Motion, Acceleration, Energy of Motion

Chapter 2

MotionChapter 2.1

2.1 MotionObjectives

Compare frames of reference. Distinguish between speed and

velocity. Calculate when a moving object will

arrive at a given point. Make a graph to solve a distance-

time problem.

2.1 MotionImagine you are

traveling across the ocean on a ship. You see another ship getting closer.

Are both ships moving or is only the ship you are on moving?

2.1 MotionIt isn't always easy

to decide when objects are moving.

You judge motion in relation to stationary objects. This is called a frame of reference.

Frame of ReferenceA frame of

reference is a place or object that you assume is fixed.

You observe how objects move in relation to that frame of reference.

What does this

photographs tell you about the motion of

thecar?

You use the earth’s surface as your frame of reference most of the time. However, you experience moving frames of reference when you ride in a vehicle, such

as a car, a bus, or an elevator.Two persons with the same frame of

reference see the motion of an object the same way. Both boys on the bus see the ball drop straight

down.Two persons with different frames of reference see an object’s motion differently.

Both boys on the bus see the ball drop straight down. To a bystander, the ball moves in two directions—forward with the bus & down toward its

floor.

MOTION – Reference Frames

http://www.youtube.com/watch?v=7FYBG5GSklU

Measuring Motion Speed is the

distance an object travels in a certain amount of time.

To calculate speed, you divide the distance traveled by the time it took to travel that distance.

Speed = Distance/Time

If the cyclist rode his bike 8 km to school in 20 minutes, you

can calculate his speed in km/h as follows:

Speed : 8 km/20 min x 60 min/1 h = 24 km/h

Velocity The speed and the

direction of an object's motion are called velocity.

Since a moving object always travels in some direction, velocity is a more accurate term for describing motion.

A weather vane indicates wind direction. If it blows at 40 km/h what is its velocity?

Constant Speed A moving object that doesn't change its

speed travels at constant speed. Constant speed means equal distances are

covered in an equal amount or time.

A distance-time graph of

constant speed forms a straight line.

Average Speed A distance-time graph of a marathon

shows that the runner's speed changed several times. To find the speed of the runner

during the entire race, you need to calculate his average

speed.Average speed is equal to the

total distance traveled divided by the total time.

What was the average speed of the marathon

runner?

Practice Problems pg.36 1. A kayak races 100 m in 50 s. What is its

speed?

Speed = distance / time

100 m/ 50 s = 2 m/s

Now do 3 & 53. Distance=3m/s x 35s = 105 mDistance = 3m/s x 60s/min x 60 min = 10,800 m 5. 7.5 km / 5h = 1.5 km/h

Relativity and Space -Time

We know observations of motion depend on your frame of reference.

Yet, experiments show the speed of light is always the same, regardless of the motion of the light source or the motion of the observer!

To understand this, think about a rocket docked on the earth and another rocket traveling directly toward the sun at great speed…

The light from the sun reaches both rockets at the same speed (300 million meters/s).

Relativity and Space -Time

The constant speed of light is the basis for Albert Einstein's special theory of relativity.

He reasoned that space and time are connected into one whole, called space-time.

You are constantly traveling through this combination of space and time.

When you stand still, you travel only through time.

Relativity and Space -Time

When you move, you travel through space and time.

If you could move at close to the speed of light, you would travel quickly through space and slowly through time.

http://www.youtube.com/watch?v=KYWM2oZgi4E&feature=related

Check and Explain pg. 38

Answer questions 1 & 2. Use complete sentences.

1. A frame of reference is usually fixed and is used to describe the motion of objects relative to it.

2. the velocity of the bird include the direction the bird is moving.

AccelerationChapter 2.2

2.2 AccelerationObjectives

Define operationally the acceleration of an object.

Relate motion in a circle to acceleration.

Contrast acceleration and constant speed.

Make a graph showing acceleration.

AccelerationAny change in velocity is called

an acceleration. Acceleration can be speeding

up, slowing down, or changing the direction of the motion.

All of these accelerations, or velocity changes, require an outside force.

Acceleration Imagine you are competing as a speed skater at

the Winter Olympics.

