Physics 113 –Morning Session, Summer 2007
Participation Points #3
To be turned in at the end each
discussion session, stapled on top of Participation Points #2 and #1
Student’s name _____________________ Total Previous Points
accumulated:
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-You
should try to answer these questions before the instructor goes over them.
-TA (grading, tutoring, solution manual,…)
information:
-Tutoring
schedule (in room S-4-073):
5.4- Free-body diagram.
Draw the free-body diagram for the ski racer down a frictionless slope of angle
θ with a horizontal force holding him in place. Also draw a convenient
reference frame in this case (most vectors pointing along its axes). Then write
down the equations along the two axes.
5.5- Free-body diagram.
Draw the free-body diagram for the ski racer going down a slope of angle θ
and coefficient of kinetic friction μk. Choose a convenient
reference frame (most vectors pointing along its axes). Then write down the
equations.
5.6- Circular Motion. Mass
m1 going in a circular motion on a flat table, it is connected via a
rope of negligible mass to mass m2 through a hole at the center of
the circular path, hanging under the table. Draw the free-body diagrams for the
two masses, and indicate which force is producing the radial acceleration of m1.
Note that we use only inertial frames and there is no such centripetal force in
these frames.
6.1- Work
and Kinetic Energy. If I jump from an aircraft
or from a table, the gravity force on me is the same, mg, so why is it that I
would get hurt (or…) in the first case and not the second. One observation, the
speed I will reach just before hitting the ground is much larger in the first
case (the terminal speed, from the example we did, ~64m/s) than in the second
case (~10m/s if it takes 1s). So why a larger final speed leads to greater
damage? Because we all stop when hitting the ground, when kinetic energy has to
convert into work against the ground, and you know that work is force times the
“stopping distance”. Since this distance is very small, the force applied on
the ground is huge and so is the reaction force on me! The force increases as
the final speed squared.
Assume you are
“parachuting” into the ocean and the stopping distance is 1m, compare the
stopping forces when you enter it vertically and at a 300 angle with
the horizontal.
6.2- Varying
Force. A spring with spring
constant k=200N/m. How much work does it take to stretch the spring (a) 10cm
from equilibrium (b) from 10cm to 20cm from equilibriums.
6.3- Power. The power output of a runner can be modeled as
P=m(bv-c), with m and v the runner’s mass and speed, and b=4.27 J/kg/m and
c=1.83 W/kg. Find the work done by a 60 kg runner who runs a 10 km race at a
speed of 18 km/h.
7.1- Conservation
of energy. The catapult:
height when vertical (when the ball will leave the spoon) is 1.1m; weight mass
is 1kg, ball mass is 0.1 kg. Calculate the initial height for the weight to hit
a target 2.8m away
7.2- Conservation
of Mechanical Energy. A
falls sliding down a frictionless track from a height h (see figure). What is
the minimum h such that it can make it around the loop?
7.3- Potential Energy Curves. A particle fall
sliding down a frictionless track shown, from point A. Give its speed at points
B and approximate location of its right hand turn-point.