Physical Physical ScienceScience

Chapter 10Chapter 10

ForcesForces

Objectives

Describe what a force isExplain how balanced and unbalanced forces are related to motion

The Nature of ForceThe Nature of Force By definition, a By definition, a ForceForce is a is a pushpush or a or a pullpull..

A Push

Or

A Pull

Just like Velocity & Acceleration Forces have

both magnitude and direction

components

Just like Velocity & Acceleration Forces have

both magnitude and direction

components

Balanced & Unbalanced Balanced & Unbalanced ForcesForces

With a Balanced force – opposite and equal forces acting on the same object result in NO motion of the object

Unbalanced forces Unbalanced forces – two or more – two or more forces of unequal strength or forces of unequal strength or direction acting upon on an object direction acting upon on an object results in the motion of the objectresults in the motion of the object

A. Force

Balanced Forces

forces acting on an object that are opposite in direction and equal in size

no change in velocity

A. Force Net Force

unbalanced forces that are not opposite and equal

It causes velocity to change (object accelerates)

F = ma or a = F/m (Newton’s 2nd Law)

Ffriction

W

Fpull

Fnet

NN

Types of forces and how they act:

Objectives/Learning targets: Describe the nature of forces and

how they act on objects Determine the relationship

between elastic force and stretch distance

Types of Forces Magnetic Gravitational Centrifugal Friction Electro-static Elastic Chemical *All Push or Pull on an Object

Elastic Force & Hooke’s Law

Felastic = k XWhere X = distancek is a constant based on the type of

material

Felastic depends directly on the distance stretched

Friction

ObjectivesDescribe what friction is

Explain what determines the friction force between two objects

B. Friction

1. Force that opposes motion between two surfaces in contact.

2. Amount depends on:

a. Kinds of surfaces in contact.

b. Amount of force pressing surfaces together. Something that weighs more will have greater friction.

3. Friction is caused by microwelds

4. Types of friction:

a. Static (usually the greatest)

b. Sliding

c. Rolling (usually the least)

d. Fluid Friction (air or water resistance)

C. Air resistance (drag force)

1. Force that opposes motion of objects through air

2. Pushes up on falling objects

3. Affected by object’s speed, size, shape

4. Without drag force, all objects fall at the same rate

5. Terminal velocity is the max speed at which an object can fall

Gravity

ObjectivesIdentify the factors that affect gravitational force between two objects

Explain why objects accelerate during free fall

D. Gravity

1. Attraction between objects

2. Weakest force in universe

3. Farthest range

4. Directly proportional to the masses of the objects

5. Inversely proportional to the squares of the distance between

Gravitational force is plotted versus distance Gravitational force is plotted versus distance from Earth’s center.from Earth’s center.

Gravity and Distance: The Inverse-Square Law

Gravitational Forces F = G(mGravitational Forces F = G(m11mm22)/d)/d22

MM11

M2

2 d

d

1/2 d

d

M2 M2

M2

M2

2 M2

2 M2

M1

2 M1

2 M1

M1

M1

M1

F

2F

2F

4F

F

¼ F

4F

E. Gravitational Acceleration

1. g = 9.8 m/s/s or 9.8m/s2 on Earth

2. FWEIGHT = m x g

3. All objects fall with the same g

4. Weight is NOT the same as mass

Weight is the Pull of Gravity

• Weight is a measure of the pull of gravity.

• Weight is measured with a spring that is compressed or stretched.

Another Problem

• Weight changes when the pull of gravity changes

• The Earth is six times the mass of the moon.

• On Earth the astronaut weighs 185 lbs.

• Moon has 1/6th the gravity so the astronaut weighs only 31 lbs.

• Weight is not about the astronaut, it’s about what object they’re standing on.

Weight Changes in Many Situations

Skiers “unload” (flex downward at the knees) to reduce the effects of weight.

F. Free Fall (Weightlessness)

1. As long as an object is free falling, nothing exerts an upward force

2. With no upward force, FW = 0 N

A rubber stopper is going around in a circle attached by a string.

