Download Chapter 10 Energy Work Simple Machines

Chapter 10
Energy Work Simple Machines
DEMO: What factors affect energy P257
10-1
Energy and Work
WORK—is equal to the constant force exerted on an object in the direction of work times the objects
displacement
W  F d
UNITS: Nm = Joule (J)
Kinetic Energy—is equal to ½ mass of the object times the speed of the object squared.
KE 
1 2
mv
2
UNITS: kgm2/s2 = joule (J)
Work ENERGY Theorem—work is equal to the change in kinetic energy
W  KE
Kinetic energy is energy of motion. The kinetic energy of an object is the energy it possesses because of its
motion. The kinetic energy of a point mass m is given by
Energy as the capacity for doing work is a convertible currency. To give something kinetic energy you
must do work on it. This development uses the concept of work as well as Newton's second law and the
motion equations. It is a special case of the work-energy principle, a powerful general principle of nature
CALCULATION OF WORK:
.
Forces in Car Crashes
Stopping Distance for Auto
Assuming proper operation of the brakes, the minimum stopping distance for an
automobile is determined by the effective coefficient of friction between the tires and the
road. The friction force of the road must do enough work on the car to reduce its kinetic
energy to zero (work-energy principle). If the wheels of the car continue to turn while
braking, then static friction is operating, while if the wheels are locked and sliding over
the road surface, the braking force is a kinetic friction force.
To reduce the kinetic energy to zero:
so the stopping distance is
Show calculation
Note that this implies a stopping distance independent of vehicle mass. It also implies a
quadrupling of stopping distance with a doubling of vehicle speed.
Even though the stopping distance for a large
truck should be the same as that for a car, there
are real-world cases where a truck has more
difficulty stopping. Runaway truck ramps are
placed on long downgrades like this one on
Monteagle in Tennessee. Deep, loose gravel
provides a stopping force greater than that
obtained from just a tire against the road.
Stopping a Truck
Suppose a large truck and a small truck are
moving with equal momentum. Which will
require more work to stop? Which will require
more force to stop in a given distance?
Truck Collision
In a head-on collision:
Which truck will experience the greatest force?
Which truck will experience the greatest impulse?
Which truck will experience the greatest change in momentum?
Which truck will experience the greatest change in velocity?
Which truck will experience the greatest acceleration?
Which truck would you rather be in during the collision?
Assignments
Problem Solving Page 260-261
Practice problems 261 #1-3
POWER—is equal to the work divided by the time
P
W
t
UNITS:
Watts (J/s)
Power Calculation
10-2 Machines
Benefits of machines: Multiplies force, Work out is always less than work in
Mechanical Advantage
MA 
Fr
Fe
Ideal mechanical Advantage
IMA 
de
dr
EFFICIENGY
e
Wout
 100
Win
e
MA
 100
IMA
Click on any machine for further details.
CHPTER 11 ENERGY AND ITS CONSERVATION
Types of energy:
Mechamical energy
1. kinetic energy
2. rotational kinetic energy
3. potential energy
4. elastic potential energy
Conservation of energy:
Energy in collisions: