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Tuesday- Physics
Con A homework
Newton’s laws
Homework Question #1
• A record of travel along a straight path is
as follows:
– Start from rest with constant acceleration of
2.77 m/s2 for 10 s
– Constant velocity for next 2.05 min
– Constant negative acceleration (-9.47 m/s2)
for 4.39 s
• Find the overall displacement fro the
complete trip.
Answer #1
Part one:
Vi = 0, a = 2.77, (t = 10) identifier = vf
D = vit + ½ at2
d = _____
Vf = vi +at
vf = ______
Part two:
Vf from the first part = vi and vf in the second part
_________
a=0
t = 2.05 min x 60 = 123 sec
Several equation to chose from… d = ½(vi +vf)t
v=
Answer #1
• Part C:
vf of the second part = vi of the third
a = -9.47, t = 4.39
D = vit + ½ at2
d = __________
Overall displacement = the sum of D’s from
each part
Problem #2
• Killer whales routinely jump 7.5 meters
above the water. What is their velocity as
they leave the water in order to achieve
this feat?
Answer #2
•
•
•
•
7.5 = d, a = -9.8 (freefall), vf = 0
Want vi
Identifier is time
Vf2 = vi2 + 2ad
Problem #3
• Two students are on a balcony 19.6 m above the street.
One student (student A) throws the ball vertically
downward at 14.7 m/s at the same instant the other
student (student B) throws the ball vertically upward with
same speed. The second ball just misses the balcony
on the way down.
a) What is the difference in their time in the air?
b) What is the velocity of each ball the instant before
it hits the ground?
c) How far apart are the balls 0.8 s after they are
thrown?
What do we know
• Student A for the whole flight
D = -19.6 m, a = -9.8 m/s2, vi = -14.7 m/s
Want t (part a) and vf (part b)
Since solving for t would mean using the quadratic
formula…
Solve for vf first
Identifier is then t so use Vf2 = Vi2 + 2ad
Final velocity = ________
(remember downward motion)
Student A
• Once final velocity is found, use to find t:
• Ant equation with t is possible, simplest is
Vf = Vi +at
t = __________
Student B
• Vi = +14.7, a = -9.8, d = -19.6
• Repeat the procedure done to find vf, t for
student A
• Vf = ________,
t = ________
Part C
To answer part c:
• find the overall displacement between balls
• Find d of each
• D of student A
You know a = -9.8, t = 0.8 s, vi = -14.7 , want d
Final velocity is the identifier
Use d = vit + ½ at2 d =__________
Part C
• D of student B
You know a = -9.8, t = 0.8 s, vi = +14.7 , want d
Final velocity is the identifier
Use d = vit + ½ at2 d =__________
How far apart means add the magnitudes
Newton’s Laws of Motion
• 3 Laws
• Describes how motion is governed by
forces
What is the First Law of
motion?
First Law
• An object at rest, stays at rest
• An object in motion, stays at the same
constant rate
• Unless acted upon by a net outside force
Inertia
• The measure of an object’s resistance to
change in motion
• Related to how massive the object is
• Larger the mass, the more inertia
Auto Accidents
Questions
• Will you be able to continue to hold some
one or some thing in an auto accident
• What will happen to Fluffy if the animal is
in the back window-well of your sedan
during a crash
• What happens to your body in a frontal
crash versus a rear end collision?
What is the 2nd law of motion?
2nd law
• If there is a change in motion
(acceleration)
• Acceleration = net force / mass
• Rewrite as the equation:
Fnet = ma
3rd Law
• For every action force
• there is an equal but opposite reaction
force
• Fist-fight
• Any interactions between objects