PHY115 – Sault College – Bazlurslide 1 Newton’s Third Law of Motion

PHY115 – Sault College – Bazlur slide 1

Newton’s Third Law of Motion


Newton’s Laws of Motion1. Newton’s First Law of Motion

– Every object will continue in a state of rest or with constant speed in a straight line unless acted upon by an external force.

2. Newton’s Second Law of Motion – When a net force act on an object, the object accelerates

in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the mass. Thus, a F/m or, a F/m

3. Newton’s Third Law of Motion – Whenever one object exerts a force on a second object,

the second object exerts an equal and opposite force on the first.


ForceA force is not a thing in itself but an interaction

between one thing and another. Force is an interaction between two objects.

Action force: hand on wall

Reaction force: wall on hand

You can not push on something

without being pushed back.


ForceIf you push on a wall with your fingers, more is

happening than you pushing on the wall.

The wall is also pushing on you.

How else can you explain the bending of your fingers?

Your fingers and the wall push on each other.

There is a pair of forces involved: – your push on the wall and – the wall's push back on you.

These forces are equal in magnitude and opposite in direction and comprise a single interaction.

In fact, you can't push on the wall unless the wall pushes back.


Who is pushingForce is an interaction between two objects.

Action force: hand on wall

Reaction force: wall on hand

You can not push on something without being pushed back.


Action Reaction


Force


Force Tension on the spring is 100 N + 100 N = 200 N

As a whole the balance is not moving as it has two equal and opposite forces and they are canceling out.

Any of these weight is not moving as the balance is pulling back with 100 N on each of them.


Force Tension on the spring is 100 N.

As a whole the balance is not moving as it has two equal and opposite forces and they are canceling out.

Any of these weight is not moving as the balance is pulling back with 100 N on each of them.


An interaction requires a pair of forces Consider a boxer's fist hitting a massive punching bag.

Fist hits the bag (and dents it) while the bag hits back on the fist (and stops its motion).

In hitting the bag there is an interaction with the bag that involves a pair of forces.

The force pair can be quite large.

An interaction requires a pair of forces acting on two objects.


An interaction requires a pair of forcesBut what if hitting a piece of tissue paper?

The boxer's fist can only exert as much force on the tissue paper as the tissue paper can exert on the fist.

Furthermore, the fist can't exert any force at all unless what is being hit exerts the same amount of force back.

An interaction requires a pair of forces acting on two objects.


Other examples: You pull on a cart and it accelerates.

But in doing so, the cart pulls back on you, as evidenced perhaps by the tightening of the rope wrapped around your hand.

One thing interacts with another; – you with the cart


Other examples: A hammer hits a stake and drives it into the ground.

In doing so, the stake exerts an equal amount of force on the hammer, which brings the hammer to an abrupt halt.

One thing interacts with another; – you with the cart, or – the hammer with the stake.


Newton's 3rd Law of Motion

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


Examples of Newton’s 3rd Law The hand exerts an action force on the putty.

– The putty changes shape.

The putty exerts a reaction force on the hand.– The hand also changes shape.


Examples of Newton’s 3rd Law Try standing on a skateboard Facing a partner on

another skateboard with equal masses.

Give your partner a gentle push. What happens?– The partner accelerates away from you.

The partner exerts a reaction force on you.– You also accelerate in the

opposite direction.

Both of you move even though one person pushed the other.

Thus both an action and reaction force exists.


Examples of Newton’s 3rd Law Through the ball away from you.

As you exert a force forward on the ball, you accelerate backward.

– The ball must exert a reaction force back on you.– Similar to a rocket.


Examples of Newton’s 3rd Law In order to move, the rocket expels gaseous molecules.

The rocket exerts a large force on the molecules to expel them at a very high speed.

At the same time, the gaseous molecules exert a forward force of equal magnitude on the rocket.

Expelled molecules do not need to push against the ground or the atmosphere to propel the rocket.

If it was the case, the rocket would not accelerate in the vacuum of outer space.


Newton's 3rd Law of MotionNewton'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.


Defining Your SystemAn interesting question often arises; since action and reaction

forces are equal and opposite, why don't they cancel to zero?

To answer this question we must consider the system involved.

Consider the force pair between the apple and orange.

A force is exerted on the orange by the apple and the orange accelerates.


Defining Your SystemWe'll first consider the system to be the orange, and define it by a dashed line

surrounding it.

Note that there is an external force acting on the system—which is provided by the apple.

The fact that the orange simultaneously exerts a force on the apple, which is external to the system, may affect the apple (another system), but not the orange.

