Newton’s 3rd Law

3rd Six Weeks

Golf and Newton’s 3rd Law

Newton’s 3rd Law of Motion

• The Law states: Whenever one object exerts a force upon a second object, the second object exerts an equal and opposite force on the first object

• Also known as the “law of action and reaction”

• When a force occurs in nature, there is always a reaction force.

• F1 = -F2

Newton’s 3rd Law

• Newton's third law is often stated thus: “To every action there is always opposed an equal reaction.”

• In any interaction there is an action and reaction pair of forces that are equal in magnitude and opposite in direction.

• Neither force exists without the other—forces come in pairs, one action and the other reaction.

• The action and reaction pair of forces makes up one interaction between two things.

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

A pair of interacting forces is known as a system

Newton’s 3rd Law of Motion

• In every interaction the forces always occur in pairs.

• A pair of interacting forces is known as a system

• Without the action force, there can be no reaction force and vice versa.

• You push against the floor, the floor pushes against you. A rocket expels hot gases outward, the hot gases push back propelling the rocket upward.

The rocket recoils from the

“molecular bullets” it fires

and climbs upward

Examples of Newton’s 3rd Law in Action

The Force Exerted vs. Effect of Force• In nature, there is often a

difference between the force that is exerted and the effect of that force.

• Recall that according to the 2nd

Law, Acceleration is inversely proportional to the mass of an object.

• Furthermore, the acceleration an object experiences is directly proportional to the amount of force applied.

Remember the 2nd Law

The force exerted against the cannon

Is just as big as the force that drives the

cannonball along the barrel, yet the effect

of that force on each object differs due

to their masses.

Action and Reaction on Different Masses

• As strange as it may first seem, a falling object pulls upward on the Earth as much as the Earth pulls downward on it.

• The downward pull on the object seems normal because the acceleration of 10 meters per second each second is quite noticeable.

• The same amount of force acting upward on the huge mass of the Earth, however, produces acceleration so small that it cannot be noticed or measured.

• The Earth is pulled up by the boulder with just as much force as the boulder is pulled downward by the Earth.

Which falls toward

the other,

A or B?

Do the accelerations

of each relate

to their relative masses?

Interaction Between Rifle and Bullet

• The role of different masses is evident in a fired rifle

• The force exerted against the recoiling rifle is just as great as the force that drives the bullet. Why then, does the bullet accelerate more than the rifle?

• 2nd Law & Law of Conservation of Momentum demonstrate why even though the force on both is equal, the acceleration is different – due to differences in mass

• If we extend the idea of a rifle recoiling or “kicking” from the bullet it fires, we can understand rocket propulsion.

• Consider a machine gun recoiling each time a bullet is fired. If the machine gun is fastened so it is free to slide on a vertical wire , it accelerates upward as bullets are fired downward. A rocket accelerates the same way.

• It continually “recoils” from the ejected exhaust gas. Each molecule of exhaust gas is like a tiny bullet shot from the rocket .

Newton’s 3rd Law – Football Correlation

Do action and reaction forces cancel each other out?

• Since action and reaction forces are equal, why don’t they cancel each other out?

• Imagine a person kicking a football, if the force of the kick (action)was counteracted by the reaction of the force of the ball on the foot, it would never be moved.

• The foot and the ball act as part of an action-reaction system

• The action force and reaction force within the system are equal in magnitude and opposite in direction of each other

• However, it is clear that the football is moved when kicked.

Do action and reaction forces cancel each other out?

• The action of the foot impacting on the ball acts to accelerate it in the direction of the action since the ball is less massive.

• The ball although reacting with the identical force in the opposite direction, decelerates or acts to slow down your foot as it kicks the ball.

• The deceleration can be felt as the impact on your foot from the kick.

• If both the apple and the orange have the same mass, then the

object that can provide the most

force against the ground will

accelerate the most.

So the Earth pulling on the apple in the

“Earth-Apple System” will cause the apple

To accelerate the orange more due to the

Lower friction between its wheels & Earth

Newton’s 3rd Law: The Boxer and the Tissue Paper – insufficient inertia

• Drop a sheet of tissue paper in front of the heavyweight boxing champion of the world and challenge him to hit it in midair with a force of only 50 pounds (222 N).

• Sorry, the champ can't do it. In fact, his best punch couldn't even come close. Why is this? The tissue has insufficient inertia for a 50-pound interaction with the champ's fist. He can’t exert any more force on the tissue paper than it can exert on him – it has insufficient inertia for a great force. The tissue would be pushed or break at a low amount of force due to its composition.

Example 1: Golf Club and Golf Ball

Consider the collision between the club head and the golf ball in the sport of golf. When the club head of a moving golf club collides with a golf ball at rest upon a tee, the force experienced by the club head is equal to the force experienced by the golf ball. Most observers of this collision have difficulty with this concept because they perceive the high speed given to the ball as the result of the collision. They are notobserving unequal forces upon the ball and club head, but rather unequal accelerations. Both club head and ball experience equal forces, yet the ball experiences a greater acceleration due to its smaller mass. In a collision, there is a force on both objects that causes an acceleration of both objects. The forces are equal in magnitude and opposite in direction, yet the least massive object receives the greatest acceleration.

Example 2: Billiard Balls

Consider the collision between a moving seven ball and an eight ball that is at rest in the sport of table pool. When the seven ball collides with the eight ball, each ball experiences an equal force directed in opposite directions. The rightward moving seven ball experiences a leftward force that causes it to slow down; the eight ball experiences a rightward force that causes it to speed up. Since the two balls have equal masses, they will also experience equal accelerations. In a collision, there is a force on both objects that causes an acceleration of both objects; the forces are equal in magnitude and opposite in direction. For collisions between equal-mass objects, each object experiences the same acceleration.

Example 3: Ice Skaters

Consider the interaction between a male and female figure skater in pair figure skating. A woman (m = 45 kg) is kneeling on the shoulders of a man (m = 70 kg); the pair is moving along the ice at 1.5 m/s. The man gracefully tosses the woman forward through the air and onto the ice. The woman receives the forward force and the man receives a backward force. The force on the man is equal in magnitude and opposite in direction to the force on the woman. Yet the acceleration of the woman is greater than the acceleration of the man due to the smaller mass of the woman.

Example 3: Ice Skaters

• Many observers of this interaction have difficulty believing that the man experienced a backward force. "After all," they might argue, "the man did not move backward." Such observers are presuming that forces cause motion. In their minds, a backward force on the male skater would cause a backward motion.

• Forces cause acceleration, not motion. The male figure skater experiences a backwards force that causes his backwards acceleration. The male skater slows down while the woman skater speeds up.

• In every interaction (with no exception), there are forces acting upon the two interacting objects that are equal in magnitude and opposite in direction.

Suppose a friend who hears

About Newton’s 3rd Law says

That movement is impossible,

Because when the action and

Reaction forces are equal

And opposite, they would

Cancel each other out.

The net force on the object

Would be zero, and no matter

How hard one pushed you

Could never move!

How could you address this?

How is movement

ever possible?

A: the horse will push

Against the Earth & that will

Push the horse-cart system

forward