Newton's Laws Discussion Created for Operation Physics By Dick Heckathorn 20 October 2K + 4

I am holding an apple in my hand.

Fhand on apple Fearth on apple What forces are acting on the apple as it lies motionless in my hand? Fhand on apple Any other? No Fearth on apple

I hope you said “yes”! Is it true that all forces: come in pairs, are equal in magnitude, but opposite in direction? I hope you said “yes”!

Fhand on apple Fearth on apple Fapple on earth What is the reaction force that is equal and opposite to the gravitational force? Is it It is the hand on the apple? the apple on the earth? or Fhand on apple Fearth on apple Fapple on earth

When I let go of the apple, What force(s) are acting on it will be in free fall, What force(s) are acting on the apple as it falls? Fhand on apple Any other? No No Fearth on apple

Since there is a net force down on the apple, the apple must be accelerating downward. Fearth on apple

Since the apple is accelerating downward, could the forces acting on the apple add up to be zero? No.

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

Fhand on apple When we asked if the hand was acting on the apple after it left the hand, Fhand on apple Could one say that the apple was acting on the hand? No.

Fearth on apple Fapple on earth 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

It is the force of the apple on the earth. 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 apple.

are equal in magnitude, and We know forces: always come in pairs, are equal in magnitude, and opposite in direction.

We were not interested in Newton’s 3rd law action-reaction forces. 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.

if we want to know what an object is doing, we will concern ourselves 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 object Summary When dealing with forces, one must, at all times identify the object to which the force(s) is applied and the agent causing each force.

Object: Apple Agent: Earth Then one writes: Force earth on apple

There are paired forces never on the same object. Newton’s Third Law There are paired forces that act on different objects, never on the same object.

Forces are identified by interchanging the object and the agent. Newton’s Third Law Forces are identified by interchanging the object and the agent. Force earth on apple = Force apple on earth and are in opposite directions

Some things to think about.

beginning to overtake him and he is slipping back down. 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 pushing on the big rig? Force rig on man = Force man on rig Is the big rig pushing on the man harder than the man is pushing on the big rig? No Force rig on man = Force man on rig

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

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? No or… is the Mack truck pushing with a greater force on the Volkswagen because of its inertia and additional downhill force? No

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? No or… is the SUV exerting a larger force on the trailer since it needs to keep the trailer behind itself? No

Is there a net force on the truck? A Volkswagen is pushing a large truck down the road causing the truck to undergo a constant acceleration. Is there a net force on the truck? Yes

A Mack truck collides head-on moving at, let’s say, 50 km/hr. 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? Neither, both forces are the same.

mT . aT = mVW . aVW Is it not true that the encounter will leave the Volkswagen “ready for recycling,” while the Mack truck escapes with “minor” dents? Probably FVW on T = FT on VW mT . aT = mVW . aVW

“Newton never saw a Mack truck and therefore Newton’s third law It has been said, “Newton never saw a Mack truck and therefore Newton’s third law is most likely not applicable in this situation.” Sorry, no case has been observed where Newton’s 3rd has not been observed to be true.

A bug collides with a car A bug collides with a car. Which experience the greater force, the bug or the car? Neither, both forces are the same. Which is affected the most, the bug or the car? FB on C = FC on B mC . ac = mB . aB

That’s all tonight.

That’s all folks!