NSTA Web SeminarForce and Motion

Bill RobertsonMay 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 =

Distance traveled

Time to travel that distance

Speed

Speed at a particular

instant of time

Instantaneous

speed

=

Average and instantaneous velocity take into account

direction

acceleration =

?

?

acceleration?

acceleration =

Change in velocity

Time for the change

acceleration

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 run ?A long distance runner out 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.)

Long distance runa.She accelerated whenever she changed her 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 lawΣF = 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?