Park Physics with Sean Lally - 2017-2018

In these problems you will need the formulas for the period of a simple pendulum AND the rearranged version (solving for L) we worked out in class. 1.  What is the period of a 1-m long pendulum?  Do you remember the historical significance of this particular length of pendulum?  If not, look back in your notes from the first few days of class. 2.  If you want a Grandfather clock to swing with a 1-second period, how long should the pendulum be inside of it? 3.  Calculate the experimental length of the giant swing, assuming that someone took 2.8 seconds to swing from rest on the platform to the opposite side of the swing (on the left).

1.  Graph period of ONE swing (T, in seconds) vs. length (L, in meters).  T will be on y-axis and L will be on x-axis.  Make certain that length is converted to meters (if you used cm). 2.  Print out graph. 3.  Make a third column of data - square root of each length measurement (in meters). 4.  Think/write about this:  What makes a simple pendulum "simple"?  What is assumed to be true about the simple pendulum? 5.  If you have the video from the giant swing, try to determine the period of one oscillation (one complete swing of the person).  We are going to try to compare theoretical pendulums to the giant swing in our next class.

Review the notes on the blog (2 posts ago) and try these questions and problems (similar to what we did in class today, Monday 4/2). 1.  Give an example of 2 different conservation laws. 2.  What is the mathematical definition of momentum? 3.  What is the conservation of momentum, and when do we think about it (under what specific circumstances)? 4.  A 300-g cart moving at 40 cm/s hits another 300-g at rest.  Friction is very low.  What do you expect will happen exactly?  Give numbers if possible. 5.  A 60-kg figure skater skating at 8 m/s collides directly with a 40-kg skater initially at rest.  Friction is very low and the two become intertwined on the ice.  Calculate the speed of the two of them together after the collision.  (Hint:  First find the momentum of the 60-kg skater, and then use conservation of momentum to solve for the final speed of the two together.  This is like the velcro cart collisions.) 6...

https://youtu.be/HSLxNHPVtxY See also the previous blog post, for reading if you have time.

MOMENTUM! We come up with momentum through Newton's 2nd Law: F = ma F = m  D v/t Rearranging: Ft = m  D v So this means that (on the left side) Force x time = (on the right side) mass x change in speed. Or simply: Ft = mv This interaction of force happens during a so-called "collision" or "explosion". Ft is defined as "impulse" and mv is defined as "momentum."   Note that the above equation helps explain how boxing gloves and airbags work – they increase the time of contact, thereby decreasing the average force delivered to the object. Please watch these clips: https://www.youtube.com/watch?v=3vDQm0qIYi0 https://www.youtube.com/watch?v=y3InF19dzlM Momentum  (which is the mv part of the above equation) is another way to describe (mathematically) how objects collide or explode – generally involving contact of some sort. p = mv No new unit here, just:   kg-m/s Momentum is...

Solve these problems using the energy concepts. 1.  A lacrosse ball (0.15 kg) is thrown straight upward at 14 m/s.  How high does it go (max height)? 2.  A brick of unknown mass is dropped from 3-m above the ground.  Find the speed it has right before hitting the ground.  (Recall that the mass does not matter - show this in the calculations.)

These will NOT be collected, but an assignment given Wednesday afternoon WILL be collected during class on Friday. 1.  Define energy. 2.  Define PE and KE . 3.  What is the conservation of energy principle? 4.   Consider a 5-kg bowling ball that has been raised to 2-m above the ground.  Find the following: a.  initial PE b.  final KE right before it hits the ground c.  PE and KE right in the middle of the drop (1-m above the ground) d.  speed right before hitting the ground - treat this as an energy problem e.  does the mass matter for part d?  Explain. You may let g = 10 m/s/s for this problem, if you like.

Work, W – defined as the product of force and displacement W = F d The units are new:   newton-meter = joule (J) Work is part of the energy concept – a book-keeping technique for keeping track of real changes in a system.   For example, you “do work” on something and that work shows up as (or “transforms into”) a type of energy. Energy – the ability to do work There are several types of energy:   kinetic, potential, chemical, electrical, nuclear, geothermal, hydroelectric…… all of these can be quantified.   In essence, energy is a way of mathematically encapsulating some other manifest changes.   For example, kinetic energy (KE) is a way of describing an objects mass and velocity; potential energy (PE) describes an objects weight and location. KE = ½ mv 2 PE = mgh The units of energy are the same as work:   joules. Conservation of Energy Energy is never created or destroyed – it only changes forms.    Cons...

1. Consider 3 light bulbs, with equal resistance (30 ohms each).  They are connected to a 9-V battery. A.  Where will the total resistance of the circuit be greatest - if the bulbs are in series, parallel, or is it the same in both cases? B.   Where will the bulbs "kill" the battery soonest - if the bulbs are in series, parallel, or is it the same in both cases? C.  Full out complete tables for both cases:  when the bulbs are in series and when they are in parallel. 2.  Repeat 1C above for the case of 2 resistors (30 and 60 ohms) with a 120-V battery.  Determine everything we care about for the series and parallel cases. More practice can be created here: https://www.explorelearning.com/index.cfm?method=cResource.dspDetail&resourceID=398 Launch the "gizmo" (click on the picture) and build circuits by clicking and dragging components.  You can then drag an ammeter onto the wire and it will give you the current.  Y...

