Physics- Prelim-Module 3 Notes

7/21/2019 Physics- Prelim-Module 3 Notes

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Preliminary Physics Module 3:Moving About

Vehicles do not travel at a constant speed

1.1 Identify that a typical journey involves speed changes:

Motion can be described: as uniform motion – constantspeed/velocity and as non-uniform motion –acceleration/deceleration/change in direction

A typical journey involves speed changes, eg. speeding up

after the engine is turned on

1. !istinguish bet"een instantaneous speed and averagespeed of vehicles and other bodies:

Instantaneous speed is the speed at a particular instant of time

Average speed is distance travelled divided by time

taen! s"d/t

1.3 !istinguish bet"een scalar and vector #uantities ine#uations:

A scalar #uantity speci$es si%e but not direction

A vector #uantity speci$es si%e and direction

&calar: mass, speed, distance, 'or, energy

(ector: force, velocity, displacement, acceleration,

momentum

1.$ %ompare instantaneous speed and average speed "ithinstantaneous velocity and average velocity:

Instantaneous speed is speed at a particular instant in

time

Average speed is total distance divided by time taen

Instantaneous velocity is the velocity at a particularinstant in time

Average velocity is total displacement )vector* divided bytime taen

1.& !e'ne average velocity as: vav=∆r /∆t :

• Average velocity is calculated using the formula

vav=∆r /∆ t , 'here v " average velocity, ∆r=¿ displacement )change in position* and ∆ t " time taen



An analysis of the e(ternal forces on vehicles helps tounderstand the e)ects of acceleration and deceleration

.1 !escribe the motion of one body relative to another:

+elative velocity is the dierence bet'een the velocity ofthe object relative to the ground and the velocity of theobserver relative to the ground

. Identify the usefulness of using vector diagrams toassist solving problems:

hange in velocity " $nal velocity – initial velocity

.3 *(plain the need for a net e(ternal force to act in orderto change the velocity of an object:

orce )* is measured in e'tons )*! it is a vector#uantity

e'ton0s $rst la' of motion: an object 'ill remain at restor move at a constant velocity unless acted upon by a net

e1ternal force )inertia is the tendency of an object toresist a change in motion*

As e'ton0s $rst la' of motion states that an object 'illremain at a constant velocity unless acted upon by a nete1ternal force, a net e1ternal force is re#uired to changethe velocity of an object

.$ !escribe the actions that must be ta+en for a vehicle tochange direction, speed up or slo" do"n:

or a vehicle to change direction, speed up or slo' do'n

there must be an unbalanced net force acting on the car



.& !escribe the typical e)ects of e(ternal forces onbodies including: friction bet"een surfaces, airresistance:

riction is a force that opposes motion and allo's tyres toget traction and allo's a car to turn

Air resistance is a type of friction that opposes car motion

.- !e'ne average acceleration as: aav=∆ v

∆t =

v−u

t :

Acceleration is a vector #uantity that measures the rateof change of velocity in ms-2

a " ∆ v /∆ t " )v-u*/t

v " u 3 at

r " ut 3 4.5at62

r " 4.5)u3v*t

v62 " u62 3 2ar

. !e'ne the terms /mass0 and /"eight0 "ith reference tothe e)ects of gravity:

Mass is the #uantity of matter that an object contains,measured in ilograms

7eight is the force applied to an object due to

gravitational attraction )gravity*, measured in e'tons

7"mg )'here g is 8.9 g" or 8.9ms-2*

. 2utline the forcing involved in causing a change in thevelocity of a vehicle "hen: coasting "ith no pressureon the accelerator, pressing on the accelerator,pressing on the bra+es, passing over an icy patch onthe road, climbing and descending hills, follo"ing acurve in the road:

oasting 'ith no pressure on the accelerator: graduallythe vehicle 'ill slo' do'n until it reaches a velocity of



%ero. ;his is caused by friction from the road and airresistance

• <ressing on the accelerator - the tyres e1ert a greater

force to the road and the road applies an e#ual and

opposite reaction force to the car. ;his net force causesthe car to accelerate

• <ressing on the braes - frictional force is applied to the

'heels, the tyres apply a force to the road and the road

applies an e#ual and opposite force to the car 'hich

causes it to slo' do'n. If 'e call the direction of motion

positive applying the braes produces a net force on the

car in the negative direction.

• <assing over an icy patch on the road - if there is no

friction bet'een the road and the tyres the driver cannot

apply braing, accelerating or turning forces to the road.

=ecause there is no net force on the car it 'ould continue

in a straight line at constant velocity! the car 'ill gradually

come to rest after a long time due to air resistance

• limbing and descending hills - 'eight of the car due to

gravity, is in direction of motion. ;his means that that to

maintain a constant velocity the engine must e1ert agreater force to overcome this gravitational force pulling it

do'n! 'hen descending the force of gravity is acting in

the direction of motion, to maintain a constant speed the

braes must be applied to produce a negative force to

counteract this gravitational force acting do'n the hill

• ollo'ing a curve in the road - to drive around a corner a

net force must be applied to the car to'ards the centre of

the corner. ;urning the steering 'heel to the right causes

the tyres to push the road to the left, so that the resultingreaction force on the car is to'ards the centre of the curve

and causes the direction of the velocity to change to the

right

• ollo'ing a curve in the road re#uires acceleration to

travel at a constant speed, this is centripetal acceleration

)directed to'ards the centre of the circle* and can be

calculated using the formula: a=v2

r 'here a is the



magnitude of acceleration )ms-2*, v is the constant speed

)ms-* and r is the radius of the curve/circle )m*

• entripetal force )net force on an object travelling in a

circular path at a constant speed* is calculated by the

formula: F =ma=mv2

r 'here is the centripetal force )*,

m is the mass of the object )g*

. Interpret 4e"ton0s 5econd 6a" of Motion and relate it

to the e#uation: ∑ F =ma :

