Chapter 12 Forces and Motion. 12.1 ForcesForces and Motion 12.1 Forces A force is a push or pull that acts on an object. A force is a push or pull that

Chapter 12Chapter 12

Forces and MotionForces and Motion

12.1 Forces12.1 Forces Forces and MotionForces and Motion

12.1 Forces12.1 Forces

A force is a push or pull that acts on an A force is a push or pull that acts on an object. object.

A force can cause a resting object to A force can cause a resting object to move, or it can accelerate a moving object move, or it can accelerate a moving object by changing the object’s speed or by changing the object’s speed or direction. direction.

What is a Force?


Units of ForceUnits of Force

Force is measured in newtons (N). Force is measured in newtons (N). One newton is the force that causes a One newton is the force that causes a

1-kilogram mass to accelerate at a rate of 1-kilogram mass to accelerate at a rate of 1 m/s/s. 1 m/s/s.

A newton is equal to 1 kg-m per second A newton is equal to 1 kg-m per second per second (kg∙m/sper second (kg∙m/s22). ).


Representing ForcesRepresenting Forces

The direction and strength of a force can The direction and strength of a force can be represented by an arrow. be represented by an arrow.

The direction of the arrow represents the The direction of the arrow represents the direction of the force. direction of the force.

The length of the arrow represents the The length of the arrow represents the magnitude of the force. magnitude of the force.


Combining ForcesCombining Forces

Force arrows can be combined to show Force arrows can be combined to show the result of how forces combine. the result of how forces combine.

Forces in the same direction add together Forces in the same direction add together and forces in opposite directions subtract and forces in opposite directions subtract from one another. from one another.

The net force is the overall force acting on The net force is the overall force acting on an object after all the forces are combined. an object after all the forces are combined.


Balanced ForcesBalanced Forces

Balanced forces are forces that combine Balanced forces are forces that combine to produce a net force of zero. to produce a net force of zero.

When forces on an object are balanced, When forces on an object are balanced, the net force is zero and there is no the net force is zero and there is no change in the object’s motion. change in the object’s motion.

An unlimited number of individual forces An unlimited number of individual forces can act on an object and still produce a can act on an object and still produce a net force of zero. net force of zero.


Unbalanced ForcesUnbalanced Forces

An unbalanced force is a force that results An unbalanced force is a force that results when the net force acting on an object is when the net force acting on an object is not equal to zero. not equal to zero.

When an unbalanced force acts on an When an unbalanced force acts on an object, the object accelerates. object, the object accelerates.


FrictionFriction

All moving objects are subject to friction, a All moving objects are subject to friction, a force that opposes the motion of objects force that opposes the motion of objects that touch as they move past each other. that touch as they move past each other.

Friction acts at the surface where objects Friction acts at the surface where objects are in contact. are in contact.


There are four main types of friction: There are four main types of friction: static frictionstatic frictionsliding (sliding (kinetickinetic) friction) frictionrolling frictionrolling frictionfluid friction (fluid friction (dragdrag))


Static FrictionStatic Friction

Static friction is the friction force that acts Static friction is the friction force that acts on objects that are not moving. on objects that are not moving.

Static friction always acts in the direction Static friction always acts in the direction opposite to that of the applied force. opposite to that of the applied force.



Sliding FrictionSliding Friction

Once an object is moving, static friction no Once an object is moving, static friction no longer acts on it. longer acts on it.

Instead, a smaller friction force called Instead, a smaller friction force called sliding (sliding (kinetickinetic) friction acts on the object. ) friction acts on the object.

Sliding friction is a force that opposes the Sliding friction is a force that opposes the direction of motion of an object as it slides direction of motion of an object as it slides over a surface. over a surface.


Because sliding friction is less than static Because sliding friction is less than static friction, less force is needed to keep an friction, less force is needed to keep an object moving than to start it moving. object moving than to start it moving.


Rolling FrictionRolling Friction

Rolling friction acts on rolling objects. Rolling friction acts on rolling objects. For a given set of materials, the force of For a given set of materials, the force of

rolling friction is about 100 to 1000 times rolling friction is about 100 to 1000 times less than force of static or sliding friction. less than force of static or sliding friction.


Fluid FrictionFluid Friction

The force of fluid friction opposes the The force of fluid friction opposes the motion of an object through a fluid. motion of an object through a fluid.

Fluid friction increases as the speed of the Fluid friction increases as the speed of the object moving through the fluid increases. object moving through the fluid increases.

Fluid friction acting on an object moving Fluid friction acting on an object moving through the air is known as air resistance. through the air is known as air resistance.

At higher speeds, air resistance can At higher speeds, air resistance can become a significant force. become a significant force.


GravityGravity

Gravity is a force that acts between any Gravity is a force that acts between any two masses. two masses.

Gravity is an attractive force, that is, it Gravity is an attractive force, that is, it pulls objects together. pulls objects together.

Gravity is a Gravity is a field forcefield force in that it does not in that it does not require objects to be in contact for it to act require objects to be in contact for it to act on them. on them.

Earth’s gravity acts downward toward the Earth’s gravity acts downward toward the center of Earth. center of Earth.


