# Physics Chapter 5

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
Newton’s Third Law
A quantity that has both magnitude and direction. Examples are force, velocity, torque, electric, magnetic fields, and acceleration.
Vector quantity
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A quantity that has magnitude but not direction. Examples are mass, volume, speed, and time.
Scalar quantity
The net result of a combination of two or more vectors.
Resultant
Mutual action between objects where each object exerts an equal and opposite force on the other.
Interaction
To every action there is always an opposed equal reaction.
We can call one force the *action force* and the other the *reaction force*. Then we can express Newton’s third law in the form:
It may be as tiny as an atom or as large as the universe.
System
Cannon ball:
F
—- = *A*
*m*
Cannon:
F
—- = a
*M*
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 cannonball, and *M* the mass of the much more massive cannon. The accelerations of the cannonball and the cannon are then found by comparing the ratio of force to mass. The accelerations are:
In application of Bernoulli’s principle, the net upward force produced by the difference between upward and downward pressures. When lift equals weight, horizontal flight is possible.
Lift
Earth pulls on Moon. What is the reaction?
The action is: Earth pulls on Moon. What is the reaction?
Moon pulls on Sun.
Moon pulls on Earth.
Earth pushes on Moon.
Moon pushes on Earth.

*Give an explanation*

Moon pulls on Earth.

every object has an opposite and equal reaction on another object according to newton’s 3rd law

Which force is greater, the Earth’s pull on the Moon, or the Moon’s pull on the Earth?

-Both pulls are the same.
-Moon’s pull
-Earth’s pull

*Give an explanation*

Both pulls are the same.

Since the Moon pulling on the Earth and the Earth pulling on the Moon are a reaction pair, the strengths of the two forces must be equal to each other.

If the cannon’s recoil is the result of an equal and opposite reaction, why is the cannon’s acceleration lower than the shell’s acceleration?

-The cannon’s acceleration is not lower than the shell’s acceleration;

-The cannon has more mass than the shell.

-Not as much force is applied to it.

*Give an explanation*

The cannon has more mass than the shell.

Since the cannon has more mass than the shell, it will have a lower acceleration because both objects experience the force. This can be understood from Newton’s second law (a=F/m).

Since movement of an object or system requires that a net force be applied to it, how can this occur if all action forces have opposite and equal reaction forces?

*Give an explanation*

The action force doesn’t act on the same object as the reaction force.
Newton’s second law can be applied to each of the objects. In fact, this explains why the two objects can have different resulting accelerations, resulting in movement.
the horse is pulling a cart. What allows the horse to accelerate forward even though the cart is pulling back on the horse?
-The horse is pulling on the cart with a stronger force than the cart is pulling back on the horse.
-Friction from the ground on the horse is stronger than the sideways force the horse is exerting on the ground.
-The force of friction the ground exerts on the horse is larger than the force the cart is pulling back on the horse.
*Give an explanation*
The force of friction the ground exerts on the horse is larger than the force the cart is pulling back on the horse.
The net force exerted on the horse must be to the right if the horse accelerates to the right. The only two horizontal forces acting on the horse are friction (pushing forward) and the cart (pulling backward), so the friction force must be stronger.
Which of the following is NOT a statement or consequence of Newton’s third law?

-There can never be only one force acting on an object.
-Every force has an opposite and equal reaction.
-All forces come in pairs.
-No force can exist on its own.

*Give an explanation*

There can never be only one force acting on an object

Newton’s third law doesn’t specify how many forces can act on any one object (it just states that each of the forces will have a reaction force acting on other objects). Which force diagram could be representing a reaction pair?

*Give an explanation*

A

The magnitudes of the two force vectors are equal, and the vectors are pointed in opposite directions.

For an action force, there must be a reaction force that
-is slightly larger in amplitude than the action force.
-is slightly smaller in magnitude than the action force.
-always acts in the same direction.
-is exactly equal in magnitude.
*Give an explanation*
is exactly equal in magnitude.

every object has an opposite and equal reaction on another object according to newton’s 3rd law

An archer shoots an arrow. Consider the action force to be the bowstring against the arrow. The reaction to this force is the
-air resistance against the bow.
-friction of the ground against the archer’s feet.
-grip of the archer’s hand on the bow.
-combined weight of the arrow and bowstring.
-arrow’s push against the bowstring.

*Give an explanation*

arrow’s push against the bowstring.

every object has an opposite and equal reaction on another object according to newton’s 3rd law

A player hits a ball with a bat. The action force is the impact of the bat against the ball. The reaction to this force is the
-weight of the ball.
-grip of the player’s hand against the ball.
-air resistance on the ball.
-force that the ball exerts on the bat.
-weight of the bat.