;he acceleration of an object is proportional to the net

force and inversely proportional to the mass

e'ton0s second la' of motion: "ma, is force inne'tons )*, m is mass in ilograms )g* and a isacceleration in meters per second )m/s62*

.17 Identify the net force in a "ide variety of situationsinvolving modes of transport and e(plain theconse#uences of the application of that net force in

terms of 4e"ton0s 5econd 6a" of Motion:

• et force is the sum of all the forces acting on a body. In

transportation the net forces is in the direction of 'here

the vehicle is travelling. ;he net force depends upon the

mass and the acceleration of the vehicle, as stated by

e'tons second la' of motion. &o if the acceleration is

ept the same for all vehicles, heavy vehicles 'ill e1ert a

greater force than lighter ones

Moving vehicles have +inetic energy and energy

transformations are an important aspect in understand

motion

3.1 Identify that a moving object possesses +inetic energy

and that "or+ done on that object can increase that

energy:

>nergy can be de$ned as the capacity to do 'or. It is ascalar #uantity



7or is done 'hen an object moves in the direction of a

force applied to it. It is a scalar #uantity

89s, 'here 7 is 'or measured in joules )?*, is

magnitude of the force measured in e'tons )* and s is

displacement in the direction of the force )m*

@inetic energy is the energy associated 'ith the

movement of an object

Ek =mv

2

2, 'here > is the inetic energy of an object )?*,

m is the mass of the object )g* and v is the speed of the

object )ms-*

3. !escribe the energy transformations that occur in

collisions:

7hen a collision occurs, some inetic energy is

transferred to other objects, but the remainder of the

energy is transformed into other forms of energy. ;hey

are transformed into: potential energy of deformation:

this is stored energy in an object as a result of changing

its shape. &ometimes this energy can be transformed into

other forms 'hen the object returns to original shape.

&ound energy: is transmitted through the air because ofvibrating particles. >.g. sound energy released 'hen t'o

cars collide, ;hermal or heat energy: it is the energy that

a substance posses as a result of the random motion of

particles 'ithin the substance

3.3 !e'ne the la" of conservation of energy:

• ;he la' of conservation of energy states that energy

cannot be created or destroyed. o'ever energy can be

transferred from one object to another or transformed

from one form of energy to another

%hange of momentum relates to the forces acting on the

vehicle or the driver

$.1 !e'ne momentum as: p9mv:

Momentum is the #uantity of motion of a moving body! it

is a vector #uantity



p9mv, 'here p is momentum )g m s-*, m is mass )g*

and v is velocity )ms-*

Momentum is conserved

$. !e'ne impulse as the product of force and time:

Impulse is the product of force and the time interval over

'hich it acts, it is a vector #uantity and is measured in s

Impulse " t

Impulse is also change in momentum, therefore change in

momentum is e#ual to the product of force and time

$.3 *(plain "hy momentum is conserved in collisions in

terms of 4e"ton0s ;hird 6a" of Motion:

e'ton0s third la' of motion: for every action there is an

e#ual and opposite reaction

;he interaction bet'een t'o cars 'hich collide can besummarised as follo's: the total momentum of the systemof the t'o cars remains constant, the total change inmomentum is %ero, the change in momentum of the $rstcar is e#ual and opposite to the change in momentum ofthe second car, the force that the $rst car e1erts on the

second car is e#ual and opposite to the force that thesecond car e1erts on the $rst car

• Momentum is al'ays conserved.

• ;otal momentum before collision " total moment aftercollision: m u 3 m2 u2 " m v 3 m2 v2

• Momentum lost " momentum gained5afety devices are utilised to reduce the e)ects of changingmomentum

&.1 !e'ne the inertia of a vehicle as its tendency to remainin uniformmotion or at rest:

• Inertia is the tendency of an object to resist change in itsmotion. Inertia is not a force but it is a property of allobjects. ;he inertia of an object depends on the object0smass

&. !iscuss reasons "hy 4e"ton0s irst 6a" of Motion isnot apparent in many real "orld situations:



• e'ton0s $rst la' is not apparent in real life situations

because there 'ill al'ays be forces acting upon us or a

vehicle for e1ample: 'hen 'e are in motion in a vehicle

'e do not continue at motion and 'e 'ill eventually come

to a stop because other forces such as friction, airresistance, etc, eep us from continuing at motion

&.3 Assess the reasons for the introduction of lo" speed

<ones in built=up areas and the addition of air bags and

crumple <ones to vehicles "ith respect to the concepts

of impulse and momentum:

• &peed bumps maes it diBcult for drivers to travel at

higher speeds since they 'ould damage their cars if they

hit the bump too fast, this reduces the impulse or change

in momentum that a person 'ould e1perience in a

collision, thus reducing the impact and ris of injury

• Air bags increase the time taen for the change in

momentum of a person0s head to occur, this reduces the

impact or force e1perienced

• rumple %ones change momentum more gradually,

reducing the impact or force e1perienced

&.$ *valuate the e)ectiveness of some safety features of

motor vehicles:

&eatbelts reduce momentum more gradually, reducing

the impact or force e1perienced. If a person did not 'ear

a seatbelt in a collision, they 'ould continue to move at

the speed of the car before it began to brea and the

change in the person0s momentum 'ould occur very

suddenly 'hen they hit the 'indscreen, etc, causing

severe injury

2ther: motion graphs, E p=mgh, "here * is potential energy

>?@, m is mass >+g@, g is acceleration of gravity >.ms=@, h is

height >m@



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Physics- Prelim-Module 3 Notes