• If the string breaks, which path will the stopper follow?

• It will go straight.

• The stopper wants to go straight, but the string pulls back resulting in a circle

AB

C

Like the Stopper The Space Station Wants to Fly Off In a Straight Line

• The space station wants to fly off in a straight line into outer space.

• Gravity is the string that pulls it back, in a sense it’s constantly falling

• When this happens an infinite number of times, the result is an orbit.

Changes in Orbit• An orbiting satellite is in a

delicate balance.• If speed decreases, it will

fall faster than it flies forward, re-enter the atmosphere and fall to Earth

• If it’s speed increases it goes farther forward than it falls and increase the distance of its orbit

• Give it a big enough boost, and it will fly off into space

G. Projectile and Circular Motion

1. Projectile motion

a. Follow a curved path

b. Two types of motion are independent of one another:

1) Horizontal (based on initial velocity and inertia)

2) Vertical (based on gravity)

c. An object launched horizontally will land at the same time as an object simply dropped from the same height

2. Circular Motion

a. Objects moving in circular paths accelerate toward the center

b. Centripetal acceleration

c. Centripetal force (FC = m x aC)

d. Centrifugal force is imaginary

e. Weightlessness in orbit exists because objects are constantly falling toward Earth, but have enough forward velocity to keep them in orbit

When should the When should the pilot release the pilot release the bomb to hit the bomb to hit the target?target?

When should the When should the pilot release the pilot release the bomb to hit the bomb to hit the target?target?

AA BB CC

Are we ready!

Bombs Away!

AA BB CC

Because of gravity, many objects thrown through the air have a parabolic trajectory.

Copyright © 2010 Ryan P. Murphy

Because of gravity, many objects thrown through the air have a parabolic trajectory.

Copyright © 2010 Ryan P. Murphy

Demonstration: Which will fall the fastest if dropped at the same time?

Demonstration: Which will fall the fastest if dropped at the same time?

Demonstration: Now place the dollar on top of the book and repeat?

Objects with similar air resistance fall at the same rate. Everything falls at the same rate of

speed in a vacuum. That rate is the gravitational constant.

On earth (9.8 m/sec²)

Video! Falling Objects, Gravity, Air Resistance, on the moon with Apollo. http://www.youtube.com/watch?v

=KDp1tiUsZw8

In space, away from the gravity of Earth, you will keep going in one direction until acted upon by another force.

Ch.10 Forces

I. Newton’s Laws of Motion

“If I have seen far, it is because I have stood on the shoulders of giants.”

- Sir Isaac Newton (referring to Galileo)

A. Newton’s First Law

Newton’s First Law of Motion An object at rest will remain at rest

and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

Newton’s 3 Laws of Newton’s 3 Laws of MotionMotion

Newton’s 1st Law of MotionNewton’s 1st Law of Motion: : AKA AKA The Law of Inertia The Law of Inertia

which states which states an object at rest will remain at rest, and an an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity object in motion will remain in motion at a constant velocity until acted on by another force.until acted on by another force.

Remember: The greater the mass of an object the greater the inertia

Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.Don’t let this be you. Wear seat belts.

Because of inertia, objects (including Because of inertia, objects (including you) resist changes in their motion. you) resist changes in their motion. When the car going 80 km/hour is When the car going 80 km/hour is stopped by the brick wall, your body stopped by the brick wall, your body keeps moving at 80 m/hour.keeps moving at 80 m/hour.

2nd Law

B. Newton’s Second Law

Newton’s Second Law of Motion The acceleration of an object is

directly proportional to the net force acting on it and inversely proportional to its mass.

