Newton’s
nd
2
Law
Or Accelerating Your Dynamic
Range
Newton’s
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•
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•
nd
2
a~Fnet (direct proportion)
a~1/m (inverse proportion)
a=Fnet/m
a and Fnet in same direction
Vector equation
Law
Statics vs. Dynamics
• No net force on object
 SFx=0
 SFy=0
 SFz=0
• Motion is that of constant
speed
• Examples include car
cruising at constant speed,
airplane moving forward
at constant speed, masses
held stationary by ropes,
falling bodies after
reaching t.v.
• Net force on object
 SFx=max
 SFy=may
 SFz=maz
• Motion is that of constant
acceleration
• Examples include car
speeding up, slowing down
or changing direction,
airplane diving or climbing
or changing speed, falling
bodies before reaching t.v.
Pressure
• Pressure = force / area
• Dance skit
• Bed of nails demonstration
– More nails better than less nails
– Less force better
– More area better
Law of Falling Bodies Explained
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•
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Kindergarten Skit
Inertia greater for larger mass
Net force greater on larger mass
F/M = f/m = accel of gravity
True if NO air resistance or if
object has not reached terminal
velocity
Falling w/ Air Resistance
• Terminal Velocity (T.V)
Air resistance
– Fastest speed object falls at
weight
– Weight = air resistance => mechanical equilibrium
– Varies directly w/ mass and inversely w/ area
• Prior to terminal velocity
– W constant, A.R. increasing, F net decreasing
– A decreasing, v increasing, d fallen each second
increasing
• After terminal velocity is reached
– W=AR, F net=0
– A=0, v= t.v., d fallen each second constant
Different Masses in Freefall
Hits t.v.
b
Hits t.v.
b
a
a
b
•Which has the higher t.v.?
•Which reaches t.v. first?
•Which accelerates longer?
•Which hits ground with higher speed?
•Which hits ground first?
Check Question 1
• What is the acceleration of a 10 kg block of
ice that has a net force of 50 N placed on it?
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1) 500 m/s/s
2) 5 m/s/s
3) 1/5 m/s/s
4) 0 m/s/s
Check Question 2
• The road pushes the tires of a car with a
force of 5000 N. Air resistance pushes in
the opposite direction with a force of 1000
N. If the car has a mass of 2000 kg, what is
the acceleration of the car?
– 1) 2 m/s/s
– 3) 5 m/s/s
2) 2.5 m/s/s
4) 1/2 m/s/s
Check Question 3
A styrofoam and golf ball are
dropped from rest at the
same height. Answer the
following
1) Which has the greater
TV?
2) Which accelerates
longer?
3) Which has the greater
velocity when it hits the
ground?
4) Which hits the ground
first?