CHAPTER 2

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THINK ABOUT IT….

If Wile E. Coyote and a boulder fall off a cliff at the same time, which do you think will hit the ground first? Would it matter if the cliff were very high or particularly low? How could Mr. Coyote slow down his fall?

SECTION : GRAVITY AND MOTION

Gravity and Falling Objects - ActivityGravity and Acceleration

Demo 1: Which will hit the ground first, second, third, a basketball, baseball, ping pong ball?

   Prediction:    Observation: 

LEFT

Acceleration Due To Gravity

Acceleration = the rate of velocity changing over time.A falling object accelerates at a constant rate. The object falls farther and faster each second than it did the second before

RIGHT - NOTES

Velocity of Falling Objects

Equation:

V = g x t

Example Problems:

EX 1: A penny at rest is dropped from the top of a tall stairwell. What is the penny’s velocity after it has fallen for 2 s?

 

  

 

EX 2: The same penny hits the ground in 4.5s. What is the penny’s velocity as it hits the ground?

Table of Formulas

RIGHT - NOTES

Velocity of Falling Objects

EX 3: A marble at rest is dropped from a tall building. The marble hits the ground with a velocity of 98 m/s. How long was the marble in the air?

 

 

EX 4: An acorn at rest falls from an oak tree. The acorn hits the ground with a velocity of 14.6 m/s. How long did it take the acorn to land?

Air Resistance and Falling Objects

Demo1: Which falls faster?

Crumpled paper or flat sheet of paper?

 

 

Why did this happen?

Demo

Falling Water

Trial 1 Observation:

Trial 2 Observation:

What differences did you observe in the behavior of water during the two trials?

In trial 2, how fast did the cup fall compared with how fast the water fell?

Activity - LEFT

Spider MapActivity - LEFT

gravity

9.8 m/s2

force

Air resistance

Opposes gravityforce

Terminal velocity

Constant velocityGravity = air resistance

Free Fall No air resistance

Only gravity

orbi

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g

Object moves forward

Object in free fall

Unbalanced force

Objects in circular path

Forces and Motion

Centripetal force

Projectile Motion

Objects in curved pathObject accelerates vertically downward

Spider MapActivity - LEFT

gravity

9.8 m/s2

force

Air resistance

Opposes gravityforce

Terminal velocity

Constant velocityGravity = air resistance

Free Fall No air resistance

Only gravity

Forces and Motion

Air Resistance and Falling Objects

Air Resistance - force that opposes the motion of objects through air

RIGHT - NOTES

Acceleration Stops at Terminal Velocity

• As the speed of a falling object , air resistance .

• Air resistance until it = gravity.

• Terminal velocity – the constant velocity of a falling object when the air resistance = gravity

RIGHT - NOTES

An Introduction to Skydiving

Free Fall Occurs When There Is No Air Resistance

Free fall – the motion of a body when only the force of gravity acts it

Two places that have no air resistance and free fall:• Vacuum • Space

RIGHT - NOTES

Orbiting Objects are in Free Fall

• An object is in orbit for two reasons:• It is moving forward• It is in free fall

RIGHT - NOTES

LEFT - PICTURE

• Centripetal force - the unbalanced force that causes objects to move in a circular path

Ex. The moon orbits the earth, the earth around sun

Space exploration: What is an orbit?

The swing set myth

Orbiting and Centripetal ForceRIGHT - NOTES

Projectile Motion and Gravity

Projectile Motion and Gravity

Projectile Motion- the curved path that an object follows when thrown.

Horizontal Motion - When you throw a ball, the forward force your hand exerts on the ball

Vertical Motion- Pulled down towards the earth by gravity

RIGHT - NOTES

LEFT - PICTURE

QUICK LAB – PENNY PROJECTILE MOTION

1. Position a ruler and two pennies on a desk.

2. Hold the ruler by the end on the desk, move the ruler quickly so it knocks the penny off the table and so that the other penny also drops. Repeat.

3. Which penny travels in a projectile motion?

4. In what order do the pennies hit the ground?

LEFT - ACTIVITY

SECTION 2: NEWTON’S LAWS OF MOTION

Newton’s First Law of Motion

An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.

What does this mean?

