FORCESFORCES episode III episode IIIThe return of The return of NewtonNewton

PhysicsMr. Maloney

© 2002 Mike Maloney

ObjectivesObjectives

You will be able to …describe what happens when all forces are

balanced out.apply concepts of equilibrium to problems.observe and describe the consequences of

Newton’s 1st Law.

© 2002 Mike Maloney

Inertia questionsInertia questions I put a book on a table and what happens? I slide a puck across the ice what happens? An astronaut gets pushed away from the shuttle

out in deep space what happens? A magician pulls a table cloth out from under

some plates and glasses, what happens?

In all these cases the motion, whether the object is moving or at rest, stays constant.

How can this be .. aren’t there forces?

© 2002 Mike Maloney

Net force is zero?Net force is zero?

What if the net force acting on an object is zero? This could mean there are no forces acting

on the object Or the forces cancel each other out.

© 2002 Mike Maloney

Newton’s 1Newton’s 1stst Law ( Law (InertiaInertia))

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced NET force.

It describes what happens when there is NO NET force acting on something.

© 2002 Mike Maloney

Newton’s 1Newton’s 1stst Law Law MATHEMATICALLYMATHEMATICALLY

AT REST TENDS TO STAY AT RESTVelocity = 0No net outside force [F=0]Acceleration (change in v) = 0Object stays stationary

© 2002 Mike Maloney

IN MOTION STAYS IN MOTION …Velocity ≠ 0No net outside force [F=0]Acceleration (change in v) = 0Object is in constant velocity motion

(speed and direction)

Newton’s 1Newton’s 1stst Law Law MATHEMATICALLY MATHEMATICALLY (part2)(part2)

© 2002 Mike Maloney

Newton’s 1Newton’s 1stst Law Law (Consequences) (Consequences)

An object at rest will stay there if you do not disturb it

A moving object’s motion will not change (no a, no v and no change in direction) if there is no outside net force acting on it.

When the net force acting on something is zero, the object is said to be in equilibrium.

Where have you heard that before?

© 2002 Mike Maloney

Newton’s 1Newton’s 1stst Law Law (Consequence Examples) (Consequence Examples)

Pulling table cloth off table.Card flickHammering head with books Car crash into a wallHow is Homer’s crash messed up?Dancing Ball-Guy

© 2002 Mike Maloney

Extra Note on newton’s 1Extra Note on newton’s 1stst law law Another consequence of Newton’s first law is that there is

no difference between constant velocity motion and no motion … they both are cases of 0 acceleration.

This seems weird, but think about driving in a car. When you are at rest sitting in the seat you don’t really feel

anything. When you speed up you can “feel” the force speeding you up as

the seat pushes on you. But when you get to 55 and set the car on cruise control, you do

not feel that force anymore. You are moving at a constant velocity and no force is need to keep you moving.

In a force sense, 0 velocity is just another type of constant velocity, nothing special about it.

© 2002 Mike Maloney

Extra Note on newton’s 1Extra Note on newton’s 1stst law law Another consequence of Newton’s first law is that there is

no difference between constant velocity motion and no motion … they are both 0 acceleration.

This seems weird, but think about driving in a car. When you are at rest sitting in the seat you don’t really feel

anything. When you speed up you can “feel” the force speeding you up as

the seat pushes on you. But when you get to 55 and set the car on cruise control, you do

not feel that force anymore. You are moving at a constant velocity and no force is need to keep you moving.

In a force sense, 0 velocity is just another type of constant velocity, nothing special about it.

Newton’s 1st law is really just a special case of Newton’s 2nd Law, when the acceleration is equal to 0

and the net force is also 0.

© 2002 Mike Maloney

ObjectivesObjectives

Can you …describe what happens when all forces are

balanced out.apply concepts of equilibrium to problems.observe and describe the consequences of

Newton’s 1st Law.

© 2002 Mike Maloney

APPENDIXAPPENDIX

© 2002 Mike Maloney

InertiaInertia

The tendency of an object to resist a change in motion is called its inertia.

Objects with greater masses generally have greater inertias.

For our purposes, an object’s inertia is basically measured by it mass.

BACK

© 2002 Mike Maloney

Net ForceNet Force NET FORCE refers to the vector sum total

of all forces acting on an object. It is often expressed as F

For example, if there were two leftward forces of 10 lb each, the NET FORCE would be 20 lb leftward.

If there were one 10 lb rightward force and one 8 lb leftward force, the NET FORCE would be 2 lb rightward.

What about if the forces were in X and Y?

BACK

© 2002 Mike Maloney

Applied ForceApplied Force

APPLIED FORCE refers to a generic force that is acting on an object.

An APPLIED FORCE is basically any force that is not a WEIGHT, NORMAL FORCE, FRICTIONAL FORCE, SPRING FORCE, or other named force.

BACK