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 = 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 run a.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 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.

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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 CD

Split donkey carts AB CD

How does a propeller plane work? Single Airplane

How does a rocket work?

Rocket Pushes Gas

How does a jet plane work?