What do you know about What do you know about forces?forces?

In your own words, define the term “force”.In your own words, define the term “force”. Give two examples of forces.Give two examples of forces. Do you think a force affects the motion of Do you think a force affects the motion of

an object? If so, how?an object? If so, how? Describe a “real-life” situation in which you Describe a “real-life” situation in which you

encounter a force.encounter a force.

ForcesForces

What is a force?What is a force?

A force is a push A force is a push or pull exerted or pull exerted on an object.on an object.

• The object on which the force is exerted is called the system.

• The world around the object that exerts forces on it is called the environment.

systemenvironment

environment

environment

Types of ForcesTypes of Forces

Contact Force-Contact Force-a force a force that acts on an object that acts on an object only by touching it.only by touching it.

Long Range Force-Long Range Force- a a force that acts on an force that acts on an object without object without contact.contact.

The force of gravity is an example of a long range force.

• Each force has a specific, identifiable, immediate cause called the agent.

• The agent can be animate, such as a person, or inanimate, such as a desk, or a magnet.

The agent for the force of gravity is the earth’s mass.

Force is a vector quantity.Force is a vector quantity.

A force has both A force has both magnitudemagnitude and and directiondirection.. A force is represented with an A force is represented with an arrowarrow that that

points in the correct direction.points in the correct direction. The The lengthlength of the arrow is proportional to of the arrow is proportional to

the the sizesize of the force. of the force. The tail of the force vector is always on the The tail of the force vector is always on the

object, even when the force is a push.object, even when the force is a push. The symbol The symbol F F with a subscript label is with a subscript label is

used to identify the agent. used to identify the agent.

Common Types of ForcesCommon Types of ForcesForceForce SymbolSymbol DefinitionDefinition DirectionDirection

FrictionFriction FFffThe contact force that acts to The contact force that acts to oppose motion between oppose motion between surfacessurfaces

Parallel to the surface and Parallel to the surface and opposite the direction of opposite the direction of the motionthe motion

NormalNormal FFNNThe contact force exerted by a The contact force exerted by a surface on an object.surface on an object.

Perpendicular to and away Perpendicular to and away from the surfacefrom the surface

SpringSpring FFspsp A restoring force, that is, the A restoring force, that is, the push of pull a spring exerts on push of pull a spring exerts on an object.an object.

Opposite theOpposite the

displacement of the object displacement of the object at the end of a springat the end of a spring

TensionTension FFTTThe pull exerted by a string or The pull exerted by a string or rope when attached to an rope when attached to an object and pulled taut.object and pulled taut.

Away from the object and Away from the object and parallel to the string or rope parallel to the string or rope at the point of contact.at the point of contact.

ThrustThrust FFthrustthrust Forces that move objects such Forces that move objects such as rockets, planes, cars, etc.as rockets, planes, cars, etc.

In the same direction as In the same direction as the acceleration.the acceleration.

WeightWeight FFggLong range force due to Long range force due to gravitational attraction between gravitational attraction between the earth and an object.the earth and an object.

Straight down toward the Straight down toward the center of the earth.center of the earth.

Net ForceNet Force

Because forces are vectors, the total force on an Because forces are vectors, the total force on an object is the vector sum, or object is the vector sum, or resultantresultant, of all , of all forces exerted on an object.forces exerted on an object.

The vector sum of two or more forces on an The vector sum of two or more forces on an object is called the object is called the net forcenet force..

Fman ● Fwoman

Fnet

●

EquilibriumEquilibrium

If the net force on an object is zero, the If the net force on an object is zero, the object is in object is in equilibriumequilibrium..

If an object is in equilibrium, the forces are If an object is in equilibrium, the forces are balancedbalanced. (Equal in magnitude, but . (Equal in magnitude, but opposite in direction.)opposite in direction.)

An object in equilibrium will be either at An object in equilibrium will be either at rest or moving at a constant velocity.rest or moving at a constant velocity.

Drag ForceDrag Force

When an object moves through any fluid, When an object moves through any fluid, such as air or water, the fluid exerts a such as air or water, the fluid exerts a dragdrag forceforce on the moving object in the direction on the moving object in the direction opposite to its motion.opposite to its motion.

As the speed of the object increases, so As the speed of the object increases, so does the magnitude of the drag force.does the magnitude of the drag force.

The size and shape of the object also The size and shape of the object also affects the drag force, as well as the affects the drag force, as well as the temperature and viscosity of the fluid.temperature and viscosity of the fluid.

