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FORCE AND FORCE AND MOTIONMOTION
A REVISION exerciseA REVISION exercise
The mass of an object is the measure of how much how much substance is in that objectsubstance is in that object.
Every physical object has mass: you do, the Earth does, and the Sun does.
Mass is measured in
kilogramskilograms.
MassMass::
The Difference Between The Difference Between Weight and MassWeight and Mass
MassMass is the amount of
mattermatter in an object.
The mass of an object will not change as you move it from place to place.
An object's An object's massmass is is constant where ever it is.constant where ever it is.
Mass = 1 Mass = 1 kgkg
on both on both the Earth the Earth and the and the Moon.Moon.
Weight Weight is a forceforce--the force with which gravitygravity "pulls" on an object's mass.
The Earth is bigger than the Moon therefore it applies a bigger pull on objects.EarthEarth MoonMoon
An object's An object's weightweight depends upon its location .depends upon its location .
Weight = 10 Weight = 10 NN
Weight = 1.6 Weight = 1.6 NN
Weight is calculated by multiplying the massmass by the force due to gravityforce due to gravity.
Weight = m x gWeight = m x g
On the Earth:
gg is approximately 10 N 10 N kgkg-1-1
gg is approximately 1.6 N 1.6 N kgkg-1-1
On the Moon:
Christian Christian Cullen on the Cullen on the EARTHEARTHMassMass = 82 kg
WeightWeight = mass x g
= 82 x 10
= 820 N
What would What would Christian Christian Cullen’s Cullen’s massmass and and weightweight be on be on the Moon?the Moon?
Christian Christian Cullen on the Cullen on the MOONMOONMassMass = 82 kg
WeightWeight = mass x g
= 82 x 1.6
= 131.2 N
(Same as on Earth)
Forces can be:Forces can be:
Pushes
Pulls
Twists
Forces Forces in in ACTION ACTION !!
• When two rough surfacestwo rough surfaces rub together there is more friction.more friction.
Orbital sander for light sanding of wooden surfaces.
Friction is the forceforce between two surfacesrubbing together.
When two smooth surfaces rub together there is very little frictionvery little friction
FRICTIONFRICTION
There is less frictionless friction when there is a liquidliquid (e.g. oil) between the two surfaces.between the two surfaces.
Oil is added to a car engine to lubricate the parts.
There is more friction if the two surfaces are forced against each other.
Friction is a good thing:Friction is a good thing:
Just think of walking on a wet marble floor or an icy pavement with new leather soled shoes: you might slip over!
Worse still, if the tyres on your car are worn out and the road is wet and slippery, you will probably skid the car and have an accident.
FrictionFriction is the force between two surfaces rubbing together. It is
highhigh if the surfaces are dry and roughdry and rough and it is
lowlow if the surfaces are smooth and wet.smooth and wet.
Friction Force
Pulling Force
Friction is also very important for your car's brakescar's brakes to work properly.
When you put you foot on the brake pedal, some rough padsrough pads are squeezed tight against the brake discs.
This friction slows the car friction slows the car downdown. If oil gets on the discs, the brakes will not work so well.
Both your shoes and the car's tyres need good gripgood grip so their surfaces are rough.
This increasesincreases the frictionfriction between them and the ground.
Friction is a bad thing:Friction is a bad thing:
Friction inside a car engine and inside the wheel axles will slow a car down and wear outwear out the metal. To prevent this we put oiloil or even greasegrease in them. This makes their surfaces more slipperyslippery and so reduces reduces friction.friction.
When we push or pullpush or pull on an object, we create a create a force.force.
When forces are equal,equal, they are in balance.balance.
When forces are unequal,unequal, an object moves or twists. moves or twists.
At a given location on the earth and in the absence of air absence of air resistance,resistance, all objects fall with the same same uniform acceleration.uniform acceleration.
This acceleration is called the
acceleration due to acceleration due to gravitygravity and it is refered
to commonly as gg.
On Earth g = 9.8 m s-2
(Approximately 10 m s10 m s-2-2)
If all the forces are in balancebalance, the object will stay as it is - stationary or moving - at a steady speed in a straight line.
If the forces are unbalanced, it will:
start to movestart to move in the direction of the force
speed upspeed up (accelerate) slow downslow down (decelerate) change directionchange direction.
PUSHPUSH, PULLPULL and TWISTTWIST all at the same time.
When driving on the motorway you might travel several kilometres at the same speed.e.g. Travel 5 km at 100 km e.g. Travel 5 km at 100 km hh-1-1
For that short period of time you are travelling at CONSTANT SPEEDCONSTANT SPEED.
On a normal journey you speed up and slow downspeed up and slow down.
We can calculate your AVERAGE SPEEDAVERAGE SPEED for the
journey.
