Transcript Web Seminar—Force and Motion

NSTA Web Seminar
Force and Motion
Bill Robertson
May 5, 2005
Review of Newton’s first two laws and
description of motion
In-depth look at Newton’s third law
Newton’s first law
An object at rest tends to remain at
rest, and an object in motion in a
straight line tends to remain in motion
in a straight line, unless acted on by a
net external force.
Inertia
Is the property of objects that
describes how they obey
Newton’s first law.
How to describe motion
and changes in motion?
• How fast?
• In what direction?
• How are those two things
changing?
Speed?
?
speed =
?
Speed
speed =
Distance traveled
Time to travel that distance
Instantaneous
speed
=
Speed at a
particular
instant of time
Average and instantaneous
velocity take into account
direction
acceleration?
acceleration =
?
?
acceleration
acceleration =
Change in velocity
Time for the change
Two cars are rounding curves.
Both cars are traveling at a constant speed
of 40 km per hour.
Car A is rounding a very sharp curve and
car B is rounding a gradual curve.
Which of the following are true regarding
the motion of the cars?
a. Cars A and B both have zero acceleration
because their speeds are not changing.
b. Car A is accelerating more than car B
because it is changing direction at a faster
rate.
c. Car A and car B have the same acceleration
because they have the same speed.
d. Car A and car B have the same acceleration
because they have the same velocity.
Long distance
A long distance
runner out run
for a ?
workout
runs 4 km in 20 minutes, walks for a
distance of 1 km, which takes 10 minutes,
and then rests in one spot for 10 minutes.
After that, she runs another 6 km in 35
minutes.
Which of the following are true with respect
to her motion? (There is more than one
correct answer.)
a.She accelerated
sherun
changed her
Long whenever
distance
speed.
b.Her average speed for the entire trip is about
0.15 km per minute.
c.When she was at rest, you could not assign a
number to her speed or to her acceleration.
d.She never accelerates because whenever she
runs, walks, or rests, she does so at a constant
speed and direction.
Newton’s second law
ΣF = ma
ΣF: represents the net force acting on an object.
m: represents the mass of an object, which is a
numerical measure of its inertia.
a: represents the acceleration of the object.
image
Newton’s third law
• Action and reaction
• Whenever object A exerts a force
on object B, object B exerts an
equal and opposite force back on
object A.
Balloon clouds in back
Balloon clouds at bottom
1 donkey
4 donkeys
A
B
C
D
Split donkey carts
A
B
C
D
How does a propeller plane work?
Single Airplane
How does a rocket work?
Rocket Pushes Gas
How does a jet plane work?