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# Forces, Motion, & Gravity

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Forces, Motion, & Gravity
Chapter 3 Forces, Motion, & Gravity

3.1 Forces, motion, and gravity
Objectives Identify 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? Force is needed to move or put an object in motion. Often forces are balanced, therefore the motion or velocity is 0. Gravity is a “pull” between objects in the universe. Any push or pull is a force. Your arm sets the ball in motion and gravity eventually pulls it to the ground.

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 … If no other forces acts on them, both the feather and the coin should accelerate at 9.8 m/s2 In absence of air resistance all objects fall at the same rate. In vacuum feather and coin fall at the same rate.

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.

FRICTION! 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

terminal velocity 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. Gravity causes downward motion, which is opposed by the force of friction caused by air on the objects 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.

terminal velocity 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. 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. The balls projectile motion is a combination of the downward motion produced by gravity & the horizontal motion produced by the thrower. This combination of horizontal and vertical motions causes the ball to follow a curved path. Anyone who is familiar with baseball has probably heard of the curve ball or the breaking ball. A curve ball breaks because an additional force is acting on it. These balls are thrown so it spins as it travels through the air. The spin causes an unbalanced air-pressure around the ball, causing it to break away from its normal path. This unexpected spin on the ball fools the batter and the ball becomes harder to hit

Check & explain pg. 59 Give two examples of force.
pushing a lawnmower, pushing a door open, pulling a wagon, throwing a ball 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.

Section 3.2 First Law of Motion

3.2 First Law of Motion Objectives State 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. Newton is recognized as the first person to state laws about the relationship between motion and forces. The applied force pulling the disc out is not great enough to overcome the inertia of the discs stacked above it. 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 !!

Inertia 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. 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 acting on 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. INERTIA COMES FROM THE LATIN WORD MEANING IDLE 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. What would the world be like if every object set into motion stayed in motion unless it hit into something?? Fortunately when matter rubs against against other matter, the motion creates friction. Friction is present anytime objects slide, roll, or rub against each other. If you kick a soccer ball on flat ground, friction between the ball and the ground slows and stops the ball.

Sliding friction keeps the baseball player from passing the base.
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. Friction affects every object on the earth…friction opposes your efforts, but without friction you would slip and slide all over. You wouldn’t be able to hold your pencils and write. Friction affects even boats and planes as they move through water and air.

* Fluid friction also occurs when an object falls through the air
Types of Friction Fluid Friction When a boat sails through the water, fluid friction from both the air and the water opposes its motion Air, water and oils are all fluids. * 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?

Check & explain pg. 63 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.

section 3.3 Second Law of Motion

3.3 second law of motion Objectives
State 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. For both carts to have the same acceleration, what must we change about the cart on the left? Because more mass was added to the cart on the right, it's acceleration is less.

Graphing Acceleration
The 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. The 200 kg sled accelerates half as fast as the 100 kg sled. In most dog-sled races, more than one dog pulls a sled. How would each dog affect the acceleration of the sled? Each dog would increase the acceleration of the sled because each dog would increase the forces applied to the sled. 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=MA Force = 40N mass = 20kg 40 = 20 A 20 = 20 A = 2m/s2 DO PRACTICE PROBLEMS 1 & 2 pg. 66 1 N = 1 kg X 1 m/s2

Using Newton's Second Law
PRACTICE PROBLEMS 1 & 2 pg. 66 F=MA F=(40kg)(2.5m/s2) F=100 N kg + 10kg + 10kg = 60 kg 90N=(60kg)A A=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 doesn't change unless the velocity or mass changes

Momentum Momentum doesn't change unless the velocity or mass changes.
However, momentum can transfer from one object to another. For example when a rolling ball hits another, some of the momentum of the first ball transfers to the second ball.

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 This force contributes to keeping the object in circular motion without flying away.

Check & explain pg. 69 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. 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. 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)

Section 3.4 Third Law of Motion

3.4 third law of motion Objectives
Describe 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. An easy way to understand the third law of motion is to say that every action has an equal and opposite reaction. Blow up a balloon…hold it upside down…let it go… Blow up a balloon…hold it vertically…let it go… Blow up a balloon…hold it horizontally…let it go… You can see equal and opposite reactions when you let it go. The force propelling the balloon is equal and opposite to the force of the air leaving the balloon.

Newton’s third law of motion
If you push something it will always push back. The skaters' forces on each other are equal in magnitude, and opposite in direction. Although the forces are equal, the accelerations are not: the less massive skater will have a greater acceleration due to Newton's second law. If a basketball hits the ground, the basketball's force on the Earth is the same as Earth's force on the basketball. However, due to the ball's much smaller mass, Newton's second law predicts that its acceleration will be much greater. Not only do planets accelerate toward stars; but, stars accelerate toward planets.

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.

Check & explain pg. 72 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. 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.

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: Gravitational Electromagnetic Strong.. 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. Gravity keeps the planets in orbit around the sun the moon orbiting around the earth and the stars in their orbit in galaxies. The gravitational force of the moon on the earth causes the ocean tides to rise & fall. 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? Look at the difference between the electromagnetic force and the force of gravity.

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 table shows the surface gravity of each planet as it relates to the earth's gravity, which is 1.00.

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. Electric forces holds together the particles that make up all matter-the foods you eat, the clothes you wear, and everything around you.

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. Electric forces holds together the particles that make up all matter-the foods you eat, the clothes you wear, and everything around you.

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. Magnetic force and electric force interact in motors and other devices. For example, carbon granules in the receiver of your telephone change the sound of your voice into electric signals. These electric signals travel through the wires from your home to that of your friend. A magnet in your friend's telephone receiver changes the electric signals back into the sound of your voice.

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 strong interaction (strong force) is the force that holds atomic nuclei together. Remember that atomic nuclei are composed of protons and neutrons. And protons, being positively charged and acting to repel each other, require lots of energy to cause them to stick together. The strong force does this, and we sometimes refer to it as (nuclear) binding energy or nuclear glue

Nuclear forces 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. 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 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.

Check & explain pg. 79 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 your vocabulary 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 _______________ . friction Terminal velocity inertia force Universal forces Strong nuclear forces newton

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