Force
In the simplest sense, a force is a push or a pull.
A mutual action is an interaction between one thing and another.
When you push on the wall, the wall pushes on you.
The interaction that drives the nail is the same as the one that halts the hammer.
Newton's 3rd Law
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
Describes the relationship between two forces in an interaction.
One force is called the action force.
The other force is called the reaction force.
Neither force exists without the other.
They are equal in strength and opposite in direction.
They occur at the same time (simultaneously).
Newton's third law is often stated: "To every action there is always an equal opposing reaction."
It doesn't matter which force we call action and which we call reaction.
Examples
When the girl jumps to shore, the boat moves backward.
A person trying to walk on ice, where friction is minimal, may not be able to exert an action force against the ice.
Without the action force there cannot be a reaction force, and thus there is no resulting forward motion.
The dog wags the tail and the tail wags the dog.
Identifying Action and Reaction
To identify a pair of action-reaction forces, first identify the interacting objects A and B, and if the action is A on B, the reaction is B on A.
When action is A exerts force on B, the reaction is simply B exerts force on A.
Action and Reaction on Different Masses
The cannonball undergoes more acceleration than the cannon because its mass is much smaller.
F represents both the action and reaction forces; m (large), the mass of the cannon; and m (small), the mass of the cannonball.
The balloon recoils from the escaping air and climbs upward.
The rocket recoils from the "molecular cannonballs" it fires and climbs upward.
Lift
Using Newton's third law, we can understand how a helicopter gets its lifting force.
The whirling blades force air particles downward (action).
The air forces the blades upward (reaction).
This upward reaction force is called lift.
When lift equals the weight of the craft, the helicopter hovers in midair. When lift is greater, the helicopter climbs upward.
Birds and airplanes also fly because of action and reaction forces.
When a bird is soaring, the shape of its wings deflects air downward. The air in turn pushes the bird upward.
The slightly tilted wings of an airplane also deflect oncoming air downward and produce lift.
We know that Earth pulls on the moon. Does the moon also pull on Earth? If so, which pull is stronger?
They pull on each other with equal, but opposite forces. The reason the Moon is not pulled into Earth is because it's orbiting at a velocity that gives it centrifugal force that offsets the pull of gravity. We think the Moon is revolving around the Earth, but if we were on the Moon, the Earth would appear to revolve around the Moon. So Earth, too, has a centrifugal force keeping it from closing in on the Moon.
A tug of war occurs between boys and girls on a polished floor that's somewhat slippery. If the boys are wearing socks and the girls are wearing rubber-soled shoes, who will surely win, and why?
The girls will win. The force of friction is greater between the girls' feet and the floor than between the boys' feet and the floor. When both the girls and the boys exert action forces on the floor, the floor exerts a greater reaction force on the girls' feet. The girls stay at rest and the boys slide toward the girls.
A force interaction requires at least a(n)
pair of forces.
Whenever one object exerts a force on a second object, the second object exerts a force on the first that is
opposite in direction and equal in magnitude at the same time
The force that directly propels a motor scooter along a highway is that provided by the
road.
When you jump vertically upward, strictly speaking, you cause Earth to
move downward.
A system undergoes acceleration only when acted on by a(n)
net force.
If a net force acts on a horse while it is pulling a wagon, the horse
accelerates.
At a pizza shop, the cook throws the pizza dough in the air. The amount of force the cook exerts on the dough depends on the
mass of the dough.
In the interaction between a hammer and the nail it hits, is a force exerted on the nail?
Yes.
ON the hammer?
Yes.
How many forces occur in this interaction?
Two forces, action and reaction.
When a hammer exerts a force on a nail, how does the amount of force compare with that of the nail on the hammer?
They are equal and opposite according to Newton's Third Law.
When you walk along a floor, what pushes you along?
The floor pushes back on your feet.
When swimming, you push the water backward- call this action. What is the reaction force?
