Park Physics with Sean Lally - 2017-2018

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.    Consider this:   if you lift up a weight (m