Series: 4 resistors (1, 2, 3, 4 ohms) are in series with a 50-V power supply.  Find: - total resistance - total current - all voltages Draw a schematic for this problem, too. Parallel: 3 resistors (5, 10, 15 ohms) are in parallel with a 30-V power supply.  Find: - voltages - currents - total current - total resistance Draw a schematic for this problem, too.

Answers are at the bottom. 1.  Consider 2 resistors (6 and 12 ohms) in parallel with a 24-V battery.  Find the following: - schematic for the circuit - voltage and current for each resistor - total resistance for the circuit  2.  Consider 3 resistors (2, 4, 8 ohms) in parallel with a 48-V battery.  Find the following: - schematic -voltage and current for each resistor - total resistance Answers: 1.  24-V for each; 4 and 2 A; 4 ohms total 2.  48-V for each; 24, 12, and 6 A; 48/42 = 1.14 ohms

1.  The total resistance of resistors in series is their sum. R Total = R 1 + R 2 + … 2.  Current in a circuit is constant from the battery.  If resistors are in series, they all have the same current as the battery delivers.  Calculate this by: I = V/R 3.  The resistor voltages in a series circuit add up to the total battery voltage. Example problem from class: R1 = 1 ohm R2 = 2 ohms R3 = 3 ohms If all 3 resistors are in series with a 12-V battery, the total current is 2-A.  The individual resistor voltages are:  2-V, 4-V, and 6-V.  Note that these add to 12-V. Try this problem: 3 resistors (4, 5, and 3 ohms) in series with 36-V.  Find the following: - total resistance of the circuit - current through each resistor - voltage over each resistor

Circuit questions to TURN IN ON FRIDAY (in class) 1.  Describe the difference between voltage, current, and resistance.  Give the proper units, too. 2.  What is the resistance of a light bulb that allows 2 A of current through it when connected to a 12-V battery? 3.  A 5-ohm resistor is connected to a 10-volt battery. What current goes through the resistor? 4.  You have 2 bulbs in series (one right after the other) - remove one (unscrew it) and what happens to the brightness of both bulbs?  (See picture below.) 5.  You have 2 bulbs in parallel (both connected to the same voltage source) - remove one (unscrew) and what happens to the brightness of both bulbs?  (See picture below.) 6.  Draw a schematic for 2 resistors in series with a battery and switch, using the correct symbols. 7.  Recall the recent mini-lab we completed where we compared current to resistance.  Draw an approximate graph of current vs. resistance, ass...

Be prepared to start building your structure this Wednesday - 1-17-18. Bring any materials you will need for building.

Folks - so sorry to be out sick today.  Here's what I'd like you to do. First, compare your definitions of current and resistance at your table.  Then read mine and take notes. After this, you may work (with your partner, if you have one) on the electrical structure project.   I'll be available during lunch tomorrow for anyone who needs to meet.  I'll also check email throughout the day. Thanks. // Circuits 1 Thus far, we have only discussed "static" (stationary) charges.  Static charges alone are useful, but not nearly as much as charges in motion.  As you recall, electrons are the most easily moved particles.  However, for sake of ease in sign convention (positive vs. negative), we define the following: Current  (I) - the rate at which positive charge "flows" I = Q/t The unit is the coulomb per second, defined as an  ampere  (A).  Just as one coulomb is a huge amount of charge (nearly 6.3 billion billion protons), one ampere (or a...

Make sure that you recorded today's definition of voltage. Next, read up a little about the definitions of these two concepts.  Bring a brief definition, and ideally, something else in your own words. - current - resistance Happy New Year, physics phriends!

https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Charge HW to submit on Friday 1.       What exactly IS charge?  How do we think of it?  How does it relate to protons, electrons, etc.?  A way of quantifying how particles interact with each other, based on the amount (and imbalance) of protons and electrons.  Neutrons have zero charge.  Electrons have opposite charge relative to each other. 2.       Which particles in an atom are easiest to move, and why? Electrons, as they are "around" the nucleus of the atom. 3.       What does atomic number (H = 1, He = 2, etc.) mean? The number of protons in the nucleus of the atom.  4.       What are quarks  Fundamental particles, incapable of being broken up further.  There are 6 types, 2 of which (up and down) make up protons and neutrons. 5.       Coulomb’s law is an “inverse square law” – what...

https://www.surveymonkey.com/r/BQXSP6M

Intro to electricity Electricity I - Charge! Charge - as fundamental to electricity (& magnetism) as mass is to mechanics Charge is a concept used to quantatively related "particles" to other particles, in terms of how they affect each other - do they attract or repel?  If so, with what force? Charge is represented by letter Q. The basic idea - likes charges repel (- and -, or + and +) and opposite charges attract (+ and -). Charge is measured in units called coulombs (C).  A coulomb is a huge amount of charge, but a typical particle has a tiny amount of charge: - the charge of a proton is 1.6 x 10^-19 C.  Similarly, the charge of an electron is the same number, but negative, by definition (-1.6 x 10^-19 C).  The negative sign distinguishes particles from each other, in terms of whether or not they will attract or repel.  The actual sign is arbitrarily chosen. The charge of a neutron is 0 C, or neutral. But what IS charge? Charge is a bit difficul...