1. Vehicles do not typically travel at 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 speed n SI units: m/s n Other units: –Kilometres/hour (kph) –Miles per hour –Knots (nautical miles per hour)

A typical journey involves speed changes.

n A vector has magnitude and direction: v v is different to:

Scalar (magnitude only) n Distance travelled n Speed n Temperature n Mass n plus more Vector (magnitude and direction) n Displacement n Velocity n Force n Acceleration n Momentum n plus more......

Speed and velocity n Velocity can be changing even if speed is constant: v1v1 v2v2 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 displacement v =  r /  t n Do you know the difference between: –instantaneous speed –instantaneous velocity –average speed –average velocity

Relative motion n Examples: –One car following another –Girl walking on a boat or train –Canoe paddling along a flowing stream –Jet and helicopter in the sky What is a frame of reference?

2. An analysis of the external forces on vehicles helps to understand the effects of acceleration and deceleration. Qualitative understanding of force Examples: Pushing/pulling Gravitational force Electrostatic force

F = ma Mass n Qualitative understanding n What’s the difference between mass and weight? n Acceleration due to gravity is always down, so is weight n Any object thrown in the air has constant acceleration throughout its motion (a=g) aav v Plan, choose equipment or resources for, and perform a first-hand investigation to determine the mass of an object

Acceleration Rate of change of velocity Rate of change of velocity (could be a change in magnitude or direction) n Physical sensation (you can’t feel high speed travel because zero net force and zero acceleration) n Measurement: ¤accelerometer ¤data logger motion sensor ¤data logger light gates ¤ticker timer ¤stroboscopic photography ¤video analysis a =  v /  t Remember acceleration is the slope of the velocity versus time curve

Addition of vectors v +  v v vv

Weight pulls car down Road pushes up Engine pushes forward 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)

Friction n Friction always opposes motion. n Friction depends on the nature of the surfaces in contact, and how hard they are pressed 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