Newton's Laws Discussion Created for Operation Physics By Dick Heckathorn 12 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? F earth on apple F hand on apple

What is the normal force? F hand 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? F earth on apple

the hand on the apple? F earth on apple or the apple on the earth? F hand on apple F apple 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? F earth on apple

What is the reaction force that is equal and opposite to the gravitational force exerted by the earth on the apple? F earth on apple F apple on earth

Since there is a net force down on the apple, the apple must be F earth 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: always always 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 3 rd 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 3 rd 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

Newton’s Third Law Force earth on apple = Force apple on earth

You can eliminate forces from discussion if their reaction force cannot be found. When you are asked to identify the force(s) acting on an object,

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 muscles and 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 describes what is happening.

A Volkswagen pushes a Mack truck causing them to move with a constant 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 pushing with a greater force on the Volkswagen because 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 encounter will 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!