Inertia and Mass

•  Inertia (ih NUR shuh) is the tendency of an object to resist any change in its motion.

Motion and Forces

•  If an object is moving, it will have uniform motion.

•  It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it.

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Inertia and Mass

•  The velocity of the object remains constant unless a force changes it.

Motion and Forces

•  If an object is at rest, it tends to remain at rest. Its velocity is zero unless a force makes it move.

•  The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.

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Newton’s Laws of Motion

•  The British scientist Sir Isaac Newton (1642–1727) was able to state rules that describe the effects of forces on the motion of objects.

Motion and Forces

•  These rules are known as Newton’s laws of motion.

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Newton’s First Law of Motion •  Newton’s first law of motion states that an

object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it.

Motion and Forces

•  If an object is at rest, it stays at rest unless an unbalanced net force acts on it.

•  This law is sometimes called the law of inertia.

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What happens in a crash? •  The law of inertia can explain what happens

in a car crash.

Motion and Forces

•  When a car traveling about 50 km/h collides head-on with something solid, the car crumples, slows down, and stops within approximately 0.1 s.

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What happens in a crash?

•  Any passenger not wearing a safety belt continues to move forward at the same speed the car was traveling.

Motion and Forces

•  Within about 0.02 s (1/50 of a second) after the car stops, unbelted passengers slam into the dashboard, steering wheel, windshield, or the backs of the front seats.

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Safety Belts

•  The force needed to slow a person from 50 km/h to zero in 0.1 s is equal to 14 times the force that gravity exerts on the person.

Motion and Forces

•  The belt loosens a little as it restrains the person, increasing the time it takes to slow the person down.

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Safety Belts

•  This reduces the force exerted on the person.

Motion and Forces

•  The safety belt also prevents the person from being thrown out of the car.

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Safety Belts •  Air bags also reduce injuries in car crashes by

providing a cushion that reduces the force on the car’s occupants.

Motion and Forces

•  When impact occurs, a chemical reaction occurs in the air bag that produces nitrogen gas.

•  The air bag expands rapidly and then deflates just as quickly as the nitrogen gas escapes out of tiny holes in the bag.

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•  Newton’s first law of motion states that the motion of an object changes only if an unbalanced force acts on the object.

•  Newton’s second law of motion describes how the forces exerted on an object, its mass, and its acceleration are related.

Force, Mass, and Acceleration 1 Newton’s Second Law

Newton’s 2nd Law

•  Newton’s Second Law states: An object acted upon by an unbalanced force will accelerate in the direction of the force.

•  If you kick the ball, it starts moving.

•  The ball accelerates only while your foot is in contact with the ball.

Newton’s Second Law …can be written as a formula

•  In this equation, a is the acceleration, m is the mass, and Fnet is the net force.

•  If both sides of the above equation are multiplied by the mass, the equation can be written this way:

This is the more common way to write Newton’s Second Law:

Don’t write this version of the Law. There is another more common version coming up…

13 of 8 © Boardworks Ltd 2008

Newton’s Second Law

  Force is measured in newtons (N).

  Mass is measured in kilograms (kg).

  Acceleration is measured in meters per second per second (m/s2).

force = mass x acceleration

14 of 8 © Boardworks Ltd 2008

Example

Cover the quantity that you are trying to work out, which gives the rearranged formula needed for the calculation.

So to find force (f), cover up f…

A book with a mass of 2.0 kg is pushed along a table. The acceleration of the book is 0.5 m/s2. What is the net force on the book?

Newton’s 2nd Law (Example 2)

•  Newton’s second law of motion can be used to calculate acceleration.

•  For example, suppose you pull a 10-kg sled so that the net force on the sled is 5 N.

•  The acceleration can be found as follows:

Gravity & Weight

•  Gravity is the force of attraction that exists between any two objects that have mass.

•  The force of gravity depends on the mass of the objects and the distance between them.

•  Gravity is an attractive force between any two objects that depends on the masses of the objects and the distance between them.

2 Gravity

If the mass of either of the objects increases, the force between them increases .

If the objects are closer together, the gravitational force between them increases

•  No matter how far apart two objects are, the gravitational force between them never completely goes to zero.

Gravity

The Range of Gravity

•  Because the gravitational force between two objects never disappears, gravity is called a long-range force.

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Gravity & Weight

•  Weight is a force, like the push of your hand is a force, and is measured in Newtons.

•  Your weight on Earth is the gravitational force between you and Earth.

Gravity & Weight

•  The force of gravity causes all objects near Earth’s surface to fall with an acceleration of 9.8 m/s².

•  This acceleration is given the symbol g and is sometimes called the acceleration of gravity.

