1. Vehicles do not typically travel at 1. Vehicles do not typically travel at constant speed.constant speed.

Recall a range of vehicles and that time taken

- it depends on the route taken and speed travelled at various stages of the journey

Measuring speedMeasuring speed

SI units: m/sSI units: m/s Other units:Other units:

– Kilometres/hour (kph)Kilometres/hour (kph)– Miles per hourMiles per hour– Knots (nautical miles per hour)Knots (nautical miles per hour)

A typical journey involves A typical journey involves speed changes.speed changes.

A vector has magnitude A vector has magnitude and and direction:direction:

v

vis different to:

Scalar Scalar (magnitude (magnitude

only)only)

Distance travelledDistance travelled SpeedSpeed TemperatureTemperature MassMass plus more......plus more......

Vector Vector (magnitude and (magnitude and

direction)direction)

DisplacementDisplacement VelocityVelocity ForceForce AccelerationAcceleration MomentumMomentum plus more......plus more......

Speed and velocitySpeed and velocity Velocity can be Velocity can be changingchanging even if even if

speed is constant:speed is constant: v1

v2

Caution

We often use the word “velocity” when we mean “speed”, and vice-versa—especially in normal conversation. Don’t let this happen to you.

e.g.” the cyclist maintained a steady velocity around the circuit”

Velocity and displacementVelocity and displacementvv = = rr / /tt

Do Do you you know the difference between: know the difference between: – instantaneous speedinstantaneous speed– instantaneous velocityinstantaneous velocity– average speed average speed – average velocityaverage velocity

Relative motionRelative motion

Examples:Examples:– One car following another One car following another – Girl walking on a boat or trainGirl walking on a boat or train– Canoe paddling along a flowing Canoe paddling along a flowing

streamstream– Jet and helicopter in the skyJet and helicopter in the sky

What is a frame of reference?

2. An analysis of the external forces 2. An analysis of the external forces on vehicles helps to understand the on vehicles helps to understand the

effects of acceleration and effects of acceleration and deceleration.deceleration.

Qualitative understanding of force

Examples:Pushing/pullingGravitational forceElectrostatic force

FF = m = maa

Mass Qualitative understandingQualitative understanding What’s the difference between mass and What’s the difference between mass and

weight?weight? Acceleration due to gravity is Acceleration due to gravity is always downalways down, so is , so is

weightweight Any object thrown in the air has constant Any object thrown in the air has constant

acceleration throughout its motion (a=g)acceleration throughout its motion (a=g) a a

v

v

Plan, choose equipment or resources for,and perform a first-hand investigation

to determine the mass of an object

AccelerationAcceleration

Rate of Rate of change change of velocity of velocity

(could be a change in magnitude (could be a change in magnitude oror direction)direction)

Physical sensation (you can’t feel high speed travel Physical sensation (you can’t feel high speed travel because zero net force and zero acceleration)because zero net force and zero acceleration)

Measurement:Measurement:¤ accelerometeraccelerometer¤ data logger motion sensordata logger motion sensor¤ data logger light gatesdata logger light gates¤ ticker timerticker timer¤ stroboscopic photographystroboscopic photography¤ video analysisvideo analysis

a = v /tRemember accelerationis the slope of the velocity versus time curve

Addition of vectorsAddition of vectors

v + v

v

v

Weight pulls car down

Road pushes up

Engine pushes forward

Drag,friction etc. pulls back

Drag,friction etc. pulls back

(Horizontal forces only shown)

Engine pushes forward

What are 3 ways that we can make changes to a car’s velocity?(2 answers each)

FrictionFriction Friction always opposes motion.Friction always opposes motion. Friction depends on the nature of the surfaces Friction depends on the nature of the surfaces

in contact, and how hard they are pressed in contact, and how hard they are pressed together.together.

What forces and change in velocity are involved in a vehicle when you:

•coast without pressure on the accelerator? FRICTION SLOWS THE CAR DOWN

•press on the accelerator? THE DRIVING FORCE EXCEEDS THE FRICTION FORCE, CAUSING THE CAR TO SPEED UP

•press on the brakes? THE FRICTION FORCE ON THE WHEELS IS INCREASED

•pass over an icy patch? THE FRICTION FORCE IS REDUCED, SPEED IS MAINTAINED, EVEN IF THE WHEELS ARE STOPPED BY THE BRAKES

•climb or descend hills? GRAVITY OPPOSES OR INCREASES ACCELERATION

•follow a curve in the road? THE DIRECTION OF VELOCITY CHANGES, THE TURNING HAPPENS BECAUSE OF THE FRICTION FORCE ON THE ROAD