Falling ObjectsFalling Objects

Gravity causes objects to accelerate Gravity causes objects to accelerate downward, whereas air resistance acts in downward, whereas air resistance acts in the direction opposite to the motion and the direction opposite to the motion and reduces acceleration. reduces acceleration.

As objects fall to the ground, they As objects fall to the ground, they accelerate. accelerate.


If an object falls for a long time, the If an object falls for a long time, the upward force of air resistance becomes upward force of air resistance becomes equal to the downward force of gravity. equal to the downward force of gravity.

At this point, the forces acting on the At this point, the forces acting on the object are balanced. object are balanced.

Acceleration is zero and the object Acceleration is zero and the object continues falling at a constant velocity. continues falling at a constant velocity.


Terminal velocity is the constant velocity of Terminal velocity is the constant velocity of a falling object when the force of air a falling object when the force of air resistance equals the force of gravity. resistance equals the force of gravity.



Projectile MotionProjectile Motion

Projectile motion is the motion of a falling Projectile motion is the motion of a falling object after it is given an initial forward object after it is given an initial forward velocity. velocity.

Air resistance and gravity are the only Air resistance and gravity are the only forces acting on a projectile. forces acting on a projectile.


Figure 9 shows the motion of two balls Figure 9 shows the motion of two balls released at the same time. released at the same time.

Figure 9A shows that balls of different Figure 9A shows that balls of different masses fall at the same rate.masses fall at the same rate.

Figure 9B shows a ball given an initial Figure 9B shows a ball given an initial horizontal velocity falls vertically at the horizontal velocity falls vertically at the same rate as a ball with zero horizontal same rate as a ball with zero horizontal velocity. velocity.


The combination of an initial forward The combination of an initial forward velocity and the downward force of gravity velocity and the downward force of gravity causes the ball to follow a curved path. causes the ball to follow a curved path.

The two balls fall with the same vertical The two balls fall with the same vertical acceleration and strike the ground at the acceleration and strike the ground at the same time. same time.



12.2 Newton's First and Second Laws of Motion12.2 Newton's First and Second Laws of Motion Forces and MotionForces and Motion

12.2 Newton’s First and 12.2 Newton’s First and Second Laws of MotionSecond Laws of Motion


Newton’s First Law of MotionNewton’s First Law of Motion

Newton summarized his study of force and Newton summarized his study of force and motion in several laws of motion. motion in several laws of motion.

According to Newton’s first law of motion, According to Newton’s first law of motion, the state of motion of an object does not the state of motion of an object does not change as long as the net force acting on change as long as the net force acting on the object is zero. the object is zero.


Unless an unbalanced force acts on it, Unless an unbalanced force acts on it, an object at rest remains at restan object at rest remains at restan object in motion remains in motion an object in motion remains in motion

with the same speed and direction. with the same speed and direction. Newton’s first law is sometimes called the Newton’s first law is sometimes called the

law of inertia. law of inertia. Inertia is the tendency of an object to Inertia is the tendency of an object to

resist a change in its motion. resist a change in its motion.


Newton’s Second Law of MotionNewton’s Second Law of Motion

An unbalanced, or net force causes an An unbalanced, or net force causes an object’s velocity to change. object’s velocity to change.


The acceleration is directly proportional to The acceleration is directly proportional to the net force acting on the object. the net force acting on the object.

Acceleration also depends upon the mass Acceleration also depends upon the mass of the object of the object (inversely proportional)(inversely proportional)Mass is a measure of inertia of an Mass is a measure of inertia of an

object and depends on the amount of object and depends on the amount of matter the object contains. matter the object contains.


According to Newton’s second law of According to Newton’s second law of motion, the acceleration of an object is motion, the acceleration of an object is equal to the net force acting on it divided equal to the net force acting on it divided by the object’s mass. by the object’s mass.

The equation is The equation is a=F/a=F/mm or more familiarly or more familiarly F=F=mmaa..

The acceleration of an object is always in The acceleration of an object is always in the same direction as the net force.the same direction as the net force.


Newton’s second law also applies when a Newton’s second law also applies when a net force acts in the direction opposite to net force acts in the direction opposite to the object’s motion.the object’s motion.



Weight and MassWeight and Mass

Although related to each other, mass and Although related to each other, mass and weight are not the same. weight are not the same.

Weight is the force of gravity acting on an Weight is the force of gravity acting on an object. object.

An object’s weight is the product of the An object’s weight is the product of the object’s mass and acceleration due to object’s mass and acceleration due to gravity acting on it. gravity acting on it.

The equation is The equation is W=mgW=mg. .

12.3 Newton's Third Law of Motion and Momentum12.3 Newton's Third Law of Motion and Momentum Forces and MotionForces and Motion

12.3 Newton’s Third Law of 12.3 Newton’s Third Law of Motion and MomentumMotion and Momentum

A force cannot exist alone. A force cannot exist alone. Forces always exist in pairs. Forces always exist in pairs. According to Newton’s third law of motion, According to Newton’s third law of motion,

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

Newton’s Third Law


These two forces are called action and These two forces are called action and reaction forces. reaction forces.

