Download Physics 105 - BYU Physics and Astronomy

Springs and other stretchy things
Hooke’s Law:
Demo: springs and k
F

x
Springs store energy
(conservative)
Spring potential energy
P1. If a spring is compressed by 20 cm, the work done
in the first 10 cm is
A. less than
B. the same as
C. more than
the work done in the last 10 cm.
F

Hint: think of work as the average force * distance in
each part
x
Potential energy stored in spring:
1 2
PE  kx
2
Spring potential energy
A rubber slingshot with spring constant k is stretched a
distance d . A rock of mass m is shot up at a balloon
floating a height h above the rock’s lowest point, and a
horizontal distance L away. How fast is the rock
going just before it hits the balloon?
F

x
Conservation of total energy
What energy goes into his change of height?
Nonconservative
work is done on the
system by muscles,
motors, external
power, which add
to or take from the
mechanical energy
(KE+PE) of the
system.
F

x
Conservation of total energy
KE  PE  Wnonconservative
( KE f  PE f )  ( KEi  PEi )  Wnonconservative
F

x
A 1200 kg car is moving at 12 m/s down the slope. What braking
force can stop it in 100m?
Fred goes ice-blocking racing on the grass.
Starting from rest he rides down a hill with some
friction. On the way down, Fred pushes with his
hands on the ground to make himself go faster.
For each force decide how it will affect the
mechanical energy.
A. (+)Wnonconservative…mechanical energy gained
B. (-)Wnonconservative …mechanical energy lost
C. it will change only the PE
P2. Gravity _________
P3. sliding friction_________
P4. Fred’s pushing on the ground (and hence the
ground pushing him) _________
40 m
200
Fred, 50 kg (including ice), goes ice-blocking
racing on the grass. Starting from rest he rides 40
m down a hill which has a 20 slope. mk = 0.2
between the ice and grass. On the way down, Fred
pushes with his hands with an average force of 30
N forward along the slope.
1. Find the work done by Fred’s pushing (in
Joules)
2. Find the work done by friction
3. What is his speed at the bottom?
40 m
200
Power!
work F x cos 
P


time
t
1Watt =
Empire State building:
1 Horse-power=746 W
Height: 1,250 feet, 443 meters
Stories: 102
There are 1,575 steps from the building's lobby to
the 86th floor (374 m). Paul Crake holds the
record for racing these steps in 10 minutes, 15
seconds.
What average power did he expend to
lift himself? (Assume m=100 kg)
From work:
From velocity:
Power!
A car weighing 3000 N moves at a constant speed of 30 m/s
on level ground. To do this, it pushes backwards on the road
with a 5000 N force.
a) What is the power output of the car engine?
b) Where does this power go?