dynamics ( sometimes called) kinematics mechanics lecture 4 6 th november 2014

Dynamics(sometimes called)

Kinematics

Mechanics Lecture 46th November 2014

Objectives• Define basic

parameters• Describe motion

graphically• Derive two SUVAT

equations

Key WordsDistance / DisplacementSpeed / VelocityAccelerationSUVAT

Dynamics(sometimes called)

Kinematics

Mechanics Lecture 46th November 2014




equations


Check basic definitionsScalar Vector Units

Distance

How far you have gone

Displacement

Distance from home in a given direction

m

m with direction (compass etc)

Speed

How fast you are going

Velocity

How fast you are going in a given direction

ms-1

ms-1 in specified direction

Acceleration

Rate of change of velocity vector

ms-2 in specified direction




equations


Speed – easy stuff…

If an object travels at constant speed v, for distance d in time t :

v = d/t so d= v t and t=d/v

If the speed does not remain constant then v=d/t represents an average speed over time t.

• This matters for speed cameras




equations


Graphing motion

• Mini- whiteboards –• Displacement against time• Velocity against time

• What does this graph tell us?




equations


Walking graphs.D

ispl

acem

ent

Time




equations


Walking graphs.D

ispl

acem

ent

Time

What’s the story?

Set off for the bus here




equations


Getting speed from distance graphs (1)

Easy! Steady speed. v = s/t




equations


Getting speed from distance graphs (2)

Variable speed

Need to take the tangent at the time you are interested in and find its gradient.EMPA tip – 8cm * 8cm minimum




equations


Graphing motion

• What does the area under a velocity/time graph give us?

• What does this graph tell us?




equations


Card sort




equations


The equations of motion, forconstant acceleration

in a straight line

• Five parameters can describe motion…

s - displacementu - initial velocityv - final velocitya - accelerationt - time




equations


These are the four “suvat” equations:

v = u + ats = ½ (u + v) ts = u t + ½at2

v2 =u2 + 2as

So each one links four of the five parameters

no sno ano vno t




equations


Where do these come from?

• We’ll see later! • More important that you can use

them systematically

Suvat –Systematically!

Some more practice




equations


Where do the suvat equations come from?

• Try to derive first two from thinking about a and s

• The third and fourth come from combining the first two




equations





equations


Free Fall

• Galileo’s ideas

• How to examine using modern methods?




equations


Graph this (in Excel)

• ..\..\..\..\A levels\A level Physics\AS Physics\Unit 2\Dynamics\graph for NT book freefall data.xlsx




equations


Using suvat with free-fall equations

Using suvat with free-fall equations - practice




equations


Summary

• Four suvat equations

• Link the five parameters s u v a t

• Easy to use if you are systematic and consistent with coordinate systems.

• Unlikely to be tested on derivation

dynamics ( sometimes called) kinematics mechanics lecture 4 6 th november 2014

Documents

equations of motion

constant speed v

average speed

steady speed

distance graphs

displacement distance

time t

given direction ms