Hello Honors Students, Day 37! It's a great day for physics...even when I am home sick.
Your First Goals for Today:
1) To understand the idea of Energy and WORK
2) To apply the ideas of Energy and work through examples.
Take a look at the sections included, and see if you understand the things I ask from each. This is still a
learning day, even though I am out. We don't have time to waste in physics :)
You should be able to recognize the difference between POTENTIAL and KINETIC energy at this point,
and set up a table like we saw before in our example.
WORK SECTION:
Goal: To identify how to find the WORK from the change in energy or from a force. Get out your
flowcharts.
When you lift a book from the floor, you exert a force on it over a distance. When you push a crate
across the floor, you also exert a force over a distance. The application of a force over a distance is
called WORK, and results in a change in the energy of the system.
Idea: The Work is the change in energy type. You can have a + or - work.
Equations: Work is in Joules. W = Fd Where W is the Work from a force, F is the amount of Force, and d
is the distance it traveled. NOTE: If the force is OPPOSITE motion, Work is NEGATIVE.
Work = Change in KE
AND
Work = Change in GPE
Example One: You push on the wall with a force of 600N. Did you do work?
Answer: Nope! It did not move, so no d, no force.
Example Two: You slowly lift a 2kg book at constant velocity of 3m. How much work did you do on the
book?
Idea: AT CONSTANT VELOCITY, the forces are BALANCED. A weight of 20N is down, you push 20N up.
So the FORCE is 20N of you, and the distance is 3m. 20*3 = 60J. Positive because you pushed up and it
went up.
How much work did the Weight (gravity) do? Well, its force is 20N down, and it still went 3m, so 60 J.
But it is negative because Weight is down and the book went up.
AT THIS POINT: You should be OK with the idea of Work with forces, and have the basics of how to
determine if work is + or -.
For Evidence of What You Should Be Able to Do:
Try them both, and see how well you do
Results:
Notice the type of questions asked. Both mathematical and conceptual. For 10 especially, in this isolated
system the TOTAL energy (called SYSTEM ENERGY) is the same at each point. The kinetic and potential
makeup of that amount changes, but the total is the same.
CASE 1 for both clues involves a table (Either MOMENTUM or ENERGY)
Example One:
Example 2:
So Now: Look at the following problems. DONT SOLVE THEM YET! Instead, look at each one and write if
you use MOMENTUM/IMPULSE 1 or 2, or ENERGY/WORK 1 or 2 from earlier to get it.
1) Victoria, my 5.45kg cat likes to climb to the top of my bookcase. If she fell, she would be
going 5.69m/s right before she hit the ground.
a. How tall is the bookcase?
b. How much work was done to Victoria for the fall?
2) On another occasion, Victoria (still 5.45kg) jumps onto an uncovered litter box at a
speed of 4m/s and makes a .1m deep impression in the litter before stopping.
With what force did Victoria hit the sand?
3) An angry panda is playing tennis. A .45kg ball is moving right at 15m/s and is then hit left
at 16m/s by the panda. If the ball hits the racket for .4 seconds, what is the force on the
ball?
4) A tiny ball is on a string starts 1.2m in the air. It swings down and comes up again. How
fast is it moving .9m off the ground?
5) While playing around with your best friend, you throw a .2kg ball at 6m/s towards their
3.63kg stationary head. The ball bounces backwards at 4m/s.
a. How fast is your friends head moving after?
b. If the ball was in contact with their head for .15 sec, how much force was
present?
6) About 50,000 years ago a 4.5x107 kg asteroid hit the Earth and created the Barringer
crater, which is 180m deep. The meteor hit the Earth (5.9742x10 24 kg) at 20,000 m/s
according to some data.
a. How much force did the meteor hit the Earth?
Do that, then try Number One AND JUST ONE for a numerical answer. The rest are practice IF YOU
WANT.