forces, motion, & gravity. objectives identify some forces that act in everyday situations. ...

CHAPTER 3Forces, Motion,

& Gravity

3.1 FORCES, MOTION, AND GRAVITY

ObjectivesIdentify some forces that act in

everyday situations.Determine the force of gravity in SI

units.Generalize about how friction affects

motion.Observe projectile motion in

everyday objects.

FORCES AROUND YOU How do you think force and motion are

related?

Why is gravity called a force?

When you throw a ball, what happens to it and why?

FORCES AROUND YOU A force is a push or pull that starts, stops,

or changes the direction of an object.

Force transfers energy to an object.

• To lift the wheelbarrow, the man must first apply force to lift it.

• He must then apply force in another direction to push the wheelbarrow.

FORCE AND GRAVITY The amount and

direction of most forces can be measured.

A spring scale is one kind of instrument used to measure force.

The spring scale, shown on the right, measures the force of gravity on an object.

SI UNIT - NEWTON The SI unit for force is called a newton. The newton is named for Sir Isaac

Newton, the scientist who explained how force and motion are related.

One newton (N) is the amount of force needed to cause a 1-

kg mass to accelerate at a rate of 1 m per second for each

second of motion.

FORCE AND GRAVITY GRAVITY DETERMINES WEIGHT!

The weight of an object depends on the force that pulls the object toward the earth.

You know this force as gravity!

Since the force of gravity on the earth is 9.8 m/s2, the weight of a 1-kg object is 9.8 N.

THE SPRING SCALE

FALLING OBJECTS If you drop a feather

and a coin at exactly the same time from a second story window, which one will reach the ground first?

Recall that the acceleration of gravity is 9.8 m/s2 …

FALLING OBJECTS When an object moves

through air, its motion is opposed by the force of friction.

Friction occurs when the surfaces of any kind of matter move past each other.

Friction from the air affects the motion of a falling object by acting against the force of gravity.

FALLING OBJECTS What kind of force might

act differently on the feather and the coin?

FRICTION!

SKILLSBUILDER Measuring with a Spring Scale - Go to

page 37 Record your predictions & measurements in

your graph.Measurement in Newtons

OBJECT PREDICTED FORCE (N)

MEASURED FORCE (N)

1

2

3

4

5

A falling object gains speed until the force from air friction, which acts upward, equals the downward force of gravity.

When the upward and downward forces are equal, the object reaches terminal velocity.

At terminal velocity, an object's velocity becomes constant.

TERMINAL VELOCITY

What forces are acting on both objects that affect the speed of the falling feather & coin?

• All falling objects reach terminal velocity if given enough time.

• However, the time required to reach terminal velocity varies.

• Dense objects with little surface area, such the coin, must fall for several seconds before reaching terminal velocity.

• Less dense objects with a lot of surface area, such as the feather, reach terminal velocity much faster.

HOW WOULD CHANGING THE SHAPE OF AN OBJECT AFFECT ITS TERMINAL VELOCITY?

Decreasing the surface area of an object will decrease the force of air friction acting on it, and increase its terminal velocity.

TERMINAL VELOCITY

Try it with a tissue…

PROJECTILE MOTION If you throw a ball for your dog to fetch,

the ball follows a curved path to the ground.

This path is called projectile motion.

http://www.brainpop.com/science/motionsforcesandtime/force/

1. Give two examples of force.

pushing a lawnmower, pushing a door open, pulling a wagon, throwing a ball

2. Describe how you can use a spring scale to measure the force of gravity on an object.

The weight of an object is a measure of the force of gravity on an object – measured in Newtons. The force pushes on the object which stretches the spring. The spring scale has calibrations on it that measures the force in Newtons as well as its weight in grams.

CHECK & EXPLAIN PG. 59

SECTION 3.2

First Law of Motion

3.2 FIRST LAW OF MOTION

ObjectivesState Newton's first law of motion.