The force on the orange is not canceled by the force on the apple. So in this case the action and reaction forces don't cancel.

When the orange is the system (within the dashed line) an external force provided by the apple acts on it. Action and reaction forces do not cancel and the system accelerates.


Defining Your SystemIf, however, we consider the system to enclose both the orange and apple, the

force pair is internal to the orange-apple system.

Then the forces do cancel each other.

The apple and orange move closer together but the system's “center of gravity” is in the same place before and after the pulling. There is no net force and therefore no net acceleration. Relatively they are not moving.

Similarly, the many force pairs between molecules in a golf ball may hold the ball together into a cohesive solid, but they play no role at all in accelerating the ball. A force external to the ball is needed to accelerate the ball.

When both the orange and apple compose the system (both within the dashed line) no external force acts on it. Action and reaction are within the system and do cancel to zero. Zero net force means no acceleration of the system.


Defining Your Systemthe system enclosed both the orange and apple can only move if there is an

external force.

That external force may be when the floor pushes back on the apple.

An external horizontal force occurs when the floor pushes on the apple (reaction to the apple’s push on the floor). The orange-apple system accelerates.


Defining Your SystemNewton’s 2nd law describes the effect of forces on a single object.

Newton’s 3rd law describes the effect of forces between two objects.

Newton’s Second Law of Motion – When a net force act on an object, the object accelerates

in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the mass. Thus, a F/m or, a F/m

1. Newton’s Third Law of Motion – Whenever one object exerts a force on a second

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

Remember the action and reaction forces are on the different objects.


Defining Your SystemIn general, when Body A inside a system

interacts with Body B outside the system, each can experience a net force.

Action and reaction forces don't cancel.

You can't cancel a force acting on Body A with a force acting on Body B.

Forces cancel only when they act on the same body, or on the same system.

Action and reaction forces always act on different bodies.


Action and Reaction on Different MassesAs 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.


When a ball drops on the earth

The action force that the earth exerts on the ball is equal in size , but opposite in direction, to the reaction force that the ball exerts on the Earth.

The reason we see the acceleration of the ball, but not the earth, is because the ball has less mass than the Earth.


Action and Reaction on Different MassesThe role of different masses is evident in a fired rifle.

When firing a rifle there is an interaction between the rifle and the bullet.

A pair of forces acts on both the rifle and the bullet. The force exerted on the bullet is as great as the reaction force exerted on the rifle; hence, the rifle kicks back.

Since the forces are equal in magnitude, why doesn't the rifle recoil with the same speed as the bullet?


Action Reaction In analyzing changes in motion, Newton's second law reminds us

that we must also consider the masses involved.

Suppose we let F represent both the action and reaction force, m the mass of the bullet, and the mass of the more massive rifle.

The accelerations of the bullet and the rifle are then found by taking the ratio of force to mass.

The acceleration of the bullet is given by

while the acceleration of the recoiling rifle is

We see why the change in motion of the bullet is so huge compared to the change of motion of the rifle.


Rocket 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.


Rocket 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.


Rocket Consider a balloon recoiling when air is expelled.

If the air is expelled downward, the balloon accelerates upward.


Verification of 3rd Law














Verification of 3rd Lawa = 2 m/s2

m = 6 kg

Fnet = am = 2 x 6 = 12 N




Application of 3rd Law






Summary of Newton's Three Laws An object at rest tends to remain at rest; an object in motion tends to remain in motion at

constant speed along a straight-line path. This tendency of objects to resist change in motion is called inertia. Mass is a measure of inertia. Objects will undergo changes in motion only in the presence of a net force.

When a net force acts on an object, the object will accelerate. The acceleration is directly proportional to the net force and inversely proportional to the mass. Symbolically, a = F/m. Acceleration is always in the direction of the net force. When objects fall in a vacuum, the net force is simply the weight, and the acceleration is g (the symbol g denotes that acceleration is due to gravity alone). When objects fall in air, the net force is equal to the weight minus the force of air resistance, and the acceleration is less than g. If and when the force of air resistance equals the weight of a falling object, acceleration terminates, and the object falls at constant speed (called terminal speed).

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force of the first. Forces come in pairs, one action and the other reaction, both of which comprise the interaction between one object and the other. Action and reaction always act on different objects. Neither force exists without the other.


VectorsAny quantity that requires both magnitude and direction for a

complete description is a vector quantity.

Examples of vector quantities include force, velocity, and acceleration.

A quantity that can be described by magnitude only, not involving direction, is called a scalar quantity. Mass, volume, and speed are scalar quantities.

