Motion

Motion Motion a change in an objecta change in an object’’s positions position

Motion

Linear motion:Linear motion: motion in a single motion in a single dimension (in a line).dimension (in a line).When you talk about an object’s motion, you are referring to a rate

Rate:Rate: A quantity divided by time A quantity divided by time - tells how quickly something - tells how quickly something happenshappens

Motion is RelativeIf something is relative, it depends on

the frame of referenceFrame of Reference:Frame of Reference: point of view point of view of the observerof the observer

When we discuss the motion of something, we describe its motion relative to something else.

Usually, when we discuss the speeds of things on Earth, we mean the speed with respect to the Earth’s surface.Ex: how quickly a dog runs past a tree

SpeedSpeed:Speed: the distance covered per unit of the distance covered per unit of time.time.•A measure of how fast something is moving.

• the rate at which distance is covered. Ex: 100 km/hr, 55 mph, 30 m/sEquation: s = d / ts = d / t

s = speed d = distance (m, km, cm, mi, ft) t = time (s, hr, day, minute, year, millenium )

Instantaneous speed • the speed at any given instant

– Ex: speedometer

Average speedAverage speed the total distance covered divided by the time it takes to cover that distanceAverage speed does not indicate changes in the speed that may take place during a trip.

BOTH instantaneous and average speeds indicate the rate at which distance is covered.

Speed practice

• If a nascar driver was able to drive 600 miles in 3 hours, what was his average speed?

Speed practice 2

• What is the speed of a car that travels 20 miles in 40 minutes?

Discuss with a neighbor how motion and Discuss with a neighbor how motion and speed are relatedspeed are related

Check Your UnderstandingCheck Your Understanding

Discuss with a different Discuss with a different neighbor how instantaneous neighbor how instantaneous speed and average speed speed and average speed are similar are similar andand how they are how they are differentdifferent

Velocity

• When we say that a car travels 60km/hr, we are indicating it’s speed. When we say that a car is traveling 60km/hr to the north, we are indicating it’s velocity.

Velocity:Velocity: speed in a given speed in a given directiondirection

•Ex: 100 km/hr East, 55 mph North, 30 m/s Southwest•Equation: v = d / tv = d / t

Speed is a description of how fast an object moves; velocity is how fast it moves AND in what direction .

Check Your UnderstandingCheck Your Understanding

The speedometer of a car moving The speedometer of a car moving northward reads 100 km/h. It passes northward reads 100 km/h. It passes another car that travels southward at another car that travels southward at 100 km/h. Do both have the same 100 km/h. Do both have the same speed? Do they have the same speed? Do they have the same velocity? Discuss with a neighborvelocity? Discuss with a neighbor

Both cars have the same speed, but they have opposite velocities because they are moving in opposite directions.

Constant Velocity• Constant velocity requires both constant

speed and constant direction. • Motion at constant velocity is in a straight

line at constant speed.

Changing Velocity• Constant speed and constant velocity are

not the same thing. • A body may move with constant speed

around a curved path, but it does not move with constant velocity because the direction changes at every instant.

Let’s do the math• We’ve practiced calculating speed when given

distance and time, but can you find distance if given speed and time?– Of course you can, you just need to rearrange the

speed formula so that you are solving for distance

– Try it – calculate distance if given:• Speed = 8 m/s • T = 2 s

• You can also find time if given speed and distance, just rearrange the speed formula to solve for time

• Try it: find time if given:– S = 6 km/s, d = 30 km

Warm up – write your answers on paper from the red bucket

1.  How are velocity and speed similar?2.  How are they different?3.  What must happen for velocity to remain constant (hint, there are two things)?

4. Draw the magic triangle for speed

5. Write the formula to solve for time if given speed and distance

*When you finish, get out your homework from last night and your motion notes

Position - Time graphs

• Position-Time graphs show the distance covered over an elapsed time– Aka Distance-Time

graphs and Displacement-Time graphs

• Time is always the independent variable

• Position (distance) is always the dependent variable

• The slope of a Position-Time graph is equal to velocity– Slope = rise/run– Slope = position / time – Velocity = position / time

• The steeper the slope, the faster the velocity– A positive slope is forward

motion– A negative slope is moving

backwards– A zero slope is NOT moving at

all

Finding speed from a graph• To determine speed

from this graph, do the following:– Pick two points– Determine how much

distance was covered between the two points

– Determine how much time passed between the two points

– Plug those #s into the speed equation and solve

Check Your UnderstandingCheck Your UnderstandingDiscuss with a

neighbor: Which person is moving faster, the red or blue jogger? What does it mean to move faster in terms of distance?

The red jogger. The red jogger’s line has a steeper slope and therefore a faster speed.

Check Your UnderstandingCheck Your Understanding

Are both joggers moving forwards or backwards?

Forwards. The slope is positive, meaning that the distance increases over time.