Average SpeedAverage Speed =
Distance Distance travelledtravelledTime takenTime taken
e.g. Distance from Raumati Beach to Wellington is 50 50 kmkm
Time to travel from Raumati Beach to Wellington is 45 45 minutes.minutes.The journey will have fast bits and The journey will have fast bits and
slow bits. You will travel faster in slow bits. You will travel faster in the four lane parts. You might be the four lane parts. You might be slowed down by the slowed down by the Paramata Paramata BridgeBridge. You may have to stop for . You may have to stop for a train at a train at McKay’s CrossingMcKay’s Crossing. You . You will not be able to travel at a will not be able to travel at a constant speed.constant speed.
Now set out your work as follows to calculate the AVERAGE SPEEDAVERAGE SPEED for the whole journey.
Show all your Show all your working !working !
dd
VV tt
For us SpeedSpeed and VelocityVelocity are the same thing.
Let:
VV= velocity
dd = distance
tt = time
VV = ddtt
dd = VV x x t t
tt = dd
The 3 equations you need The 3 equations you need are:are:
VV
e.g. Distance from Raumati Beach to Wellington is 50 50 kmkm
Time to travel from Raumati Beach to Wellington is 45 45 minutes.minutes.d = 50 d = 50 kmkm
t = 45 t = 45 minutesminutes = 0.75 hr= 0.75 hr
Average SpeedAverage Speed =
ddtt
Average SpeedAverage Speed =
50500.750.75 66.7 km h-
1
=
We can represent a journey in graph formgraph form.
What will the slope of the graph tell us about our trip?
Distance time graph
0
5
10
15
20
25
30
35
40
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
Time (s)
Dis
tan
ce (
m)
A
B C
D
E
A = fast constant speed B = Slower constant speed C = at rest D= medium constant speed E = at rest
The SLOPE SLOPE of the graph gives us an indication of the speedspeed.
The steepersteeper the slopeslope the greatergreater the speedspeed.
A horizontal linehorizontal line tells us that the object is at restrest (or stationary)From the graph you can calculate the speedspeed.
Distance Time Graph
0
10
20
30
40
50
60
0 1 2 3 4 5 6 7 8
Distance (m)
Tim
e (s
)
Slope =
RISERISERUNRUN
Slope =
305
RISERISE
RUNRUN
= 6 ms-1
Time (s)
Dis
tan
ce
(m)
Distance Time GraphDistance Time Graph
Distance Time Graph
0
10
20
30
40
50
60
0 1 2 3 4 5 6 7 8
Time (m)
Dis
tan
ce
(m
)
The SLOPE of the graph indicates The SLOPE of the graph indicates SPEED.SPEED.
The steeper the slope the
faster the speed.
Distance Time GraphDistance Time Graph
An object that is changing its velocity is acceleratingaccelerating.
Something that slows down or speeds up, is said to accelerateaccelerate.
All accelerations are caused by forcesforces.
A force is basically a pushpush or a pullpull on an object.
Something that is speeding up has a positive positive acceleration, acceleration, and something that slowing down (decelerating) has a negative negative acceleration. acceleration.
The slopeslope of a velocity timevelocity time shows whether an object is acceleratingaccelerating.
AcceleratinAccelerating at a g at a constant constant raterate
Travelling Travelling at a at a constant constant speedspeed
DeceleratinDecelerating at a g at a constant constant raterate
V V V
t t t
Acceleration
Change in velocity
t
=
=Final V - Initial V
Change in time
e.g. A car speeds up from rest to e.g. A car speeds up from rest to
24 ms 24 ms-1 -1 in 3 secondsin 3 secondsInitial Velocity = 0 ms-1
Final Velocity = 24 ms-1
Time taken = 3 s
Acceleration t
=Final V - Initial V
Initial Velocity = 0 ms-1
Final Velocity = 24 ms-1
Time taken = 3 s
3= 24 - 0
= 8 ms-2
e.g. A car slows down from 35 e.g. A car slows down from 35 msms-1 -1
to 10 msto 10 ms-1 -1 in 5 secondsin 5 seconds
Acceleration t
=Final V - Initial V
I V = 35 ms-1
F V = 10 ms-1
t = 5 s
5= 10 - 35
=
-5 ms-2
5- 25
=
1. The weightlifter is pushing up with a bigger force than the weight of the bar.
2. The forces are unbalancedunbalanced so the bar acceleratesaccelerates up.
A weightlifter A weightlifter is lifting up is lifting up the bar.the bar.
1. The weightlifter is using a force equal to the weight of the bar.
2. The forces are balancedbalanced so the bar remains stationary.stationary.
A weightlifter A weightlifter is holding the is holding the bar above his bar above his head.head.
1. The weightlifter is using a force less than the weight of the bar.2. The forces are unbalancedunbalanced so the bar acceleratesaccelerates down.
A weightlifter A weightlifter is lowering is lowering the bar to the the bar to the floor.floor.
