How Can You Describe Motion?

Chapter 1, Lesson 2; pF14-21

Observing Motion

• Observing Motion– Change in object’s position compared to objects

at rest

• Describing motion scientifically– Measure position or distance– Measure time

• Relationship between distance and time– Speed

Identifying Frame of Reference

• Sometimes it is difficult to tell which object is in motion– Ex. Heliocentric model vs. Geocentric model• Helio = “sun”• Geo = “Earth”• Centric = “center”

– See sun move across our sky– Moon move across the sky– Stars move across the sky

• People think Earth is the center

Identifying Frame of Reference

• People use Earth to measure motion– Earth’s equator rotating • 1675 km/hr (1041 mi/hr)

– Earth orbits sun • 100,000 km/hr (62,000 mi/hr)

– Solar system orbits galaxy center • 792,000 km/hr (492,000 mi/hr)

• Sometimes necessary to use more distant frames of reference

Identifying Frame of Reference

• Motion is observed in comparison to objects that seem to be standing still– Ex.• Block falls from table• Moves past top of the table• Changing distance from the top of the table• Table is the frame of reference

• Frame of reference: something you do not expect to move

Identifying Frame of Reference

• Can’t observe motion without a frame of reference– Sitting in car,“feel” like you are moving forward

• Car beside you moved backwards

– Sitting in plane, “feel” like you aren’t moving?• Look outside and you are moving toward a runway

• Observing motion depends on observer’s frame of reference– Person walking east in west bound train

• Motion of walking person inside train• Motion of walking person outside train

Calculating Speed

• Measuring motion often requires measuring time– Speed: measure of how far something moves

during a period of time• Speed is a measurement at one instant of time

– Cars speed up and slow down– Glaciers & dolphins speed up & slow down too

• Ratio of distance moved to elapsed time– Examples / Units:

» Humans: 65 km/hr» Light: 300,000 km/s» San Andreas Fault: 2 cm/yr

Calculating Speed

• Difficult to measure speed in an instant of time– Average speed: measure of the total distance the

object has moved divided by total elapsed time• Drive from Prairie City to Chicago is 315 mi• The trip takes about 5 hrs• The average speed is 63 mi/hr

Velocity

• Velocity: speed of an object in a particular direction– Why do the following cars have different velocities?

• Ex 1: Two cars pass each other in opposite lanes of 2-way highway. Both cars traveling at 88 km/hr?– Different directions

• Ex. 2: Two cars pass each other in same lane of 2 lane highway. One car traveling at 60 km/hr the other at 80 km/hr?– Different speeds

– What will happen to the following cars?• Ex.3: Two cars traveling at the same velocity?

– Will always be same distance from each other

Velocity

• Scientists sometimes use arrows to represent velocity– Length represent speed measurement– Direction it points represents direction

measurement

• Velocity is circular motion constantly changing– Direction is constantly changing– Speed may remain constant

Velocity

• Changes in velocity caused by applied forces– Car speeds up, slows down, changes direction• Force has acted on it• Your body moves backwards, or forwards, or to the

side– Your body resists the force or change in motion

The Laws of Motion

• Evolution of the laws– Aristotle • Moving objects needed a force, push or pull, to keep it

moving

– Galileo• Moving objects keep moving in straight line until

something interfered with it

– Newton• Built on Galileo’s work

The Laws of Motion

• Laws of motion give us a way to measure & predict motion & the effect of forces– Hold true on Earth• Used to describe how objects move on Earth

– Laws can only be tested under controlled conditions– Too many other forces acting on objects

– Hold true in rest of the universe• Used to describe how planets move around the sun

The First Law of Motion

• First Law of Motion– “An object at rest tends to stay at rest, and an

object in motion tends to stay in motion in a straight line until an outside force acts on it”• Objects don’t change velocity unless acted on by force

– Examples:» Hockey puck sitting on ice» Puck sliding across the ice

– Objects in motion slow down on Earth» Outside force, friction, acts on them

The First Law of Motion

• “Law of Inertia”– Inertia: tendency of an object to resist a change in

its motion• Property of car that makes it difficult to start rolling• Property of car that makes it difficult to stop rolling

Second Law of Motion

• Second Law of Motion– “Force = Mass x Acceleration”– Unbalanced forces change motion of objects• Unbalanced force: force without an equal force in the

opposite direction• Changes velocity – speed, direction, or both• Acceleration: rate of change in velocity

– Depends on mass of object & size of force» Greater force = greater acceleration» Greater mass = smaller acceleration

Second Law of Motion

• Unbalanced forces change motion of objects– Examples:• Object at rest

– Object moves in direction of force

• Object moving in same direction as force– Object moves faster in same direction

• Object moving in the opposite direction as force– Object slows down or stops

Second Law of Motion

• Unbalanced forces change motion of objects– Examples:• 1 person pushing on back of car might get it to move• 2 people pushing can get it rolling faster• 1 person pushes from from – 1 from back may cancel

out

– Rate of change in car’s velocity depends on:• Mass of car• Size of forces applied to it

– Different results when you push on toy car vs. real car

Second Law of Motion

• Unbalanced forces change motion of objects– Amusement Park Examples:• Bumper car bumps into stationary car slowly?

– Stationary car moves a little bit

• Bumper car bumps into stationary car fast– Stationary car moves a lot more

• Acceleration depends on force applied by car in motion• Bumper car bumps into real car

– Bumper car moves backwards (car = more mass & inertia)

Third Law of Motion

• Third Law of Motion– “For every action force, there is an equal and

opposite reaction force”• Forces always come in pairs

– Examples: (Action force?) (Reaction force?)» Pushing on desk / Pulling of Door» Sit on a chair» Skateboarder pushes with a toe» Swimmer pushes on water

– Gravitational forces» Parachutist falling to Earth

Summary

• Motion is a change in the position of an object• Speed is a measure of an object over time• Velocity is a measure of speed in a direction• There are three laws that describe motion– An object at rest stays at rest and an object in motion

stays in motion– Unbalanced forces change the velocity of an object. The

rate of this change in velocity or acceleration depends on the size of the force and the mass of the object

– For every action force, there is an equal and opposite reaction force

Homework

• Read Lesson 2, Chapter 1, pF14-21• Answer ?s on Google Form