Identify inertia and explain how to overcome it.

Define operationally how friction occurs in an everyday situation.

Observe Newton's first law of motion.

NEWTON’S FIRST LAW OF MOTION

Newton observed that an object at rest stays at rest until an outside force causes it to move.

He also observed that an object in motion continues to move in the same direction until a force stops it or changes its direction.

Newton stated his observations as the first law of motion.

Why do the disc boxes at the top of the stack stay in place when the girl pulls

out the lower one?

NEWTON’S FIRST LAW OF MOTION

AN OBJECT AT REST WILL REMAIN AT REST AND AN OBJECT IN MOTION WILL

REMAIN IN MOTION UNLESS ACTED ON BY AN

OUTSIDE FORCE !!

Your body is at rest, and it will remain at rest until some outside force moves it.

Your body resists change; you have inertia.

Inertia is the tendency of an object to resist any change in motion.

INERTIA

To overcome your inertia and move out of the

chair, you must apply some kind of force to your body. This force must be greater than

the forces that are actingon your body.

OBJECTS IN MOTION The first law of motion

and inertia also applies to objects in motion.

Inertia can be overcome only by the application of some type of force.

When you pedal a bike, you accelerate to make the bike

move forward. When the biker stops pedaling, the bike and the rider keep moving forward in the

same direction. This is due to inertia.

FRICTION …When matter rubs against other matter,

the motion creates friction. Friction works in the opposite direction to

the force of motion. Friction is an outside force that resists

motion when two surfaces come in contact.

If you kick a soccer ball on flat ground, friction

between the ball and the ground slows and stops

the ball.

TYPES OF FRICTION Sliding Friction

When two solid surfaces slide over each other, sliding friction acts between them.

Sliding friction keeps the baseball player from passing the

base.

TYPES OF FRICTION

Fluid FrictionWhen a boat sails

through the water, fluid friction from both the air and the water opposes its motion

* Fluid friction also occurs when

an object falls through the air

TYPES OF FRICTION Rolling Friction

When an object rolls over a surface, the friction produced is called rolling friction

The tread on a tire affects the

amount of rolling friction

to be overcome.

Which tire will create more

rolling friction?

1.What is Newton’s first law of motion?

Newtons first law of motion is also the law of inertia – an object will remain at rest or in motion unless acted upon by an outside force.

2.Explain how a dog overcomes inertia when it gets up after napping on the floor.

The dog uses its muscles to apply a force needed to overcome its napping position.

3.Explain how friction works to keep bicycle tires from sliding on the road.

The weight of the rider and bike force the tires against the road surface which creates friction and keeps the tires from slipping.

CHECK & EXPLAIN PG. 63

SECTION 3.3

Second Law of Motion

3.3 SECOND LAW OF MOTION

ObjectivesState Newton's second law of motion

Calculate the force of a moving object by using Information about its mass and acceleration.

Identify the relationship between momentum, mass, and velocity.

Make a model to show how force acts during circular motion.

ACCELERATED MOTION Newton understood that

force, mass, and acceleration are related.

He showed that the motion of an object changes, or accelerates, when a force acts on it.

If you know the mass and the acceleration of an object, you can determine its force.

This relationship is stated in Newton's second law of motion.

NEWTON SECOND LAW OF MOTION

THE NET FORCE ON AN OBJECT EQUALS ITS

MASS TIMES ITS ACCELERATION.

ACCELERATED MOTION

FORCE, MASS, AND ACCELERATION

A direct relationship exists between force (f) mass (m) and acceleration (a).

The boy in the picture applies the same amount of force to both carts.

Because more mass was added to the cart on the right, it's acceleration is

less.

For both carts to have the same acceleration, what must we change about

the cart on the left?

GRAPHING ACCELERATIONThe graph shows the relationship between force and acceleration during a dog-sled

race.Notice that as the

force the dogs apply to the sled

increases, the acceleration of the sled also increases.

