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WELCOME TO MR. T’S CLASS!
• A few observations to get us started.
Dear Students,
I know when you are texting in class. Seriously,
no one just looks down at their lap and smiles for
no reason.
Sincerely, Mr. T
• ~ Expectations ~
REVISITING NEWTON’S
AND 2ND LAWS
SPH3U
Forces / Dynamics Unit
ST
1
REVIEW
• What is the first law?
• An object will move at constant velocity unless
acted upon by an external, unbalanced net
force.
• What is the second law?
• When an external, unbalanced net force acts on
an object, the object will accelerate in the same
direction as the force.
SAMPLE PROBLEM:
• A two man canoe team is practicing to compete
in a race. One man has a mass of 70 kg, the
other a mass of 75 kg, and the canoe has a
mass of 20 kg. One can exert an average force
of 400 N [forward] and the other 420 N [forward]
on the canoe using the paddles. The water
resists the motion with a force of 380 N. What is
the initial acceleration of the canoe?
VERTICAL MOTION:
• A skydiver is jumping out of an airplane. The air
resistance acting on the skydiver during the first
few seconds of the jump is 251 N. The
acceleration of the skydiver during this time is
5.96 𝑚 𝑠 2 [down]. Calculate the mass of the
skydiver.
IN GROUPS OF 2 OR 3, FIND THE SOLUTION:
• In the 1978 movie “Superman,” there is a scene where
Lois Lane falls from a helicopter crash at the top of a
building. She falls from rest for 9.00 s before Superman
catches her. a) Ignoring air resistance, determine her
velocity (in km/h) at the moment Superman catches
her.
• (320 km/h is the answer, but as a note, because of air
resistance she would probably have reached an actual
terminal velocity of about 200 km/h)
• b) In 2.0 s Superman has brought her 47 kg body to a
complete stop. Determine the average force he must
have exerted on her body to accomplish this, and
explain how you think this would affect her.
• (2100 N. Although this amount of force would not be
lethal, it would likely cause her some injuries.)
WHAT IS G-FORCE?
• The g-force (with g from gravitational)
associated with an object is its acceleration
relative to free-fall.[1][2] This acceleration
experienced by an object is due to the vector
sum of non-gravitational forces acting on an
object free to move.
•
From http://en.wikipedia.org/wiki/G-force
Standing on the Moon at its equator
Standing on the Earth at sea level–standard
Saturn V moon rocket just after launch
Bugatti Veyron from 0 to 100 km/h in 2.4 s
Space Shuttle, maximum during launch and reentry
High-g roller coasters[8]:340
0.1654 g
1g
1.14 g
1.55 g†
3g
3.5–6.3 g
Top Fuel drag racing world record of 4.4 s over 1/4
mile
4.2 g
Formula One car, maximum under heavy braking
5+ g
Luge, maximum expected at the Whistler Sliding
Centre
5.2 g
Formula One car, peak lateral in turns [20]
5–6 g
Standard, full aerobatics certified glider
+7/-5 g
Apollo 16 on reentry[21]
7.19 g
Typical max. turn in an aerobatic plane or fighter jet
9–12 g
Maximum for human on a rocket sled
46.2 g
Death or serious injury likely
> 50 g
Sprint missile
100 g
Brief human exposure survived in crash[16]
> 100 g
Space gun with a barrel length of 1 km and a
muzzle velocity of 6 km/s,
as proposed by Quicklaunch (assuming
constant acceleration)
1,800 g
Shock capability of mechanical wrist
watches[22]
> 5,000 g
Current formula one engines, maximum
piston acceleration [23]
8,600 g
Rating of electronics built into military
artillery shells[24]
15,500 g
9 × 19 Parabellum handgun bullet (average
along the length of the barrel)[25]
31,000 g
9 × 19 Parabellum handgun bullet, peak[26]
190,000 g
Mean acceleration of a proton in the Large
Hadron Collider[27]
190,000,000 g
• A package of mass 10.0 kg is being lowered by
a rope at an acceleration of 1.50 m/s 2 [down].
Calculate the tension in the rope. Use g = 9.81
m/s2.
a) 83.1 N [down]
b) 83.1 N [up]
c) 113 N [down]
d) 113 N [up]
e) 2.48 N [up]
• A sled of mass 12.0 kg has an initial velocity of
6.00 m/s [forward] and slows to a stop due to
friction after a displacement of 3.00 metres.
Determine the coefficient of friction between the
sled and the ground.
a) 0.412
b) 0.512
c) 0.612
d) 0.712
e) 0.812
• A 15.0kg box is sitting on the floor. The coefficient
of static friction is 0.40 and the coefficient of kinetic
friction is 0.30.
• a) Sketch the free body diagram of this box
(assume for only this question that it is moving).
• b) Determine if the box will move if I push with a
force of 20N. If it does move, determine its
acceleration.
(Since static friction can be as high as 59N, no it will not move.)
• c) Determine if the box will move if I push with a
force of 70N. If it does move, determine its
acceleration.
(Accelerates at 1.7 𝑚
𝑠 2 .)