Motion & Forces Describing Motion Motion Speed & Velocity.

Motion & ForcesMotion & Forces

Describing MotionDescribing Motion MotionMotion Speed & VelocitySpeed & Velocity

Newton’s First LawNewton’s First Law

Newton’s First Law of MotionNewton’s First Law of Motion An object at rest will remain at rest An object at rest will remain at rest

and an object in motion will continue and an object in motion will continue moving at a constant velocity unless moving at a constant velocity unless acted upon by a net force.acted upon by a net force.

motionmotion

constant velocityconstant velocity

net forcenet force

MotionMotion

Problem:Problem: Is your desk moving?Is your desk moving?

We need a We need a reference pointreference point...... nonmoving point from which motion is nonmoving point from which motion is

measuredmeasured

MotionMotion

MotionMotion Change in position in relation to a Change in position in relation to a

reference point.reference point.

Reference point

Motion

Relative MotionRelative Motion

If you are sitting in a chair reading If you are sitting in a chair reading this sentence, you are moving.this sentence, you are moving.

You are not moving relative to your You are not moving relative to your desk or your school building, but you desk or your school building, but you are moving relative to the other are moving relative to the other planets in the solar system and the planets in the solar system and the Sun.Sun.

MotionMotion• DistanceDistance

• Distance describes how far an Distance describes how far an object has moved. object has moved.

• The SI unit of length or distance The SI unit of length or distance is the meter (m). Longer is the meter (m). Longer distances are measured in distances are measured in kilometers (km).kilometers (km).

• Shorter distances are measured Shorter distances are measured in centimeters (cm).in centimeters (cm).

MotionMotion• Displacement is the distance and

direction of an object's change in position from a reference point.

• Suppose a runner jogs to the 50-m mark and then turns around and runs back to the 20-m mark.

• The runner travels 50 m in the original direction (north) plus 30 m in the opposite direction (south), so the total distance she ran is 80 m.

MotionMotion

Problem:Problem: You are a passenger in a car stopped You are a passenger in a car stopped

at a stop sign. Out of the corner of at a stop sign. Out of the corner of your eye, you notice a tree on the side your eye, you notice a tree on the side of the road begin to move forward.of the road begin to move forward.

You have mistakenly set You have mistakenly set yourselfyourself as as the reference point.the reference point.

Speed & VelocitySpeed & Velocity

SpeedSpeed rate of motion rate of motion distance traveled per unit timedistance traveled per unit time

timedistance

speed

sdt

Calculating SpeedCalculating Speed

The SI unit for distance is the meter The SI unit for distance is the meter and the SI unit of time is the second and the SI unit of time is the second (s), so in SI, units of speed(s), so in SI, units of speed

Sometimes it is more convenient to Sometimes it is more convenient to express speed in other units, such as express speed in other units, such as kilometers per hour (km/h).kilometers per hour (km/h).

Speed & VelocitySpeed & Velocity

Instantaneous SpeedInstantaneous Speed speed at a given instantspeed at a given instant Speed odometer readingSpeed odometer reading

Average SpeedAverage Speed the total distance traveled divided by the total distance traveled divided by

the total time of travelthe total time of travel

time totaldistance total

speed avg.

Changing Instantaneous Changing Instantaneous SpeedSpeed

When something is speeding up or When something is speeding up or slowing down, its instantaneous slowing down, its instantaneous speed is changing.speed is changing.

If an object is moving with constant If an object is moving with constant speed, the instantaneous speed speed, the instantaneous speed doesn't change.doesn't change.

To determine direction you need to To determine direction you need to know the velocityknow the velocity

Speed & VelocitySpeed & Velocity

Problem:Problem: A storm is 10 km away and is moving at A storm is 10 km away and is moving at

a speed of 60 km/h. Should you be a speed of 60 km/h. Should you be worried?worried?

It depends on the It depends on the storm’s direction!storm’s direction!

Speed & VelocitySpeed & Velocity

VelocityVelocity speed in a given directionspeed in a given direction can change even when the speed is can change even when the speed is

constant!constant!

CalculationsCalculations Your neighbor skates at a speed of 4 m/s. Your neighbor skates at a speed of 4 m/s.

You can skate 100 m in 20 s. Who skates You can skate 100 m in 20 s. Who skates faster?faster?

GIVEN:

d = 100 mt = 20 ss = ?

WORK:s = d ÷ t

s = (100 m) ÷ (20 s)

s = 5 m/s

You skate faster!sdt

CalculationsCalculations Sound travels 330 m/s. If a lightning bolt Sound travels 330 m/s. If a lightning bolt

strikes the ground 1 km away from you, how strikes the ground 1 km away from you, how long will it take for you to hear it?long will it take for you to hear it?

GIVEN:

s = 330 m/sd = 1km = 1000mt = ?

WORK:t = d ÷ s

t = (1000 m) ÷ (330 m/s)

t = 3.03 ssdt

Graphing MotionGraphing Motion slope =

steeper slope =

Straight line =

flat line =

Single point = instantaneous speedinstantaneous speed

Distance-Time Graph

A

B

faster speedfaster speed

constant speedconstant speed

no motionno motion

speedspeed

Graphing MotionGraphing Motion Who started out faster?Who started out faster?

A (steeper slope)A (steeper slope) Who had a constant speed?Who had a constant speed?

AA Describe B from 10-20 min.Describe B from 10-20 min.

B stopped movingB stopped moving Find their average speeds.Find their average speeds.

A = (2400m) ÷ (30min) A = (2400m) ÷ (30min) A = 80 m/min A = 80 m/min

B = (1200m) ÷ (30min) B = (1200m) ÷ (30min) B = 40 m/min B = 40 m/min

Distance-Time Graph

A

B

0

100

200

300

400

0 5 10 15 20

Time (s)

Dis

tan

ce (

m)

Distance-Time Graph

Graphing MotionGraphing Motion

Acceleration is Acceleration is indicated by a indicated by a curve on a curve on a Distance-Time Distance-Time graph.graph.

Changing slope = Changing slope = changing velocitychanging velocity

Graphing MotionGraphing Motion

0

1

2

3

0 2 4 6 8 10

Time (s)

Sp

ee

d (

m/s

)

Speed-Time Graph

Specify the time period Specify the time period when the object was...when the object was...

slowing downslowing down 5 to 10 seconds5 to 10 seconds

speeding upspeeding up 0 to 3 seconds0 to 3 seconds

moving at a constant moving at a constant speedspeed 3 to 5 seconds3 to 5 seconds

not movingnot moving 0 & 10 seconds0 & 10 seconds