A vector quantity is nicely represented by an arrow.

When the length of the arrow is scaled to represent the quantity's magnitude, and the direction of the arrow shows the direction of the quantity, we refer to the arrow as a vector.


VectorsAdding vectors that act along parallel directions is simple enough:

if they are in the same direction, they add; if they are in opposite directions, they subtract.

The sum of two or more vectors is called their resultant.


VectorsTo find the resultant of two vectors that don't act in exactly the

same or opposite direction, we use the parallelogram rule.

Construct a parallelogram wherein the two vectors are adjacent sides - the diagonal of the parallelogram shows the resultant. In Figure the parallelograms are rectangles.


VectorsWhen two equal-length vectors at right angles to each other are

added, they form a square.

The diagonal of the square is the resultant, √2 times the length of either side.


Force VectorsOne is 30 newtons and the other is 40 newtons.

Simple measurement shows the resultant is 50 newtons


Which rope has the greater tension?Nellie Newton hanging at rest from a pair of ropes that make different angles

with the vertical.

Investigation will show there are three forces acting on Nellie: – her weight,

– a tension in the left-hand rope, and

– a tension in the right rope.

Because the ropes hang at different angles, the rope tensions will be different from each other.


a step-by-step solutionBecause Nellie hangs in equilibrium, her weight must be supported by tensions

in the ropes, which must add vectorially to equal her weight.

Use of the parallelogram rule shows that the tension in the right-hand rope is greater than the tension in the left rope.

If you measure the vectors you'll see that tension in the right rope is about twice the tension in the left rope.

How does tension in the right rope compare with her weight?


Velocity VectorsSpeed is a measure of “how fast”; velocity is a

measure of both how fast and “which direction.”

If the speedometer in a car reads 100 kilometers per hour you know your speed.

If there is also a compass on the dashboard, indicating that the car is moving due north, for example, you know your velocity—100 kilometers per hour north.

To know your velocity is to know your speed and your direction.


Velocity VectorsConsider an airplane flying due north at 80 kilometers per hour relative to the

surrounding air.

Suppose that the plane is caught in a 60-kilometer-per-hour crosswind (wind blowing at right angles to the direction of the airplane) that blows it off its intended course.

Velocity vectors scaled so that 1 centimeter represents 20 kilometers per hour. Thus, the 80-kilometer-per-hour velocity of the airplane is shown by the 4-centimeter vector and the 60-kilometer-per-hour tailwind is shown by the 3-centimeter vector.

The diagonal of the constructed parallelogram (rectangle in this case) measures 5 cm, which represents 100 km/h. So the airplane moves at 100 km/h relative to the ground, in a direction northeast.


resulting velocitiesHere we see a top view of an airplane being blown off course by

wind in various directions.

With pencil and using the parallelogram rule, sketch the vectors that show the resulting velocities for each case.

In which case does the airplane travel fastest across the ground? Slowest?


Components of VectorsJust as two vectors at right angles can be combined into one resultant vector,

in reverse any vector can be “resolved” into two component vectors perpendicular to each other.

These two vectors are known as the components of the given vector they replace.

The process of determining the components of a vector is called resolution.

Any vector drawn on a piece of paper can be resolved into a vertical and a horizontal component.


Vector resolutionA rectangle is drawn that has V as its diagonal.

The sides of this rectangle are the desired components, vectors X and Y.

In reverse, note that the vector sum of vectors X and Y is V.


Resultant velocity ?Consider a motorboat that normally travels 10

km/h in still water.

If the boat heads directly across the river, which also flows at a rate of 10 km/h, what will be its velocity relative to the shore?


Resultant velocity ?Consider a motorboat that normally travels 10

km/h in still water.

If the boat heads directly across the river, which also flows at a rate of 10 km/h, what will be its velocity relative to the shore?


Then answer these questions Here we see top views of three motorboats crossing a river. All

have the same speed relative to the water, and all experience the same water flow. Construct resultant vectors showing the speed and direction of the boats.

(a) Which boat takes the shortest path to the opposite shore?

(b) Which boat reaches the opposite shore first?

(c) Which boat provides the fastest ride?


Components of VectorsWith a ruler, draw the horizontal and vertical components of the

two vectors shown.

Measure the components and compare your findings with the answers given below.

(Left vector: the horizontal component is 3 cm; the vertical component is 4 cm. Right vector: the horizontal component is 6 cm; the vertical component is 4 cm.)

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PHY115 – Sault College – Bazlurslide 1 Newton’s Third Law of Motion