Friday 10 February 2012
Class meeting
Topics
Textbook sections
Ponderables
Mini-labs (deliv.)
Lab
Demonstrations
Mini-lectures
Quiz
Other
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PHYS 116 SCALE-UP
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Kinetic energy & work
Chap. 7, sections 7-1 to 7-7
Chancellor’s Jello™
Spring, gravitational and kinetic energies (abstract)
Work & kinetic energy, with CQs
No quiz (exam previous evening)
Mini-lecture on work and energy, with CQs (10 min)
Mini-lab: Spring, gravitational and kinetic energies (60 min)
o Equipment: for each group: projectile launcher, meter stick
Ponderable: Chancellor’s Jello™ (30 min)
CQs on Work and Energy
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Friday 10 February 2012
PHYS 116 SCALE-UP
SPRING, GRAVITATIONAL AND KINETIC ENERGIES
Pre-lab: (submitted via Sakai and written in your notebook) A spring with spring constant k is
positioned vertically and then compressed from its equilibrium length by a distance Δx. A block of mass m
is placed on top of the spring, and is launched into the air when the spring is released. (This is similar to
the projectile launcher you used earlier in the semester and will use in this mini-lab.) To what maximum
height hmax will the block travel? Show how you arrived at your answer.
Deliverable: You are to write an abstract (which would be part of a full lab report were you to produce
one). It is due (submitted via the Sakai site) by the next class meeting.
Explorations
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Using the projectile launcher, shoot a ball vertically and determine the maximum height hmax
reached by the projectile for the two available values of the compression (the low and medium
settings—not the high setting). Make sure you get the zero position for the compression correct!
Based on your pre-lab calculation, how can hmax and Δx be combined into an expression that
remains constant as hmax and Δx change? Do your data agree with this result?
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Friday 10 February 2012

PHYS 116 SCALE-UP
Measure the mass of the projectile and use your measurements of hmax and and Δx to calculate a
value of the spring constant k for your launcher, together with its uncertainty. Then predict the
value of hmax for Δx corresponding to the high compression setting (but do not do the experiment
yourself). We will compare the values obtained by various groups, and an instructor will perform
the experiment.
Applications
1. A block of mass 0.5 kg sliding on a horizontal frictionless surface has an initial speed of 3 m/s when it
encounters a spring that is attached to a wall. If by the time the block comes to rest the spring has
compressed by 20 cm, what is the spring constant k of the spring? What happens to the block
afterward?
2. Consider a projectile launcher placed horizontally on the edge of a frictionless table. When the spring
is compressed by 1.2 cm and then released, the ball lands on the floor at a distance of 1.9 m from the
edge of the table. What compression of the spring is needed so that the ball lands on the floor at a
distance of 2.3 m from the edge of the table?
CHANCELLOR’S JELLO™
To raise money to offset the budget cuts imposed by the Legislature on the University, you want to have
Chancellor Thorp bungee jump from a crane. He agrees to do so if contributions can be found to restore
funding for 10 course sections. To add some interest, the jump will be made from 44 m above a 2.5 m
deep pool of Carolina Blue Jello™. A 30-m long bungee cord will be attached to the Chancellor’s ankle.
First you must convince him that your plan is safe for a person of his mass, 70 kg. Having been a
chemistry major at UNC, the Chancellor took Physics 116 so he knows that as the bungee cord begins to
stretch, it will exert a force which has the same properties as the force exerted by a spring. Your plan
has the Chancellor stepping off a platform and being in free fall for the 30 m before the cord begins to
stretch. You must determine the elastic constant (i.e. the spring constant) of the bungee cord so that it
stretches only 12 m, which will just keep the his head out of the Jello™.
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