The water pushing you forward.
If the action is a bowstring acting on an arrow, identify the reaction force.
The arrow pushes back on the bowstring.
When you jump up, the world really does recoil downward. Why can ́t this motion of the world be noticed?
The world has such a large mass that is acceleration downwards is negligible.
When a rifle is fired, how does the size of the force of the rifle on the bullet compare with the force of the bullet on the rifle?
The forces are the same, equal and opposite.
How does the acceleration of the rifle compare with that of the bullet? Defend your answer.
The rifle will recoil backwards with less acceleration because it has a larger mass. F = M x a
How can a rocket be propelled above the atmosphere where there is no air to "push against"?
The rocket doesn ́t push against the air, the rocket pushes against the hot gases coming out of its thrusters. The hot gases push back propelling the rocket forward.
In the interaction between an apple and an orange, how many forces are exerted on the apple?
One. The orange pulling on the apple.
On the orange?
One, the apple pulling the orange.
Are these forces equal in strength?
Yes.
Are these forces opposite in direction?
Yes.
Consider the orange system. Do action and reaction forces cancel each other in the orange system?
No. They act on different objects - the apple and the orange.
Does the orange system accelerate?
Yes, towards the apple.
Consider the orange-apple system. Do action and reaction forces cancel each other in this system
Yes, if you take them both together. One force pulls to the right and another equal and opposite force to the left.
Do the orange and apple accelerate away from each other, or do they remain together?
They remain together.
In the horizontal direction, how many forces are exerted on the cart?
Two, the horse pulling the cart forward and friction pulling the cart backwards.
What is the net horizontal force on the cart?
Force of pull of horse forward - Force of friction on cart backwards
How many horizontal forces are exerted on the horse.
Two: floor pushing horse forward, and cart pulling back
the horse (due to friction of the cart).
What is the net horizontal force on the horse?
Ffp - Fpc
How many horizontal forces are exerted by the horse on the other objects?
Two. One on the ground and one on the cart.
How many horizontal forces are exerted on the horse-cart system?
Two: Fp and Ff. The force pulling the system forward and the friction holding the system back.
What is the net horizontal force on the horse-cart system?
Fp -Ff
In order to increase its speed, why must the horse push harder against the ground than it pulls on the wagon?
Otherwise the net force would be zero and no acceleration would happen.
If you hit a wall with a force of 200 N, how much force is exerted on you?
200 N is exerted back on you.
Why can ́t you hit a feather in midair with a force of 200 N?
It cannot exert a reaction force of 200 N back on you.
How does the saying "You get what you give" relate to Newton's third law?
If you give a hug you get one back. If you kiss somebody, they kiss you back. Eye for an eye, tooth for a tooth. If you hit somebody, they hit you back with an equal and opposite force.
Your weight is the result of the gravitational force of Earth on your body. What is the corresponding reaction force?
Your body pulling on the earth
Why can you exert a greater force on the pedals of a bicycle if you pull up on the handlebars?
The handlebars push down with the same force as your pull. This force is transmitted to the pedals.
Consider the two forces acting on a person who stands still, namely the downward pull of gravity and the upward support of the floor. Are these forces equal and opposite? Do they comprise an action-reaction pair? Why or why not?
Yes, they are the only forces acting on a non accelerating person; no they are 2 interactions - person-earth and floor-person.
If you walk on a log that is floating in the water, the log moves backward. Why?
You push forward on the log. The log pushes backward on you. This backward force makes the log go backward.
Why is it easier to walk on a carpeted floor than on a smooth, polished floor?
One can exert a greater horizontal force on a carpet than a polished floor due to friction. So you get a greater reaction force.
If you step off a ledge, you accelerate noticeable toward Earth because of the gravitational interaction between you and Earth. Does Earth accelerate toward you as well? Explain.