This means that there is a natural tendency of objects to keep on doing what they're doing. All objects resist changes in their state of motion. In the absence of an unbalanced force, an object in motion will maintain this state of motion.

RIGHT - NOTES

NEWTON’S FIRST LAW OF MOTIONWhat is the motion in this picture? Forward motion – rolling

What is the unbalanced force in this picture? Rock

What happened to the skater in this picture? Fall off the skateboard

 

What other force do you know that can stop an object from moving?

Friction, gravity

LEFT - PICTURE

NEWTON’S FIRST LAW OF MOTION

Newton’s 1st Law is also the reason you wear a seatbelt!

Newton’s 1st Law is also known as inertia

Inertia – the tendency of an object to resist being moved, or if the object is moving to resist a change in speed or direction until an outside force acts on it

Does mass have an affect on inertia? Yes mass has an affect on inertia

Would you rather catch a baseball or bowling ball?

RIGHT - NOTES

EXPLORING INERTIALEFT - PICTURE

NEWTON’S SECOND LAW OF MOTIONAcceleration is produced when a force acts on a mass.

The greater the mass, the greater the force needed to accelerate the object.

So, Heavier objects need more force to move the same distance as lighter objects

NEWTON’S 2ND LAW OF MOTION

Newton’s 2nd Law can be expressed as a mathematical equation:

or FORCE = MASS times ACCELERATION

SI Unit for force = Newtons (N)

NEWTON’S 2ND LAW OF MOTION

Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s2. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

NEWTON’S 2ND LAW OF MOTION

What is the acceleration of a 3 Kg mass if a force of 14.4 N is used to move the mass?

What force is necessary to accelerate a 1,250 Kg car at a rate of 40 m/s2?

NEWTON’S THIRD LAWRIGHT - NOTES

…When an object pushes another object it gets pushed back in the opposite direction equally hard

…For every force there is a reaction force that is equal in size, but opposite in direction.

For every action there is an equal and opposite re-action.

NEWTON’S THIRD LAW

Action – rocket pushes down on the ground with the force of its engines

Reaction - the ground pushes the rocket up with an equal force.

Newton’s Cradle

RIGHT - NOTES

SECTION 3: MOMENTUM

Momentum - Depends on the object’s mass and velocity.

The more momentum an object has, the harder it is to stop the object or change directions.

Momentum = mass x velocity

p = m x v (Kg m/s) (Kg) (m/s)

RIGHT - NOTES

What would have more momentum? A bowling ball or a basketball rolled at the same velocity?Bowling ball b/c it has more mass

 A tractor trailer or a Honda civic at the same velocity?Tractor trailer

MOMENTUMMomentum = mass x velocity

p = m x v (Kg m/s) (Kg) (m/s)

Example:

A 100 Kg car falls off a cliff from rest and hits the ground with a velocity of 35 m/s. What is the car’s momentum when it hits the ground?

A 35 Kg bowling ball was thrown at Tommy at a velocity of 15 m/s. What was the bowling ball’s momentum?

RIGHT - NOTES

MOMENTUM PROBLEMS

What is the momentum of an ostrich with a mass of 120 Kg that runs with a velocity of 16 m/s north?

  What is the momentum of a 6 Kg cat that is moving at 10 m/s

down the alley toward the mouse?    An 85 Kg man is jogging with a velocity of 2.6 m/s to the north.

Nearby, a 65 Kg person is skateboarding and is traveling with a velocity of 3 m/s north. Which person has a greater momentum? Show your work.

RIGHT - NOTES

SECTION 3: MOMENTUMRIGHT - NOTES

Momentum and Velocity and Mass

• If you increase velocity, then momentum increases

• If you increase mass, then momentum increases

Collisions

• If a moving train collides with a train at rest and the trains move together, you momentum of both is equal

• If a heavy object (bowling ball) collides with a light object (bowling pin), the heavy object’s momentum decreases

LAW OF CONSERVATION OF MOMENTUM

When two objects collide, their combined momentum remains the same after collision

Newton’s third law of motion = Conservation of Momentum

  5Reaction ForceAction Force

RIGHT - NOTES

The action force is the cue ball, the reaction force is the #5 billiard ball exerting force back

 

 

CONCEPT MAP – GRAVITY REVIEW

Gravity

LEFT - DRAW