Transcript P221_2009_week5

Extra Office Hours/Help
• EXAM I is on Wednesday in class!!!
• Chapter 1 – 6. One 8.5”x11” sheet of paper (one
side) allowed for notes.
• Tomorrow’s discussion sections with Aditi will be
help sessions for the exam: come with
questions!
• Office hours:
– DVB Monday 3:00 to 4:00.
– Aditi Raval Monday forum: 1:30 to 2:30
SW 331: 2:30 to 3:30
– DVB Tuesday 3:00 to 5:00.
DVB- Review/Summary
(* -> topics that I think we have emphasized the most)
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UNITS, (dimensional analysis and checking your answers)
Newton’s Laws: F=ma ; Free body diagrams*
Interpreting graphs
Kinematics*: Big 3, def’s of a(t),v(t) etc., free fall
Vectors: components*, adding*, products (. & x)
2-D motion*: Projectiles, relative motion,
centripetal acceleration
• Friction and Drag
• To date we’ve had 9 lectures covering new
material, look carefully at each an glean the 2-3
key points, write review questions, …
DVB- Exam details
• NEXT WEDNESDAY (11 Feb.)
• 4 multiple choice questions followed by 2 multipart (4 or 5) problems for a total of 13 individual
questions. Partial credit is available for all.
• If part b uses answer from part a and a is wrong
you can still get full credit for b!!
• Some of the 13 are very straight-forward, a few
are more challenging.
• Show your work and use a PEN not a pencil!!
• Questions at back of chapter and CALM are
pretty good practice in addition to the problems
from the chapters.
DVB’s Formula sheet
• Newton II and III
• K1, K2, K3; g = 9.80 m/s2
• Vector components (if you don’t know trig
well), dot and cross products (2 each).
• Centripetal acceleration
• Friction: Static, kinetic, drag
• Definitions of instantaneous and average
velocity and acceleration.
• VAB = VAC + VCB NOTE: VCB = - VBC
• Any notes I’d need on drawing FBD’s
CALM suggestions
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Free Body Diagrams (8)
Drag/friction (8)
Vectors (7)
Relative Motion (5)
Uniform Circular motion (4)
Kinematics (3)
Projectile Motion (3)
17 no-answer
Sample Exam question
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A soccer player kicks a ball with an initial velocity Vo at
an angle of 18.0o above the horizontal and 18.0 m from
the goal line. The ball passes the goal line exactly
0.920 seconds after leaving the player’s foot. Neglect
drag throughout the ball’s
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What is the acceleration (magnitude and direction) that the ball
experiences as it crosses the goal line (but before it hits the
net at the back of the goal) ?
What is Vo (the magnitude of the initial velocity at which the
ball was kicked)?
How far above the ground is the ball when it crosses the goal
line?
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6-
Chapter 6 Examples
Assume
throughout that
the rotation is at
a constant rate.
Q0: What is the
direction of the net
force on the ball?
4Vag
Vpg
Vpa
Let’s take the x axis to point East and the y axis to point North, and consider
the diagram shown above. Then we have:
|Vpa| = 500kph ; Vpa = [500 sin(20) i + 500 cos(20) j ] kph.
The problem tells us that the ground velocity is Vpg = (800km/2hr) j = 400 kph j
But we know that the velocity of the plane with respect to (wrt) the ground
must be the VECTOR sum of its velocity with respect to the air (VPa) and the
velocity of the air wrt the ground (i.e. the wind speed and direction which is
what the question is asking us to find). Write the vector sum:
400 kph j = [500 sin(20) i + 500 cos(20) j ] kph + Vag
So we have that the wind must have components: Vx= -500 sin(20) = -171 kph
and Vy = 400 - 500cos(20) = - 69.8 kph.
Therefore the wind speed is 185kph and it is in the direction 22.2o S of W
More Chapter 4 and 5 Examples
5-
Chapter 6 Examples
Chapter 7 examples
CALM:
Estimate the amount of work performed on a 1200
kg car as it accelerates from rest to 1.2m/s2 over
a distance of 150 m. (poorly phrased question)
(25 correct, 4 incorrect; 26 no ans.)
• The square root of 2a(150) equals 19 as the
velocity. 0.5 times 1200 kg times 19 equals,
216600 Newtons (Correct number, incorrect
units, and confusing description)
• W=F*d F=m*a F=1200kg*1.2m/s2=1440N
d=150m W=1440*150=216000J (correct, if one
assumes that the force is constant)
• A weight-lifter performs the following three steps in his
lift: 1). He raises the barbell to his chest 2). He holds it at
his chest for 10 sec. 3). He lowers the barbell back to the
ground. Rank algebraically the work performed by
gravity in each of the three steps (label them W1, W2
etc.). Repeat the question for the work performed by the
weight-lifter. (15 correct; 7 incorrect; 5 uncertain; 28 no
answer)
• 1) W1 moves the bar only in a y-direction, his force of
pulling bar up is greater than the force of gravity pulling
the weight down. 2)W2 the lifter feels the force of gravity
pulling the bar back down. thus to keep the bar from
going down he must provide a normal force to keep it
from moving for 10 seconds. 3) W3 the lifter only puts
the bar back down to the ground, so only force working
is directly down thanks to gravity (the answer is ???)
• W1=mgh W2=-1/2g(100) W3=-mgh (1 and 3 are true for
the lifter, but these appeared to be for gravity, and W2 is
wrong).
Chapter 7 examples
B). What is the mass of the block? C). How fast is it moving after it has
traveled down the 0.50m under the influence of the force Fr, assuming that
it started from rest?