How does the acceleration of the 200 kg sled compare to the

acceleration of the 100 kg sled?

USING NEWTON'S SECOND LAW

Even without doing calculations, Newton's second law helps you understand how

force, mass, and acceleration are related.

Recall Newton's formula for the second law of motion.

USING NEWTON'S SECOND LAW

Sample Problem:A dog has a mass of 20 kg. If the dog is

pushed across the ice with a force of 40 N, what is its acceleration?

F=MAForce = 40N mass = 20kg

40 = 20 A20 = 20 A = 2m/s2

DO PRACTICE PROBLEMS 1 & 2 pg. 66

USING NEWTON'S SECOND LAW

PRACTICE PROBLEMS 1 & 2 pg. 66F=MA

F=(40kg)(2.5m/s2)F=100 N

--------------------------------------40kg + 10kg + 10kg = 60 kg

90N=(60kg)A 60 60A=1.5m/s2

MOMENTUM A rolling marble can be stopped more

easily than a bowling bowl moving at the same velocity. Both objects have inertia of motion or momentum.

The bowling bowl has more momentum…why?

MOMENTUM All objects have inertia of motion, or

momentum. The momentum of a moving object is related to

its mass and velocity. A moving object has a large momentum if it has

a large mass, a large velocity, or both.

MOMENTUM Momentum doesn't change unless the

velocity or mass changes. However, momentum can transfer from

one object to another.

CLASS ACTIVITY Choose a sports activity to

demonstrate the transfer of

momentum.

In each instance how was momentum transferred from one object to another??

Balls change direction and or speed…players may fall after colliding with another player or ball…

FORCES IN CIRCULAR MOTION

You learned that an object in circular motion has centripetal acceleration.

For example, if you twirl an eraser on a string, the tension on the string pulls the eraser inward, toward the center…The inward tension on the string is centripetal force.

Centripetal force is any force that causes an object to follow a circular path

http://www.youtube.com/watch?v=yyDRI6iQ9Fw

1. How does the force applied to an object affect its acceleration?

The greater the force applied to an object, the greater the acceleration if the mass remains constant.

2. If an eagle and a bumblebee are both traveling at 16 km/hr., which has more momentum? Explain.

The eagle because the eagle has more mass.

3. Calculate: A 28 kg meteor hits the surface of the moon at 130 km/s. What is the meteor’s momentum? M=mv

3, 640, 000 kg*m/s (remember to convert km/s to m/s because velocity is measured in m/s)

CHECK & EXPLAIN PG. 69

SECTION 3.4

Third Law of Motion

3.4 THIRD LAW OF MOTION

ObjectivesDescribe Newton's third law of motion.

Distinguish between balanced and unbalanced forces.

Predict how forces interact in everyday situations.

Infer how the third law of motion is applied.

NEWTON’S THIRD LAW OF MOTION

When one object exerts a force upon a second

object, the second object exerts an equal

and opposite force upon the first object.

NEWTON’S THIRD LAW OF MOTION

NEWTON’S THIRD LAW OF MOTION

• For example, The rockets of the space shuttle force burning gases downward through the exhaust vents.

• In response to these downward forces, the shuttle system moves upward.

•When one object exerts a force upon a second object, the second object exerts an equal and opposite force upon the first object.

BALANCED AND UNBALANCED FORCES

When forces on an object are unbalanced motion occurs.

This crew team uses Newton’s third law of motion to move it’s boat.

The boat moves in the opposite direction of the oars with a force that is equal to that of the oars as they push against the water.

http://www.brainpop.com/science/motionsforcesandtime/newtonslawsofmotion/preview.weml

1. What is Newton’s third law of Motion?

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

3. Predict: What are the action and reaction forces acting when someone jumps from a canoe to a riverbank? Explain why the jumper falls into the water?

The force exerted by the jumper makes the boat move instead of the jumper who falls before reaching the bank.

