Newton's Laws Discussion

Created for Operation PhysicsBy

Dick Heckathorn12 October 2K + 4

I am holding

an apple in my hand.

What forces are acting on the apple as it lies motionless in my hand?

Fearth on apple

Fhand on apple

Is it true that all forces:

come in pairs,

are equal in magnitude,

but opposite in direction?

I hope you said “yes”!

What then is the reaction force that is equal and opposite to the gravitational force exerted by the

earth on the apple?

Fearth on apple

the hand on the apple?

Fearth on apple

or

the apple

on the earth?

Fhand on apple Fapple on earth

The reaction force to earth on apple is…

When I let go of the apple,

it will be in free fall?

What forces are acting on

the apple as it falls?

Fearth on apple

What is the reaction force

that is equal and opposite

to the gravitational force exerted

by the earth on the apple?

Fearth on apple

Fapple on earth

Since there is a net force down on the apple, the apple must be

Fearth on apple

accelerating downward.

Since the apple is accelerating,

could the forces acting on the apple

add up to be zero?

No.

Once again, what then is the force that is equal and opposite

to the gravitational force

exerted by the earth on the apple?

It is the force of the apple on the earth.

Note…It is not acting on the ball.

We know forces:

alwaysalways come in pairs,

are directed away from

each other,

are equal in magnitude, and

opposite in direction.

When we examined the apple accelerating as it fell, we wanted to know only the

forces acting on the apple.

We were not interested in Newton’s 3rd law

which deals with

action-reaction forces.

When dealing with

the laws of Newton,

if we want to know what an object is doing,

we will concern ourselves

with only the forces

acting on the object.

We will not concern our self with Newton’s 3rd law.

When dealing with forces,

one must, at all times identify

the agent

(cause of the force)

and the object

(to which the force is applied).

Summary

Agent:

earth

Object:

apple

Then one writes:

Force earth on apple

Agent:

apple

Object:

earth

Then one writes:

Force apple on earth

Newton’s Third Law

There are paired forces

that act on

different objects,

never on the same object.

Newton’s Third Law

Forces are identified by

interchanging the

agent and

the object.

Force earth on apple = Force apple on earth

and are in opposite directions

Some things

to

think about.

A man is trying to push

a big rig up a steep hill,

but the big rig is

beginning to overtake him and

he is slipping back down.

Is the big rig

pushing on the man

harder than

the man

is pushing on the big rig?

No

A student flexes his musclesand pushes against the wall.

Does the wall push back?Yes

How does the wall know enough to push back?

Does Newton’s law cause it?

No, it just describeswhat is happening.

A Volkswagenpushes

a Mack truckcausing them to move

with aconstant velocity.

Does the Volkswagen exert a larger force on Mack truck

since it is moving the truck

in a forward direction?or…

does the Mack truck exert a larger force on the Volkswagen

since it is much more massive?Neither, they are the same magnitude

There’s more.

Next

the Volkswagen

is pushing

the Mack truck uphill.

Is the Volkswagen

pushing on the truck

with a greater force

since it has to overcome the additional downward force

of the Mack truck?

or…is the Mack truck pushingwith a greater force on the

Volkswagenbecause of its inertia

and additional downhill force?

A SUV

with a heavy trailer

is slowly

driving down

a steep incline.

Is the trailer

exerting a larger force

on the SUV

since it would like to roll down the incline faster

than the slow-moving SUV?

or…

is the SUV exerting

a larger force on the trailer

since it needs to

keep the trailer behind itself?

A Volkswagen is pushing a large truck down the road causing the

truck to undergo a constant acceleration ‘a’.

Is there a net force on the truck?

What can you conclude

about the force of the

Volkswagen on the truck

compared to the force of the

truck on the Volkswagen?

A Mack truck collides head-on

with a Volkswagen, both

moving at, let’s say, 50 km/hr.

During the collision, which of the two vehicles exerts the larger

force on the other?

Is it not true that the encounterwill leave the Volkswagen

“ready for recycling,”while the Mack truck escapes

with “minor” dents?Why or why not?

“Newton never saw a Mack truck and therefore Newton’s third law is most likely not applicable in this situation.”

That’s all folks!