Gravity

Earth’s Gravitational Acceleration

•  By Newton’s second law of motion, the force of Earth’s gravity on an object (i.e., its weight) is the object’s mass times the acceleration of gravity.

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Gravity

Weight 2

Recall Newton’s Second Law when calculating weight:

force = mass x acceleration

•  Weight and mass are not the same.

Gravity

Weight and Mass

•  Weight is a force, and mass is a measure of the amount of matter an object contains.

•  Weight and mass are related. Weight increases as mass increases.

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Gravity

Weight and Mass

•  The weight of an object can change, depending on the gravitational force on the object.

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•  The table shows how various weights on Earth would be different on the Moon and some of the planets.

Gravity

Weight and Mass 2

•  You’ve probably seen pictures of astronauts and equipment floating inside the space shuttle.

Gravity

Weightlessness and Free Fall

•  They are said to be experiencing the sensation of weightlessness.

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Do not attempt to write an explanation of weightlessness in your outline until the teacher directs you to do so at the end of this set of slides….

•  However, for a typical mission, the shuttle orbits Earth at an altitude of about 400 km.

Gravity

Weightlessness and Free Fall

•  According to the law of universal gravitation, at 400-km altitude the force of Earth’s gravity is about 90 percent as strong as it is at Earth’s surface.

•  So an astronaut with a mass of 80 kg still would weigh about 700 N in orbit, compared with a weight of about 780 N at Earth’s surface. …So he is NOT weightless in space!

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•  So what does it mean to say that something is weightless in orbit?

Gravity

Floating in Space

•  When you stand on a scale, you are at rest and the net force on you is zero.

•  The scale supports you and balances your weight by exerting an upward force.

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•  The dial on the scale shows the upward force exerted by the scale, which is your weight.

Gravity

Floating in Space

•  Now suppose you stand on the scale in an elevator that is falling.

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•  If you and the scale were in free fall, then you no longer would push down on the scale at all.

Gravity

Floating in Space

•  The scale dial would say you have zero weight, even though the force of gravity on you hasn’t changed!

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•  Similarly, a space shuttle in orbit is in free fall, but it is falling around Earth, rather than straight downward.

Gravity

Floating in Space

•  Everything in the orbiting space shuttle is falling around Earth at the same rate, in the same way you and the scale were falling in the elevator.

•  Objects in the shuttle seem to be floating because they are all falling with the same acceleration.

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You may now write an explanation of weightlessness in your outline.

PROJECTILE MOTION

•  If you’ve tossed a ball to someone, you’ve probably noticed that thrown objects don’t always travel in straight lines. They curve downward.

Gravity

Projectile Motion

•  Earth’s gravity causes projectiles to follow a curved path.

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•  When you throw a ball, the force exerted by your hand pushes the ball forward.

Gravity

Horizontal and Vertical Motions

•  This force gives the ball horizontal motion. •  No FORCE

accelerates it forward once it’s in the air, so its horizontal velocity is constant, if you ignore air resistance.

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•  However, once you let go of the ball, gravity can pull it downward, giving it vertical acceleration.

Gravity

Horizontal and Vertical Motions

•  The ball has constant horizontal velocity but increasing vertical velocity.

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•  Gravity exerts an unbalanced force on the ball, changing the direction of its path from only forward to forward and downward.

Gravity

Horizontal and Vertical Motions

•  The result of these two motions is that the ball appears to travel in a curve.

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•  If you were to throw a ball as hard as you could from shoulder height in a perfectly horizontal direction, would it take longer to reach the ground than if you dropped a ball from the same height?

Gravity

Horizontal and Vertical Distance 2

•  Surprisingly, it wouldn’t.

Gravity

Horizontal and Vertical Distance

•  Both balls travel the same vertical distance in the same amount of time.

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Circular Motion- Centripetal Force

•  When a ball enters a curve, even if its speed does not change, it is accelerating because its ___________ is changing.

Gravity

Centripetal Force and Acceleration

•  When a ball goes around a curve, the change in the direction of the velocity is toward the center of the curve.

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direction

The word centripetal means “center seeking”

Circular Motion

•  If you are constantly accelerating, there must be a force acting on you the entire time.

•  The force exerted is the centripetal force and always points toward the center of the circle.

•  Acceleration toward the center of a curved or circular path is called centripetal acceleration.

Gravity

Centripetal Force and Acceleration 2

•  Imagine whirling an object tied to a string above your head.

Gravity

Gravity Can Be a Centripetal Force

•  The string exerts a centripetal force on the object that keeps it moving in a circular path.

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Gravity

Gravity Can Be a Centripetal Force •  In the same way, Earth’s gravity exerts a

centripetal force on the Moon that keeps it moving in a nearly circular orbit.

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