = ma
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Inclined Plane.
Elevator
Horizontal Pulley
Connected masses.
Atwood’s machine.
Inclined Plane
2. The 5-kg box above is pushed up a 20o incline at
constant speed.
• Sketch the free body diagram.
• Calculate the pushing force if m, the coefficient
of friction is 0.49. Remember Ff = mFn.
3. The box pictured below is accelerating down a
25o incline at 1.5 m/s2.
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Sketch the free body diagram.
Designate the downhill direction as positive.
Write the equation of Fnet.
Calculate m.
Elevator Forces
Elevators.
Scale
measures
normal force.
Not weight.
Sketch free body on man in elevator.
• Write the equation for Fnet.
• Fnet = SF = F1 + F2 …
ma = F1 + F2 …
4. A 100 kg man stands on a scale in an
elevator. Use g = 10 m/s2.
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What will the scale read when the elevator:
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Goes up at constant velocity.
Accelerates up at 2 m/s2.
Accelerates down at -3 m/s2.
Accelerates down at -10 m/s2.
The scale reads the normal force.
a) at constant velocity, a = 0 ma = 0
SF = 0 so 0 = Fn – mg
mg = Fn
–Fn ~ (100kg) (10 m/s2) 1000 N
He feels the same
b) ma = Fn - mg for a =2 m/s2.
(100 kg)(2m/s2) + (1000 N) = 1200 N
He feels heavier.
c) ma = Fn - mg
(100kg)(-3m/s2) + (100)(10 m/s2) =
700 N
He feels lighter.
• d) ma = Fn - mg
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ma + mg = Fn
100 kg (-10 + 10) m/s2
Fn = 0 He feels weightless!
He is in freefall.
Read Hamper 2.2
Problems in Force #1-4.
• Problem Sheet “Problems in Force 2” Sketch
all free body diagrams with that.
• Do pg 29 #5, 6.
Linear Momentum
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Product mass x velocity
Property of object moving in straight line
p = mDv
Vector quantity that is conserved.
Don’t forget to include sign for velocity when
calculating.
• S pbefore = S pafter.
• Impulse = Dp. Relates Force to momentum.
Momentum Change &
Newton’s 2nd Law
• F = ma
• F = mDv
Dt
• FDt =mDv m (vf - vi) for const mass.
• FDt = Dp
Dp = Change in momentum
Force N
force - t graph:
Dp Impulse is area under curve
For constant force = FDt.
Non-Constant Force
Force vs. time graph. The area under the curve =
impulse or Dp change in momentum.
Impulse involves time
Work involves distance!
Work = Fd cos q.
The force must be parallel to the
distance moved (0o or 180o)
Work: force x distance moved in direction
of force. W = F cos q.
For varying force = area under curve on F vs.
d graph.
Work is a scalar measured in Joules.
• Work done on an object causes changes in
energy in by the amount of work done.
• Ex: if 300 Joules of work is done stopping a
moving object, 300 J of KE was converted to
thermal E, and sound.
A grenade is launched into the air and
explodes into hundreds of pieces at the
top of its arc. How does the total KE and
total momentum compare just before and
just after the explosion?
• KE
• Momentum
more, less, the same.
more, less, the same.
Positive & Negative Work
• Work can be pos, 0, neg depending on q:
If
0° ≤ q < 90°
If a = 90°
If
cos q = +
cos q = 0
90° < q ≤ 180°
W is positive .
W is 0.
cos q is neg
W is neg.
Types of Energy
Mechanical
• KE
• PEg
• PE elas
Non- Mechanical
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Heat
Light
Sound
Nuclear
Chemical
Electro-magnetic.
Conservation of Energy
• Holds true for all energy but – KE not conserved
necessary…. E can be converted to other types.
• Momentum is a single quantity. It must be
conserved!!
• To find final velocity don’t assume KE is
conserved unless a collision is elastic.
We can relate KE to momentum
• See text for derivation.
Power = rate work gets done/E transformed.
The kWh is a unit of energy equivalent to
1 kW of power expended for 1 h of time.
Use 1000 watts for 1 hour, that's a kilowatt-hour.
• kWh are units of energy.
• 1000 J/s (1 h) (3600 s/h) = 3.6 x 106 J
Efficiency is ratio of amount of work,
Energy, power we get out compared to
amount put in.
Often expressed as %.
Ex: A car engine has eff of 20% and produces
25 kJ of useful work/sec. How much energy is
converted to heat per sec?
Handout Power & Efficiency.