The forces are equal in size and opposite The forces are equal in size and opposite in direction. in direction.

The forces do The forces do notnot cancel each other cancel each other because they act on different objects. because they act on different objects.


MomentumMomentum

Momentum is the product of an object’s Momentum is the product of an object’s mass and its velocity. mass and its velocity.

The momentum for any object at rest is The momentum for any object at rest is zero. zero.

Momentum is calculated by multiplying an Momentum is calculated by multiplying an object’s mass (in kg) and its velocity (in object’s mass (in kg) and its velocity (in m/s). m/s).


The formula is The formula is pp=m=mvv. . Momentum is measured in kg∙m/s.Momentum is measured in kg∙m/s.


Conservation of MomentumConservation of Momentum

Under certain conditions, collisions obey Under certain conditions, collisions obey the law of conservation of momentum. the law of conservation of momentum.

In physics, conservation means that In physics, conservation means that something has a constant value.something has a constant value.Conservation of momentum means that Conservation of momentum means that

momentum does not increase or momentum does not increase or decrease. decrease.


A closed system means other objects and A closed system means other objects and forces cannot enter or leave the system. forces cannot enter or leave the system.

Objects Objects withinwithin the system can exert forces the system can exert forces on one another. on one another.


According to the law of conservation of According to the law of conservation of momentum, if no net force acts on a momentum, if no net force acts on a system, then the total momentum of the system, then the total momentum of the system does not change. system does not change.

In a closed system the loss of momentum In a closed system the loss of momentum of one object equals the gain in of one object equals the gain in momentum of another object momentum of another object momentum is conservedmomentum is conserved





12.4 Universal Forces12.4 Universal Forces Forces and MotionForces and Motion

12.4 Universal Forces12.4 Universal Forces

Observations of planets, stars, and Observations of planets, stars, and galaxies strongly suggest four different galaxies strongly suggest four different forces exist throughout the universe. forces exist throughout the universe.

The four universal forces are The four universal forces are electromagnetic electromagnetic strong nuclear strong nuclear weak nuclear weak nuclear gravitationalgravitational


Electromagnetic ForcesElectromagnetic Forces

Electric and magnetic forces are two Electric and magnetic forces are two different aspects of the electromagnetic different aspects of the electromagnetic force. force.

Electromagnetic force is associated with Electromagnetic force is associated with charged particles. charged particles.

Electric force and magnetic force are the Electric force and magnetic force are the only forces that both attract and repel. only forces that both attract and repel.


Electric ForcesElectric Forces

Objects with opposite charges attract each Objects with opposite charges attract each other. other.

Objects with like charges repel each other. Objects with like charges repel each other.


Magnetic ForcesMagnetic Forces

Magnetic forces act on certain metals, on Magnetic forces act on certain metals, on the poles of magnets, and on moving the poles of magnets, and on moving charges. charges.

Magnets have two poles that attract each Magnets have two poles that attract each other. other.

Like poles repel each other. Like poles repel each other.


Nuclear ForcesNuclear Forces

Two forces, the strong nuclear force and Two forces, the strong nuclear force and the weak nuclear force, act within the the weak nuclear force, act within the nucleus to hold it together. nucleus to hold it together.


Strong Nuclear ForceStrong Nuclear Force

The strong nuclear force is a powerful The strong nuclear force is a powerful force of attraction that acts only on the force of attraction that acts only on the neutrons and protons in the nucleus. neutrons and protons in the nucleus.

Although this force acts over only Although this force acts over only extremely short distances, it is 100 times extremely short distances, it is 100 times stronger than the electric force of repulsion stronger than the electric force of repulsion at these distances. at these distances.


Weak Nuclear ForceWeak Nuclear Force

The weak nuclear force is an attractive The weak nuclear force is an attractive force that acts only over a range that is force that acts only over a range that is shorter than the strong nuclear force. shorter than the strong nuclear force.

The existence of beta decay indicates that The existence of beta decay indicates that there must a force much weaker than the there must a force much weaker than the strong nuclear force.strong nuclear force.


Gravitational ForceGravitational Force

Gravity is the weakest universal force. Gravity is the weakest universal force. Gravitational force is an attractive force Gravitational force is an attractive force

that acts between any two masses. that acts between any two masses. Newton’s law of universal gravitation Newton’s law of universal gravitation

states that every object in the universe states that every object in the universe attracts every other object. attracts every other object.

F=G(mF=G(m11mm22/d/d22)) Value of G is 6.67Value of G is 6.67×10×10-11-11N∙mN∙m22/kg/kg22


Gravity Acts Over Large Gravity Acts Over Large DistancesDistances

The gravitational force between two The gravitational force between two objects is proportional to their masses. objects is proportional to their masses.

Gravitational force decreases with the Gravitational force decreases with the square of the distance between the square of the distance between the objects. objects.

Gravity is the weakest universal force but Gravity is the weakest universal force but it is the most effective force over long it is the most effective force over long distances. distances.

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Chapter 12 Forces and Motion. 12.1 ForcesForces and Motion 12.1 Forces A force is a push or pull that acts on an object. A force is a push or pull that