CHECK & EXPLAIN PG. 72

SECTION 3.5

Universal Forces

3.5 UNIVERSAL FORCES Objectives

Identify the four universal forces.Describe how each universal force

affects everyday experiences.Generalize on the importance of

universal forces.Classify everyday forces according

to the types of universal forces.

3.5 UNIVERSAL FORCES While traveling through the

universe, you experience several kinds of forces.

These four universal forces are:GravitationalElectromagneticStrong..and weak nuclear forces

GRAVITATIONAL FORCE Newton's law of universal

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

Objects of any size are pulled toward each other by gravitational force.

For example, the earth's gravitational force on your body pulls you toward the ground.

You don't notice the gravitational force between small objects because it is the weakest of the universal forces.

Planets, stars, and galaxies are so large that their gravitational force affects other objects in

space.

LAW OF UNIVERSAL GRAVITATION

The strength of gravitational force depends on the amount of mass in an object and the distance between objects.

What would happen to objects on the earth if the force of gravity was as strong

as the electromagnetic force?

GRAVITY AND THE SOLAR SYSTEM Gravitational force

keeps the earth and other objects in the solar system in orbit around the sun.

Each planet moves in an elliptical orbit around the sun.

THE EARTH-MOON SYSTEM

Newton realized the gravitational force that applies to objects on the earth also applies to the moon.

He proposed that the moon travels around the earth because the earth pulls on the moon.

The combined gravitational force of the sun and the moon (above) produces large spring

tides.

ELECTROMAGNETIC FORCES

Electromagnetic forces are really two different forces that are closely related – electric force and magnetic force.

Both of these forces may attract or repel.

ELECTROMAGNETIC FORCES

Electric force exists between charged particles.

Objects with different charges attract each other, and objects with the same charge repel each other.

The electric force is much stronger than the gravitational force.

ELECTROMAGNETIC FORCES Magnetic force acts between two

magnets. Magnetic force attracts or repels just as

electric force attracts and repels.

The magnets moving this train

can be seen along the outer

edges of the track.

NUCLEAR FORCES An atom is

composed of electrons orbiting around a nucleus of protons and neutrons.

Electric forces keep electrons in orbit around the nucleus.

The forces that hold the particles in the nucleus together are nuclear forces.

NUCLEAR FORCES One type of nuclear

force is called the strong force.

Protons and neutrons are made of even smaller particles called quarks.

The strong force holds the nuclear particles together by holding the quarks together.

The other type of nuclear force is the weak forces.

The weak force holds together the particles within neutrons and protons.

In neutrons, the weak force is easily overcome, and the neutrons decay to form different atoms.

Nuclei that decay in this way are called radioactive.

NUCLEAR FORCES

Measurement of radioactive decay is used to reveal the age of the materials used in ancient artifacts.

Some kinds of nuclear decay release harmful radiation that can affect people and the environment.

Safe, permanent disposal of radioactive materials is a worldwide problem.

NUCLEAR FORCES

1. Name the four universal forces.

Gravitational

Electromagnetic

Strong nuclear forces

Weak nuclear forces

2. Explain how each universal force might apply when you warm food in an oven.

Gravity keeps food in one place; electromagnetic forces operate the oven; nuclear force holds the atoms together in the food.

CHECK & EXPLAIN PG. 79

1. The force that opposes motion is _______________.

2. When the upward and downward forces on a falling object are equal, the object reaches _____________.

3. When your body is at rest, you have _____________.

4. A push or pull that starts or stops the motion of an object or changes its direction is called a(n) ______________ .

5. Gravitation, electromagnetic, and strong and weak nuclear forces are ________________ .

6. The forces that hold together the particles in the nucleus of an atom are called __________________ .

7. The SI unit for force is _______________ .

CHECK YOUR VOCABULARY

Terminal velocity

Universal forces

inertia

force

friction

Strong nuclear forcesnewton