F = ma

Newton’s Second Law of Motion Newton’s Second Law of Motion aka aka F=maF=ma Force = mass x accelerationForce = mass x acceleration

Can be written as:Can be written as: F=ma ; a= F/m ; m= F/aF=ma ; a= F/m ; m= F/a

What is the basic unit for mass? What is the basic unit for mass? KilogramKilogram What is the basic unit for acceleration? What is the basic unit for acceleration? Meter/sec/secMeter/sec/sec Therefore the basic unit for Force is Therefore the basic unit for Force is

(kilogram)( meter/sec/sec)(kilogram)( meter/sec/sec) An object with a mass of 1 kg accelerating at 1 m/s/s has a force of 1 An object with a mass of 1 kg accelerating at 1 m/s/s has a force of 1

Newton Newton

Newton’s 3 Laws of Newton’s 3 Laws of MotionMotion

Newton’s 2Newton’s 2ndnd Law & Force of Law & Force of GravityGravity

Everyone has heard of the FORCE of gravityEveryone has heard of the FORCE of gravity Gravity: the force that pulls objects towards each Gravity: the force that pulls objects towards each

otherother Since gravity is a force it also obeys Newton’s Since gravity is a force it also obeys Newton’s

second law: F=masecond law: F=ma

With this experiment, Galileo proved Aristotle wrong

Since objects fall at the same speed, their

acceleration is the same.All objects accelerate at the rate. Here on Earth the rate

is:

Ag=9.8 m/s2

OrAg=32 ft/s2

With this experiment, Apollo 15 astronauts proved Galileo right.(link to You Tube)

Air resistance keeps things from falling equally

F=maF=ma Weight is the force of gravity acting on an object’s mass.Weight is the force of gravity acting on an object’s mass. Therefore weight is a type of ForceTherefore weight is a type of Force The formula for weight: Weight = mass x AThe formula for weight: Weight = mass x Agg

SinceSince A Agg= 9.8 m/s= 9.8 m/s2 2 then then

Weight = mass x 9.8 m/sWeight = mass x 9.8 m/s22 Got it? Got it?

Newton’s 2Newton’s 2ndnd Law & Weight Law & Weight

Remember:1 newton = 0.22

pounds

Your weight Your weight on other on other planetsplanets

& 3 different & 3 different types of types of

starsstars

2nd Law (F = m x a) How much force is needed to accelerate a

1400 kilogram car 2 meters per second/per second?

Write the formula F = m x a Fill in given numbers and units F = 1400 kg x 2 meters per second/second Solve for the unknown 2800 kg-meters/second/second or 2800 N

If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

Check Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object?

12 N = 3 kg x 4 m/s/s

2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.

16 N = 3.2 kg x 5 m/s/s 

3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

66 kg-m/sec/sec or 66 N

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

 9800 kg-m/sec/sec or 9800 N

C. Newton’s Third Law

Newton’s Third Law of Motion When one object exerts a force on a

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

Newton’s 3Newton’s 3rdrd Law of Motion Law of Motion:: For every action there is an equal & opposite reaction.For every action there is an equal & opposite reaction.

If an object is not in motion, then all forces acting on it are balanced and the net force is zero!If an object is not in motion, then all forces acting on it are balanced and the net force is zero! Friction – the force that one surface exerts on another when the two rub against each other.Friction – the force that one surface exerts on another when the two rub against each other.

Newton’s 3 Laws of Newton’s 3 Laws of MotionMotion

Sliding friction

Rolling friction

Fluid friction

Momentum Momentum An object’s momentum is directly related to both An object’s momentum is directly related to both

its mass and velocity.its mass and velocity. Momentum = mass x velocity Momentum = mass x velocity For some reason, maybe because mass is For some reason, maybe because mass is

designated as “m” in formulas, momentum is designated as “m” in formulas, momentum is designated as “designated as “pp”. ”.

Therefore: Therefore: pp = mv = mv The unit for The unit for mass is kgmass is kg, the unit for , the unit for velocity is velocity is

meter/secondmeter/second, therefore the unit for , therefore the unit for momentum is momentum is kg m/seckg m/sec

Conservation of MomentumConservation of Momentum:: When two or more objects interact (collide) the When two or more objects interact (collide) the

total momentum before the collision is equal to total momentum before the collision is equal to the total momentum after the collisionthe total momentum after the collision

The momentum of an object is in the same direction as its velocity.

The more momentum a moving object has, the harder it is to stop.