Terminal VelocityTerminal Velocity An object in free fall has very An object in free fall has very

little velocity at the start, and little velocity at the start, and therefore a small drag force.therefore a small drag force.

The downward force of gravity The downward force of gravity is much stronger than the is much stronger than the upward drag force so the upward drag force so the object accelerates downward.object accelerates downward.

As the ball’s velocity increases, As the ball’s velocity increases, so does its drag force.so does its drag force.

Eventually the drag force Eventually the drag force equals the force of gravity. equals the force of gravity. When this happens there is no When this happens there is no net force and no acceleration.net force and no acceleration.

The constant velocity that is The constant velocity that is reached when this state of reached when this state of equilibrium is achieved is equilibrium is achieved is called the called the terminal velocityterminal velocity..

gravity

Air resistance

gravity

Air resistance

Terminal velocity

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

An object that is at rest will remain at rest An object that is at rest will remain at rest or an object that is moving will continue to or an object that is moving will continue to move in a straight line with constant move in a straight line with constant speed, if the net force acting on that object speed, if the net force acting on that object is zero.is zero.

Newton’s first law is often called the Newton’s first law is often called the Law Law of Inertiaof Inertia..

InertiaInertia is the tendency of an object to is the tendency of an object to resist change.resist change.

Newton’s First LawNewton’s First Law

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

If the net force on an object is not equal to zero, If the net force on an object is not equal to zero, the motion of the object will change.the motion of the object will change.

A change in motion is called A change in motion is called accelerationacceleration.. Experiments show that the acceleration of an Experiments show that the acceleration of an

object is object is proportional to the net forceproportional to the net force exerted on exerted on the object and the object and inversely proportional to the massinversely proportional to the mass of the object being accelerated.of the object being accelerated.

This is a statement of Newton’s second law and This is a statement of Newton’s second law and can be written as an equation.can be written as an equation.

Newton’s Second Law a = FNewton’s Second Law a = Fnetnet/m or /m or F=maF=ma

Measuring ForceMeasuring Force

One unit of force causes a 1 kg mass to One unit of force causes a 1 kg mass to accelerate at 1 m/saccelerate at 1 m/s22..

Because force is equal to mass times Because force is equal to mass times acceleration, F=ma, one force unit has the acceleration, F=ma, one force unit has the dimensions 1 kgdimensions 1 kg●●m/sm/s22

This unit in the SI system is known as the This unit in the SI system is known as the newton, Nnewton, N..

Using Newton’s LawsUsing Newton’s Laws

Consider a ball falling in midair:Consider a ball falling in midair: If it is touching nothing and air resistanceIf it is touching nothing and air resistance is is

neglected, the only force acting on it is neglected, the only force acting on it is FFgg..

The ball’s acceleration is The ball’s acceleration is -9.8 m/s-9.8 m/s22, or g. , or g. Newton’s 2Newton’s 2ndnd law becomes law becomes F = mgF = mg. . The magnitude of the object’s weight is equal to The magnitude of the object’s weight is equal to

its mass times the acceleration it would have if it its mass times the acceleration it would have if it were falling freely.were falling freely.

Mass vs. WeightMass vs. Weight

MassMass, the amount of , the amount of matter an object matter an object contains, does not contains, does not change. (Measured in change. (Measured in kilograms)kilograms)

WeightWeight is dependent is dependent upon the acceleration upon the acceleration due to gravity and will due to gravity and will vary from planet to vary from planet to planet. (Measured in planet. (Measured in Newtons)Newtons)

What is being measured, What is being measured, mass or weight?mass or weight?

A bathroom scale contains springs.A bathroom scale contains springs. When you step on a scale, the scale exerts an upward When you step on a scale, the scale exerts an upward

force on you.force on you. Because you are not accelerating, the net force is zero.Because you are not accelerating, the net force is zero.

Therefore, that magnitude of Therefore, that magnitude of FFspsp = F = Fgg..

A spring scale, therefore measures A spring scale, therefore measures weightweight,,

not mass.not mass. If you were on another planet, the compression of the If you were on another planet, the compression of the

spring would be different, and the scale’s reading would spring would be different, and the scale’s reading would be different.be different.

Practice ProblemsPractice Problems

On Earth, a scale shows that you weigh 585 N. On Earth, a scale shows that you weigh 585 N. What is your mass?What is your mass?

What would the scale read on the moon (g=1.60 What would the scale read on the moon (g=1.60 m/sm/s22)?)?