During practice, you learned that three changes in speed were the key to winning the race:

speed up whenever possible slow down with control when necessary and change direction as smoothly as possible.

When you mastered these three things (changes in velocity), you are ready to compete.

Change in Velocity The rate at which velocity changes occur

is called acceleration.

To calculate acceleration you divide the change in velocity by the amount of time.

Acceleration = Change in velocity (final velocity – starting velocity) time

Suppose you ride a bike on a straight path to school at a velocity of 4 m/s. As you get

closer, you hear the school bell. In 3 s, you speed up to 10 m/s. How would you

calculate your acceleration?Change in Velocity = final velocity -

starting velocity10 m/ s – 4 m/ s = 6 m /s

Acceleration =

Positive Acceleration

To think about positive acceleration, imagine a car waiting at a stoplight. When the light turns from red to green, the driver steps on the accelerator, and the car speeds up. As the car moves faster, you feel the change in motion as your body is pushed back against the seat. How many seconds did

the car travel at a constant positive

acceleration?

A Graph of Positive Acceleration

Negative Acceleration

To think about negative acceleration, imagine a car slowing down. The car's velocity decreases over a certain amount of time. This type of velocity change is also called deceleration.

During what 3 second interval did the car decelerate fastest?

A Graph of Negative Acceleration

Change in DirectionAlso, whenever the

direction of a moving object changes, the velocity of the object changes.

Remember, any change in velocity is acceleration—even if the speed of the object remains the same.

Each change in direction represents a

change in velocity.

Skills Builder page 41

A Racer's AccelerationEach change in direction represents a change in velocity.

Suppose you are a race car driver. At the beginning of the race, all of the cars start from rest and accelerate straight down the race track before reaching the first corner.

Your car's speed after the first few seconds is given in the table.

Use the data in the table to plot a speed-versus-time line.

Skills Builder

Answers: No. Constant acceleration would produce

a linear graph. Final velocity, starting velocity and time. 8 m/s2 , 8 m/s2, 6 m/s2, and 4 m/s2

respectively. Greatest: 0-2 sec., Least: 3-4 sec. 26 m/s – 0 m/s = 6.5 m/s2

4s The friend will be in the lead.

Motion in a Circle An object moving in a

circle or a curve is constantly changing direction, therefore, the object is accelerating.

Acceleration caused by motion in a circle is called centripetal acceleration.

What would happen to the eraser if the girl let go of the string?

Why?

2.3 Energy of Motion

2.3 Energy of MotionObjectives

Define energy. Explain the law of conservation of energy.

Compare and contrast potential energy and kinetic energy.

Infer the gravitational potential energy of everyday objects.

Energy of MotionChanges in motion occur

constantly in the world around you. Cars move on busy highways. Tons of rock hurtle down mountainsides. Underground water is brought to the surface

for crop irrigation. What gives the cars, rocks, and

water the ability to move?

Energy and ChangeAny change in

motion requires energy.

Energy is the ability to do work.

When work is done, a change occurs.

One might say that energy is the source of change.

The archer uses energy to pull the bow's string toward her to change the string's position.

When she lets go, energy in the string

transfers to the arrow, and the arrow moves

forward.

Potential Energy

Stored energy has the ability to do work and is called potential energy.

Potential energy is associated with position.

Kinetic Energy Energy of motion is called kinetic

energy. The amount of kinetic energy depends

on the moving object's mass and velocity.

To calculate kinetic energy, you multiply one half of the object's mass times the square of its velocity.

Kinetic Energy

If the mass of a bowling ball is 4 kg and its velocity is 5 m/s, how much kinetic energy does it have?

Conservation of Energy

Energy cannot be created or destroyed.

This is known as the law of conservation of energy.

Energy can change into other forms, but the total amount of energy never changes.

Gravity and EnergyYou may infer that a rock on top of a cliff has potential energy because of its position.Once over the cliff’s edge, the force of gravity pulls the rock downward. The rock's energy is due to both its position on the cliff and the force of gravity. This type of energy is called gravitational potential energy.

Science & TechnologyTurn to page 49 in your text book.Read “SCIENCE & TECHNOLOGY” Great PotentialLook at figure 2.19Answer the question:How is a water wheel like a dam?