*Give an explanation*

-force that the ball exerts on the bat.

every object has an opposite and equal reaction on another object according to newton’s 3rd law

A baseball player bats a ball with a force of 1000 N. The reaction force that the ball exerts against the bat is

*Give an explanation*

1000 N

every object has an opposite and equal reaction on another object according to newton’s 3rd law

As a ball falls, the action force is the pull of Earth on the ball. The reaction force is the
As a ball falls, the action force is the pull of Earth on the ball. The reaction force is the
-pull of the ball’s mass on the Earth.
-acceleration of the ball.
-air resistance acting against the ball.
-none of these

*Give an explanation*

pull of the ball’s mass on the Earth

every object has an opposite and equal reaction on another object according to newton’s 3rd law

A person is attracted toward the center of Earth by a 500-N gravitational force. The Earth is attracted toward the person with a force of
-zero.
-250 N.
-1000 N.
-500 N.
*Give an explanation*
500 N

every object has an opposite and equal reaction on another object according to newton’s 3rd law

Arnold Strongman and Suzie Small each pull very hard on opposite ends of a massless rope in a tug-of-war. The greater force on the rope is exerted by

-Suzie, surprisingly.
-Arnold, of course.
-both the same, interestingly enough.

*Give an explanation*

both the same, interestingly enough

Because newton’s third law works just as well for the tension of a rope, when suzie pulls the rope from Arnold Strongman they both put an opposite and equal reaction on the rope

A Mack truck and a Volkswagen traveling at the same speed have a head-on collision. The vehicle that undergoes the greatest change in velocity will be the
-same for both.
-Volkswagen.
-Mack truck.

*Give an explanation*

Volkswagen

because although they exert the same amount of force on each other, the volkswagen has less mass, which according to newton’s second law would cause that same force to move it at a higher velocity than the truck.

A car traveling at 100 km/hr strikes an unfortunate bug and splatters it. The force of impact is
-greater on the bug.
-the same for both.
-greater on the car.

*Give an explanation*

the same for both

every object has an opposite and equal reaction on another object according to newton’s 3rd law
even though the two objects may be going different velocities after the collision, they still exerted the same amount of force on each other.

A piece of rope is pulled by two people in a tug-of-war. Each pulls with 400 N of force. What is the tension in the rope?
-zero
-800 N
-600 N
-400 N
-none of these

*Give an explanation*

400 N

If you pull the rope with 400 N of force and your friend pulls on the other side , there isn’t any *acceleration*.
With what force your friend is pulling

Two people, one twice as massive as the other, attempt a tug-of-war with 12 meters of massless rope on frictionless ice. After a brief time, they meet. The heavier person slides a distance of
4 m.
6 m.
0 m.
3 m.

*Give an explanation*

4 m

you cannot pull ‘twice as hard’ on something without feeling that same force act on you .. action and reaction being equal and opposite in every case.

Both people have the same force act on them .. but the person with twice the mass accelerates forwards with just half the acceleration of the other person (a = F/m).

This means that 2m moves only ½ the distance that 1m moves in the same time.
2m moves 1/3 of the distance while 1m moves 2/3 of 12 m

A bird sitting on the limb of a tree is moving about 30 km/s with respect to the sun. If the bird takes 1 second to drop down to a worm below, the worm would be 30 km downrange from the bird when it reached the ground. This faulty reasoning is best countered with Newton’s
second law.
first law.
law of gravitation.
third law.
none of these
first law.
The force exerted on the tires of a car to directly accelerate it along a road is exerted by the
tires.
engine.
air.
none of these
*Give an explanation*

every object has an opposite and equal reaction on another object according to newton’s 3rd law

The car tires produces an action force on the road, and the car moves forward from the friction of the road exerting a reaction force on the tires

A boxer can hit a heavy bag with great force. Why can’t he hit a piece of tissue paper in midair with the same amount of force?
*Give an explanation*
This is because the boxer can only exert as much force on the tissue paper as the tissue paper can exert on the fist
If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why do the cannonball and cannon have very different accelerations?

*Give an explanation*

This is because acceleration is equal to net force/ mass. Meaning *acceleration is inversely proportional to mass*.

This means a given force exerted on a small mass produces a large acceleration, on the other hand the same force exerted on a larger mass produces a smaller acceleration.

According to the parallelogram rule, what quantity is represented by the diagonal of a constructed parallelogram?
The vector quantity is represented by the diagonal of a constructed parallelogram.
Consider a baseball player batting a ball. Identify the action-reaction pairs when the ball is being hit.
The action force is when the bat applies force onto ball.
The action reaction force is when the ball applies force onto bat.
Consider a baseball player batting a ball. Identify the action-reaction pairs while the ball is in flight.
when the ball is in flight Earth exerts force onto ball, pulling it towards it with gravity.
The ball exerts force on the earth, by pulling the earth up toward it.

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