Announcements. Applied Physics

Transcription

1 Announcements Applied Physics Conservation of Energy Charles Dickens (196) The Energy Conservation Quiz went up this morning at 8:00 am. Don t forget to print a copy of next weeks lab. Wear comfortable shoes to class next week. We will be going up and down stairs to collect data. Homework Problems: 4.11, 4.12, 4.13, 4.14, 4.19, 4.20 KWEISI MFUME Black History Month Speaker Lecture Topic: Gender, Race, and Public Opinion Date: Thursday, February 7 Time: 7:00 p.m. Location: Don Reynolds Performance Hall Average = 69 Median = 69 Maximum = 100 Exam 1 Considering this year's Presidential election, this is a very hot topic. The lecture is FREE and open to the public. 4 2 Sponsored by: SPBC, SAB, SGA, Honors College, President s Office, and Minority Student Services

2 Types of Energy Available Mechanical Energy Work: F x distance (parallel to the force) W = Fd Kinetic: KE = (1/2)mv2 Other Thermal Chemical etc. Gravitational Potential: PE = mgh Units Again SI Units (kg m2 )/s 2 = J Other common units temperature of 1 gm of water by 1 o C cal = J calorie the amount of energy needed to raise the Food Calories Cal = kilocalories = 4186 J Work Work = Force x Distance An object must move for Work to be done. The distance must be parallel to the direction of the force. If the force and the distance are in the same direction the work is positive. If the force and the distance are in opposite directions the work is negative. Q01 How much work do you do by pushing a 5 kg box with a force of 10 N for a distance of 2 m?

3 Q02 In order to stop a moving 3 kg box. You have to apply a force of 20 N in the opposite direction of the motion of the box. It takes you 1.5 m to stop the box. How much work has been done? Kinetic Kinetic energy is the energy of motion. Anything moving has Kinetic energy. KE = (1/2)mv2 KE is always positive Consider the effect of speed on KE, what happens if you double the speed? What happens if you halve the speed? What happens if you double the mass? Q03 How much kinetic energy does a 7 kg object have if it is moving to the right at a speed of 3 m/s? Q04 How much kinetic energy does a 7 kg object have if it is moving to the left at a speed of 3 m/s

4 Potential Energy (Gravitational) PE is the energy of position. It comes in various forms; we will look at only gravitational for now. PE = mgh PE can be positive or negative depending on the choice of reference point. The amount of PE is never important!!!!!!!!! The amount of change in PE that is available is what is important. Q05 Relative to the top of a table, how much gravitational potential energy does a 2 kg weight have if it is 4 m above the table? Express your answer to the nearest Joule. Conservation of Energy Q06 How much potential energy would the 2 kg object lose if it fell from 4 m above the table to the floor, which is 1.5 m below the table? Express your answer to the nearest Joule. Energy can not be created or destroyed. The total energy of a closed system is constant. Determine the amount of energy a system starts with. Determine the amount of energy that comes into or goes out of a system. Together these must equal the energy that the system ends with.

5 The Equation: KEi + PEi + W + OE = KEf+ PEf Example Problem Suppose a 50.0 kg teenager picks up an 11.0 kg stereo and raises it m from the floor to a tabletop. (a) How much work did he do on the stereo? (b) How much work did he do to raise his own center of mass m in the process? (a) (b) W+ 0 = 0 + mgh W = (11.0kg)(9.8m/s 2 )(0.750m) W = J W+ 0 = 0 + mgh W = (50.0kg)(9.8m/s 2 )(0.400m) W = J Another Example Problem How far can an 85.0 kg baseball player slide if his initial speed is 7.00 m/s and the force of friction between him and the ground is 450 N? 0.5mv fd + 0 = mv 2 = fd (0.5mv 2 )/f = d (0.5)(85.0 kg)(7.00m/s) 2 /(450 N) = d 4.63 m = d Problem: Q07 Is the skier moving at the beginning of this problem? In other words, is there KE at the beginning?

6 Problem Q08 Does the skier start out higher or lower? In other words, is the PE greater at the beginning? Problem Q09 Is there any force doing work as the skier moves from the top of the hill to the bottom? Problem Q10 Is there any Other Energy used as the skier moves from the top of the hill to the bottom? Problem Q11 Is the skier moving at the end of this problem? In other words, is there KE at the end?

7 Problem Q12 Does the skier end up higher or lower? In other words, is the PE greater or less at the end? (Greater PE) (Less PE) Another Example Problem 0.5mvi 2 + mgh = 0.5mvf mvi 2 + mgh = 0.5mvf 2 0.5m 0.5m 0.5m vi 2 + 2gh = vf 2 (vi 2 + 2gh) 1/2 = vf [(5.0 m/s) 2 + 2(9.8m/s 2 )(10.0 m)] 1/2 = vf 14.9 m/s = vf