chapter 10 energy, work and simple machines energy the ability to produce change in itself or its’...
TRANSCRIPT
Chapter 10
Energy, Work and
Simple Machines
Energy
The ability to produce change in itself or its’ environment.
Energy of Motion:
Kinetic Energy
Kinetic EnergyNewton's 2nd Law
F = maEquation for an object in motion.
v12= v2
2 + 2ad
Fm = a
Kinetic Energy
v12= v0
2 + 2Fdm
v12 - v0
2 = 2Fdm
½m(v12 - v0
2) = Fd
Kinetic Energy
Fd = Work
∆K = Work
Kinetic Energy
½mv12 - ½m v0
2 = Fd
K = ½mv2
ΔK = Fd
WorkApplying a Force
through a distance is WORK!!
Fd = Work ΔK = Work
WorkWork is measured in
Joules(J).J = N•mJ = kg m2
s2
Example 1A 105g hockey puck is sliding across the ice. A player exerts a
4.5N force over a distance of 0.15m. How much work does the player do on the puck? What is the change in the pucks energy?
Example 1
Example 1Given: m=105g=0.105kg
F=4.5Nd=.15m
Eq: W=F dW=(4.5N)x(0.15m)
W=0.68J
Example 2A gardener pushes a lawn mower across a 20m lawn. He applies a 250N force to the handle. If the
handle makes a 30º with the ground, how much work is done
to make one pass across the lawn?
Example 2
30º
F
d
Exam
ple
2
x comp:Fx = Fcos30º
Given: d=20mF=250N at 30º
Fx = 250cos30º Fx = 217N
Eq: W = Fxd W = 217N X 20m W = 4340J
Homework 10-1Practice Problems: 1-5
Page: 227Practice Problems: 6-8
Page: 229Due: 1/27/03
Power – watts - W
Power is the rate at which work is done.
Power = work time
Power
P = W t
P = Fd t
Example 2A farmer lifts a 1.0kg bail of hay 2m. On a good day it takes the farmer 0.5s. How much power
does the farmer produce.
Exam
ple
2
Given: d = 2mm =1kgt = 0.5s
Eq: P = W t
P = mgd t
P = Fd t
P = (1kg)(9.8m/s2)(2m) 0.5s
P = 39.2 W
P = mgd t
Machines
A machine eases the load by changing either the magnitude or the direction of a force as it transmits energy to the task.
Mechanical Advantage
Fr
Fe
Mechanical Advantage
Effort Force ( Fe ) : The force you apply
to the machine.
Resistance Force ( Fr ) :The force exerted by the machine.
Mechanical Advantage
Mechanical Advantage (MA) The number of times the machine
multiplies the effort force.MA = resistance force
effort force
MA = Fr Fe
Ideal Mechanical Advantage
dr de
Fr
Fe
Ideal Mechanical Advantage
Work In = Work Out
IMA = de
dr
The Lever
Fr Felr le
MAlever = effort arm length
resistance arm length
MAlever = le
lr
Pulleys
Fixed Pulley
Movable Pulley
Mechanical Advantage of a Pulley: The number of ropes supporting the resistance weight.
MA = 1 MA = 2 MA = 3
Block and Tackle The arrangement of several pulleys.
Wheel and Axle
rw
ra
MA = radius of wheel radius of axle
MA = rw
ra
Inclined Plane
A slanted surface used to raise objects
l
h MA = l h
Efficiency
Efficiency = Work Out x 100% Work In
Efficiency = Fr x dr x 100% Fe x de
Compound Machine
MA = MA1 x MA2
1 2
Homework 10-3Practice Problems: 13-16
Page: 238Section Review: 1-4
Page: 238Due: 1/30/03
Homework 10-4Problems: 31,33,37,
40, and 41Pages: 242-245Due: 1/31/03Test: 2/4/03
Homework 10-5Problems: 48, 49, 50,
52, and 53Pages: 242-245
Due: 2/3/03Test: 2/4/03