Here are 4 types of forces4 types of forces:
• air resistance - drag:air resistance - drag: When an object moves through the air, the force of air resistance acts in the opposite direction to the motion. Air resistance depends on the shapeshape of the object and its speedspeed.
• contact force contact force (support):(support):
happens when two objects are pushed together. They exert equal and opposite forces on each other. The support forcesupport force from the ground pushes up on your feet as you push downpush down to walk forwards.
Support Force
Gravitational Force
Support Force
Gravitational Force
• gravity:gravity:
the force which pulls objects towards the Earth.
We call the pull of gravity on an object its weight.
The Earth pulls with a force of about 10 Newtons on 10 Newtons on every kilogram of massevery kilogram of mass.
• friction:friction:
the force which resists movement between two surfaces which are in contact.
Air resistance Air resistance slows the slows the shuttle.shuttle.
What is the relationship between Force, mass Force, mass and acceleration.acceleration.Force = mass x
accelerationUnits:Units: (Newtons) (kilograms) (ms-2)
FF
mm aa
mm = FF
FF = mm x x a a
aa = FF
The 3 equations you need The 3 equations you need are:are:
aa mm
Problem:Problem:A car has a mass of 700 kg.It is accelerating at 2 m/s2.
Calculate the unbalanced unbalanced forceforce causing this acceleration.
(See next slide for correct (See next slide for correct working.)working.)
1. Write down the equation. (learn it.)
2. Put in the values.
3. Work out the answer and write it down. (Do not forget the units.)(Do not forget the units.)
Force = mass x acceleration
Force = 700 kg x 2 m s-2
Force = 1400 N
What forceforce would you need to accelerateaccelerate (or decelerate) a 1500 kg1500 kg car at 3 m s3 m s-2-2?
Try this question on your own Try this question on your own paper:paper:
(See next slide for correct answer.)(See next slide for correct answer.)
What forceforce would you need to accelerateaccelerate (or decelerate) a 1500 kg1500 kg car at 3 m s3 m s-2-2?
F = m x a
= 1500 x 3
= 4500 N
Answer:
The bigger the bigger the unbalanced forceunbalanced force the bigger the acceleration.bigger the acceleration. The bigger the massbigger the mass, the smaller the smaller the accelerationacceleration.
When the mass is kept When the mass is kept constant the acceleration is constant the acceleration is directly proportional to the directly proportional to the force applied.force applied.
When the force is kept constant When the force is kept constant the acceleration is inversely the acceleration is inversely proportional to the mass of the proportional to the mass of the object.object.
What is POWER?What is POWER?
It is not about being It is not about being strong.strong.
Power is the rate of Power is the rate of doing work.doing work.
Two people do the same task. Bryce takes 30 minutes while Mariah takes 15 minutes.
Who has the greatest power Who has the greatest power output?output?
Who has the greatest power Who has the greatest power output?output?Mariah has the greater power output because she complete the same task (same amount (same amount of work)of work) in the shortest time.
Power =Power =Work Work donedoneTime Time takentaken
(Watts)(Watts)
(Joules)(Joules)
(seconds)(seconds)
WW
PP tt
PP =
WW = PP x x t t
tt =
The 3 equations you need The 3 equations you need are:are:
tt PP
Power =Power =Work Work donedoneTime Time takentaken
(Watts)(Watts)
(Joules)(Joules)
(seconds)(seconds)
WW WW
A crane lifts 600 kg of concrete a A crane lifts 600 kg of concrete a height of 20 m in 15 seconds. height of 20 m in 15 seconds. What is the power output of the What is the power output of the crane?crane?(i) What is the weight of concrete?What is the weight of concrete?
= 6000 N
Weight = mass x g= 600 x 10
(ii) What is the work done by the What is the work done by the crane?crane?Work = F x d
= 6000 x 20= 120000 J
Continue next slide
P = Wt
20
(iii) What is the power of the crane?What is the power of the crane?
= 120000 J
= 6000 W
WWPP tt
A Slide
Height = 2 m
How How much much
workwork did she did she do to do to climb up climb up to the to the top of top of the the slide?slide?Check your answer on next
slide.
Mass of girl= 20 kg
A Slide
Height = 2 m
Mass of girl= 20 kg
In climbing to the top of the slide the girl has gained
gravitationgravitational potential al potential energy. energy. (E(EP))
Gain in EP = Work Done in climbing up. = 400 J
A Slide
Height = 2 m
Mass of girl= 20 kg
= 20 x 10= 200N200N
W = F x d= 200 x 2= 400J400J
Her weightweight is the force she has to overcome to climb up.F = m x g
A Slide
Height = 2 m
Mass of girl= 20 kg
gravitationgravitational potential al potential energy. energy. (E(EP))
EP = m x g x h
=20 x 10 x 2
= 400 J
Alternate way to calculate EP
(Same answer as before)
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