Honors PhysicsHonors Physics

Newton’s First and Third Laws of Newton’s First and Third Laws of MotionMotion

ForceForce

Definition – Any push or pull on an object.Definition – Any push or pull on an object. Two types of Force:Two types of Force:

– ContactContact– FieldField

A Net Force is required to change the A Net Force is required to change the motion of a body.motion of a body.

The Net Force is the sum of all individual The Net Force is the sum of all individual forces acting on a body.forces acting on a body.

Newton’s First Law of MotionNewton’s First Law of Motion

Demonstrations.Demonstrations. An body at rest will remain at rest, and a An body at rest will remain at rest, and a

body in motion will continue in motion with body in motion will continue in motion with constant velocity, unless acted upon by a constant velocity, unless acted upon by a net external force.net external force.

What is an external force?What is an external force? In the absence of a net external force, In the absence of a net external force,

nothing changes.nothing changes.

InertiaInertia

The resistance to a change in motion.The resistance to a change in motion. A fundamental property of matter.A fundamental property of matter. It is inertia that makes a body follow the First It is inertia that makes a body follow the First

Law.Law. For that reason, the first law is also known For that reason, the first law is also known

as the Law of Inertia.as the Law of Inertia. Mass is the measurement of a body’s Mass is the measurement of a body’s

inertia.inertia.

Newton’s Third Law of Motion.Newton’s Third Law of Motion.

When body one exerts a force on body two, When body one exerts a force on body two, body two exerts and equal but oppositely body two exerts and equal but oppositely directed force on body one.directed force on body one.

Action/ReactionAction/Reaction Newton realized that he could never Newton realized that he could never

produce a single force. They always produce a single force. They always occurred in pairs.occurred in pairs.

Demonstrations.Demonstrations.

Third-Law-PairsThird-Law-Pairs

The pair of forces exist between the same The pair of forces exist between the same two bodies.two bodies.

When identifying the pairs, include only When identifying the pairs, include only those bodies.those bodies.

Don’t interpret equal forces as equal results.Don’t interpret equal forces as equal results. Why do the forces not cancel?Why do the forces not cancel?

– Consider a car pulling a trailer.Consider a car pulling a trailer.

Questions?Questions?

Assignment Questions.Assignment Questions.