The mass of an object affects the amount of momentum the object has.

For example, you can catch a baseball moving at 20 m/s, but you cannot stop a car moving at the same speed.

The car has more momentum because it has a greater mass.

The velocity of an object also affects the amount of momentum an object has.

For example, an arrow shot from a bow has a large momentum because, although it has a small mass, it travels at a high velocity.

Figure 17 Momentum An object’s momentum depends on velocity and mass. Problem Solving If both dogs have the same velocity, which one

has the greater momentum?

Sample Momentum Problems

Which has more momentum: a 3.0 kg sledgehammer swung at 1.5m/s, or a 4.0 kg sledgehammer swung at 0.9 m/s?

Read and Understand What information are you given?

Mass of smaller sledgehammer = Velocity of smaller sledgehammer = Mass of larger sledgehammer = Velocity of larger sledgehammer =

Plan and Solve What quantities are you trying to calculate?

The momentum of each sledgehammer What formula contains the given quantities

and the unknown quantity?Momentum = Mass x Velocity

Perform the calculationsSmaller sledgehammer: Larger sledgehammer:

Look Back and Check Does your answer make sense?

The 3.0 kg hammer has more momentum than the 4.0 kg one. This answer makes sense because it is swung at a greater velocity.

Momentum – 2 moving Momentum – 2 moving objectsobjects During this collision the speed of both box cars During this collision the speed of both box cars

changes. The total momentum remains constant changes. The total momentum remains constant before & after the collision. The masses of both cars before & after the collision. The masses of both cars is the same so the velocity of the red car is is the same so the velocity of the red car is transferred to the blue car.transferred to the blue car.

Momentum – 1 moving Momentum – 1 moving objectobject During this collision the speed red car is transferred During this collision the speed red car is transferred

to the blue car. The total momentum remains to the blue car. The total momentum remains constant before & after the collision. The masses of constant before & after the collision. The masses of both cars is the same so the velocity of the red car is both cars is the same so the velocity of the red car is transferred to the blue car.transferred to the blue car.

Momentum – 2 Momentum – 2 connected objectsconnected objects After this collision, the coupled cars make one object After this collision, the coupled cars make one object

w/ a total mass of 60,000 kg. Since the momentum w/ a total mass of 60,000 kg. Since the momentum after the collision must equal the momentum before, after the collision must equal the momentum before, the velocity must change. In this case the velocity is the velocity must change. In this case the velocity is reduced from 10 m/sec. to 5 m/sec. reduced from 10 m/sec. to 5 m/sec.

Let’s call it a Let’s call it a night…. night….

Take a break.Take a break.Cya Later!Cya Later!

VectorsVectors Vectors are a method used to visually show forcesVectors are a method used to visually show forces A A vectorvector is a quantity which has both magnitude (size) and direction. is a quantity which has both magnitude (size) and direction.

The The lengthlength of the arrow shows the of the arrow shows the magnitudemagnitude of the vector. of the vector. The The angleangle of the arrow shows the vector's of the arrow shows the vector's directiondirection..

Just like numbers, we can add two or more vectors together Just like numbers, we can add two or more vectors together and get a and get a net force net force called the called the resultantresultant

Adding 2 or More Adding 2 or More VectorsVectors

Add vectors A and B to get the Resultant C Add vectors A and B to get the Resultant C A + B = CA + B = C

Fig 1 - shows the magnitude & direction of the 2 vectors we are addingFig 1 - shows the magnitude & direction of the 2 vectors we are adding Fig 2 – we move the beginning of vector B to the end of Vector A, making sure to keep the magnitude & direction exactly Fig 2 – we move the beginning of vector B to the end of Vector A, making sure to keep the magnitude & direction exactly

the samethe same Fig 3 – Connect the beginning of Vector A to the end of Vector B, this is your “Resultant” C.Fig 3 – Connect the beginning of Vector A to the end of Vector B, this is your “Resultant” C.

Fig 1

Fig 2

Fig 3

Click the icon to run java script game that

allows you to add vectors