Physics 2a = Motion and Forces

Speed and Velocity and acceleration

• Prior learning• Definition and equation for speed• Key words

– Speed, distance, time, velocity, direction

Topic – Physics 2aWednesday 8th February

Physics 2a = Motion and Forces

Starter: From last lesson:

1. A cricket ball travelling at 45.0ms-1 East is hit back at 35.0ms-1 west. Determine change in velocity?

2. A tennis ball is travelling 24.0ms-1 west and is hit south at 20.0ms-1, what is the change in velocity?

Physics 2a = Motion and Forces

answers

1. V = final – initial1. 35 west – 45 East2. =35 west + 45 west3. = 80ms-1

2. V = final – initial1. 20.0ms-1 south – 24.0ms-1 west2. 20.0ms-1 south + 24.0ms-1 east3. Tan θ = opp/adj4. Tan θ = 24/20 = 1.205. -tan 1.20 = 50.2o

6. S 50.2o E or 50.2o east of south

Physics 2a = Motion and Forces

Ave Speed• Definition: How fast you are going at a particular time • OR Distance travelled divided by time taken • Units: metres per second (ms-1) • Symbol: speed.

• Example: • While exercising vigorously, it took you 12 minutes to run 4.0 km, what was your

average speed.• distance = 4 000 m speed (ave) = distance = 4000• time, t = 12 x 60 t 720• t = 720 s speed (ave) = 5.6 ms-1• Unfortunately, that is all we can tell about the speed. (scalar) For more information we

need velocity as that is a vector quantity and will give us not only a magnitude but also a direction.

Physics 2a = Motion and Forces

Instantaneous speed

• Speed at any one particular moment

• Distance moved / time taken

Physics 2a = Motion and Forces

Velocity• Let’s return to your run across the park. While your

average speed will tell you how fast, on average, you were travelling, your velocity will tell you the rate at which your displacement changes and in what direction.

• Average Velocity: • This is your total displacement (where you started to

where you finished) divided by the time it took. Average velocity explains why your exercising in the park produced a zero velocity as your displacement was zero.

• Definition: The rate of change of displacement OR Displacement divided by time taken

• Units: metres per second (ms-1) Symbol: Vav for average velocity; u for initial velocity and v for final velocity

Physics 2a = Motion and Forces

Velocity - Example• Example: You are running across the park from the

east towards the west. You have run the first kilometre towards the west then have returned 0.50 km east. If that part of the run took 4.5 minutes, what was your average velocity for that part of the run?

• 1000 m W Displacement =

500 m E s = 500 m W • s = 500 m W • t = 4.5 minutes Vav = 1.9 ms-1 west• t = 4.5 x 60• t = 270 s

270

500

t

sav v

Physics 2a = Motion and Forces

Copy and complete worked example

• Beth initially stands 200m from paula. She runs 400m towards the park. The steady run took 80s, she then rested for 20s and then jogged back to Paula in 180s.– Graph Beths movement on a displacement

time graph– Draw displacement– What distance has Beth travelled– What is Beths displacement?– At what time did Beth go past her original

starting point?

Physics 2a = Motion and Forces

Answers

• Travelled a total of 1000m (400+400+200)• Displacement is 200m• At time 220s – she is back at starting point

timedisplacement

Physics 2a = Motion and Forces

Task

• Calculate Beths average speed for the run?

• Calculate Beths instantaneous speed at 20s?

Physics 2a = Motion and Forces

Task - answers

• Calculate Beths average speed for the run?– Ave speed = dist / time– distance = 400 + 600 = 1000m– Time = 80+20+180 = 280s– Vav = 1000/280 = 3.6ms-1

• Calculate Beths instantaneous speed at 20s?– For the first 80s instantaneous speed is the same– V=s/t – 400/80– 5.0ms-1