Yes, but the accelerations produced by these equal forces are quite unequal because of the differences in the masses
Suppose you're weighing yourself while standing next to the bathroom sink. Using the idea of action and reaction, explain why the scale reading will be less when you push down on top of the sink. Why will the scale reading be more if you pull up on the bottom of the sink?
Action is you pushing down on sink - reaction is sink pushing up on you. This takes some of the burden off of the scale. If the action is you pushing up on the sink, the reaction is the sink pushing down on you. This adds burden to the scale.
When a high jumper leaves the ground, what is the source of the upward force that accelerates her? What force acts after her feet are no longer in contact with the ground?
The ground pushed upward on the jumper, which is the force that provides an upward accelerate. In the air, only the force of gravity acts on the jumper. Upward acceleration ceases and the acceleration is downward - g
hat is the reaction force to an action force of 1000 N exerted by the Earth on an orbiting communications satellite?
1000 N exerted on the earth by the communications satellite.
If action equals reaction, why isn't Earth pulled into orbit around satellites?
It is, but negligibly, due to Earth's huge mass
If a bicycle and a massive truck have a head-on collision, upon which vehicle is the impact force greater? Which vehicle undergoes the greater change in its motion? Defend your answers.
Impact force is the same. Because of the bicycle's smaller mass, the change in motion is greater for the bicycle
A speeding bus makes contact with a bug that spatters onto the windshield. Because of the sudde3n force, the unfortunate bug undergoes a sudden deceleration. Is the corresponding force that the bug exerts against the windshield greater, less or the same? Is the resulting deceleration of the bus greater than, less than or the same as that of the bug?
Magnitude is the same. The bug decelerates more because it has less mass
Some people used to think that a rocket could not travel to the moon because it would have no air to push against once it left Earth's atmosphere. We now know that idea was mistaken. What force propels a rocket when it is in a vacuum?
The force is that which the exhaust exerts. It is the reaction to the force that the rocket exerts on the exhaust gases
Since the force that acts on a cannonball when a cannon is fired is equal and opposite to the force that acts on the cannon, does this imply a zero net force and therefore the impossibility of an accelerating cannonball?
No. There is a force applied to the cannonball that causes movement as long as it is greater than all of the other forces acting on the cannonball. That force is equal to the one on the cannon, which will cause the cannon to move if it is greater than the other forces acting on it
Suppose you exert 200 N on your refrigerator and push it across the kitchen floor at constant velocity. What friction force acts between the refrigerator and the floor? Is the friction force equal and opposite to your 200-N push? Does the friction force make up the reaction force to your push?
200 N; yes; no, the reaction to your push on the refrigerator is the refrigerator's push back on you
Your teacher challenges you and your best friend to each pull on a pair of scales attached to the ends of a horizontal rope,in tug- of-war fashion, so that the readings on the scales will differ. Can this be done? Explain.
You cannot do it for 2 reasons. 1 you can pull with as much force as your friend can pull back with. 2 scales on different parts of the same rope have to have the same reading because the rope is experiencing tension as a whole everywhere, and it will be the same wherever you measure it.
A pair of 50-N weights is attached to a spring scale as shown. Does the spring scale read 0, 50, or 100 N?
50 N, the same as if the pulls were horizontal
The strong man can withstand the tension force exerted by the two horses pulling in opposite directions. How would the tension compare if only one horse pulled and the left rope was tied to a tree? How would the tension compare if the two horses pulled in the same direction, with the left rope tied to the tree?
The same; doubled (if they both pull in the same direction)
A balloon floats motionless in the air. A balloonist climbs up the supporting cable. In which direction does the balloon move as the balloonist climbs? Explain.
Downward because he pulls as he climbs
When you get up from a sitting position, do your feet push against the floor with a force equal to, more than, or less than your weight? Explain.
More than because you accelerate upward as you push
When a weightlifter jerks a barbell over his head, is the force exerted on the barbell more than, less than, or equal to the barbell's weight? Explain
More than, because the barbell accelerates upward
