1 PHY131H1F - Class 9 Today, finishing Chapter 5: • Kinetic Friction • Static Friction • Rolling without slipping (intro) • Drag Microscopic bumps and holes crash into each other, causing a frictional force. Kinetic Friction creates internal energy (thermal). A box is being pulled to the right at steady speed by a rope that angles upward. In this situation: A. n > mg. B. n = mg. C. n < mg. D. n = 0. E. Not enough information to judge the size of the normal force. Clicker Question
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PHY131H1F - Class 9 - U of T Physicsjharlow/teaching/phy131...1 PHY131H1F - Class 9 Today, finishing Chapter 5: •Kinetic Friction •Static Friction •Rolling without slipping (intro)
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PHY131H1F - Class 9
Today, finishing Chapter 5:
• Kinetic Friction
• Static Friction
• Rolling without slipping (intro)
• Drag
Microscopic bumps
and holes crash into
each other, causing a
frictional force.
Kinetic Friction
creates internal
energy (thermal).
A box is being pulled to the
right at steady speed by a
rope that angles upward. In
this situation:
A. n > mg.
B. n = mg.
C. n < mg.
D. n = 0.
E. Not enough information to judge the size
of the normal force.
Clicker Question
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Test 1 Marks Posted
You should now see your mark
out of 32 under “My Grades” on
portal.
The raw average was 19/32 =
59%.
The instructors in this course
have decided to adjust the marks
by adding 2 points to every
student’s mark, but holding the
maximum mark to 32/32 = 100%.
The mark posted on portal is your adjusted mark.
The adjusted average and median were 21/32 = 66%.
After the adjustment, 263 students (23% of the class) got less than
16/32 (fail).
After the adjustment, 72 students (6% of the class) got 32/32 =
100%.
Test 1 Marks Posted
You will receive the long-answer part of your test in
Practicals between today and Oct.20.
The multiple choice bubble sheet is stored in MP129 – if you
would like to review it, please stop by and ask April.
Check over the marking and compare it to the Test 1 Marking
Scheme posted on the portal.
If there has been a mistake in the marking or computation of
your mark, please bring the test to me or April Seeley in
MP129 and we will fix the mistake.
The deadline to report mistakes in the marking of Test 1 is
• Air resistance, or drag, is complex and involves fluid dynamics.
• For objects on Earth, with speeds between 1 and 100 m/s and size between 1 cm and 2 m, there is an approximate equation which predicts the magnitude of air resistance
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drag AvCF
where A is the cross-sectional area of the object, ρ is the
density of the air, C is the drag coefficient, and v is the
speed.
• The direction of air resistance, or Drag Force, is opposite
to the direction of motion.
• It depends on size and shape, but not mass.
Drag force in a fluid, such as air
• Air resistance, or drag, is complex and involves fluid dynamics.
• For objects on Earth, with speeds between 1 and 100 m/s and size between 1 cm and 2 m, there is an approximate equation which predicts the magnitude of air resistance
Which falls faster, a piece of paper that is uncrumpled, or
crumpled?
A. Uncrumpled (flat)
B. Crumpled
Why??? What is different?
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21
drag AvCF
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Terminal Speed
• Fdrag increases with speed
• Net force goes to zero when the object is moving fast enough so that Fdrag = mg(air resistance = weight)
• Then no net force
No acceleration
Velocity does not change
Non-Free Fall—Example
• A skydiver jumps from plane.
• Weight is the only force until air
resistance acts.
• As falling speed increases, air
resistance on diver builds up, net force
is reduced, and acceleration becomes
less.
• When air resistance equals the diver’s
weight, net force is zero and
acceleration terminates.
• Diver reaches terminal velocity, then
continues the fall at constant speed.
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Consider a heavy and light person jumping
together with same-size parachutes from the same
altitude. Who will reach the ground first?
A. The light person.
B. The heavy person.
C. Both will reach at the same time.
D. Not enough information.
Clicker Question
Free Fall vs. Non-Free Fall
Coin and feather fall with air present• Feather reaches terminal velocity very
quickly and falls slowly at constant speed, reaching the bottom after the coin does.
• Coin falls very quickly and air resistancedoesn’t build up to its weight over short-falling distances, which is why the coin hits the bottom much sooner than the falling feather.