Your new motorcycle has a mass of Your new motorcycle has a mass of

250. kg. What is its weight in newtons?250. kg. What is its weight in newtons? A 7.50 kg television is set on a spring scale. If A 7.50 kg television is set on a spring scale. If

the scale reads 78.4 N, what is the acceleration the scale reads 78.4 N, what is the acceleration due to gravity at that location?due to gravity at that location?

Apparent WeightApparent Weight

If a bathroom scale supports you-it provides the If a bathroom scale supports you-it provides the only upward force-then it reads your weight.only upward force-then it reads your weight.

How would the measurement of the scale How would the measurement of the scale change if you push down on the bathroom change if you push down on the bathroom counter while standing on the scale? Push up counter while standing on the scale? Push up on the bathroom counter? Stand with one foot on the bathroom counter? Stand with one foot on the scale and one foot off?on the scale and one foot off?

In these cases, the force exerted by the scale is In these cases, the force exerted by the scale is the the apparent weightapparent weight..

Weight in an ElevatorWeight in an Elevator If you stand on a scale in If you stand on a scale in

an elevator that is in an elevator that is in equilibrium, the scale equilibrium, the scale reads your weight.reads your weight.

If the elevator accelerates If the elevator accelerates upward, then you feel upward, then you feel heavier and the scale heavier and the scale reads a larger force. reads a larger force. Why?Why?

If the elevator accelerates If the elevator accelerates downward, then you feel downward, then you feel lighter and the scale lighter and the scale reads less. Why?reads less. Why?

If the cable holding the elevator beaks, the scale with you on it accelerates with a = -g. What would the scale read?

Which exerts the greater force?Which exerts the greater force?

You are driving down the highway and a You are driving down the highway and a bug splatters on your windshield. Which is bug splatters on your windshield. Which is greater: the force of the bug on the greater: the force of the bug on the windshield, or the force of the windshield windshield, or the force of the windshield on the bug?on the bug?

Newton’s Third Law of MotionNewton’s Third Law of Motion

All forces come in pairs. The two forces in All forces come in pairs. The two forces in a pair act on different objects and are a pair act on different objects and are equal in strength and opposite in direction.equal in strength and opposite in direction.

In other words; for every action force, In other words; for every action force, there is an equal but opposite reaction there is an equal but opposite reaction force.force.

FFA on B A on B = -F = -F B on AB on A

Which exerts the greater force?Which exerts the greater force?

You are driving down the highway and a You are driving down the highway and a bug splatters on your windshield. Which is bug splatters on your windshield. Which is greater: the force of the bug on the greater: the force of the bug on the windshield, or the force of the windshield windshield, or the force of the windshield on the bug?on the bug?

The forces are equal.The forces are equal. The deceleration of the bug is greater due The deceleration of the bug is greater due

to its smaller mass, but the forces are to its smaller mass, but the forces are equal.equal.

Practice ProblemPractice Problem

A 50.0 kg bucket is being lifted by a rope. A 50.0 kg bucket is being lifted by a rope. The rope will not break if the tension is The rope will not break if the tension is 525 N or less. The bucket started at rest, 525 N or less. The bucket started at rest, and after being lifted 3.0 m, it is moving at and after being lifted 3.0 m, it is moving at 3.0 m/s. If the acceleration is constant, is 3.0 m/s. If the acceleration is constant, is the rope in danger of breaking?the rope in danger of breaking?

Practice Problem #2Practice Problem #2

Five people are playing tug-of-war. Five people are playing tug-of-war. Anders and Alyson pull to the right with Anders and Alyson pull to the right with

45 N and 35 N respectively. Calid and 45 N and 35 N respectively. Calid and Marisol pull to the left with 53 N and 38 N, Marisol pull to the left with 53 N and 38 N, respectively. With what force and in what respectively. With what force and in what direction does Benito pull if the game is direction does Benito pull if the game is tied?tied?

Practice Problem #3Practice Problem #3

A 7.25 g bullet is fired from a gun. The A 7.25 g bullet is fired from a gun. The muzzle velocity of the bullet is 223 m/s. muzzle velocity of the bullet is 223 m/s. Assume that the bullet accelerates at a Assume that the bullet accelerates at a constant rate along the barrel of the gun constant rate along the barrel of the gun before it emerges with constant speed. before it emerges with constant speed. The barrel of the gun is 0.203 m long. The barrel of the gun is 0.203 m long. What average force does the bullet exert What average force does the bullet exert on the gun?on the gun?