FORCES
The basics
What is a force?
12/01/2017
A force is a “push” or a “pull” acting on an object. Some common examples:
________ – pulls
things downwards
_____ – acts against
anything moving
___ ________ (drag) – acts
against anything moving through air
______ – keeps things afloat
Words – upthrust, air resistance, friction, weight
There are many different names
given to forces
THRUST( OR PUSH): force applied to make
an object move
 FRICTION: force which opposes
movement, usually heat is generated
 SUPPORT (or reaction): force keeping
objects from falling against gravity
 WEIGHT:
force due to gravity
 AIR RESISTANCE: force due to friction on
an object falling through air
Forces can cause an object to



Change SHAPE
Change SPEED
Change DIRECTION
Effect of force
• Make a stationary object move
Effect of force
– Change the speed of a body
Effect of force
– Change the direction of motion of a
body without its speed
Effect
of
force
• Change the shape and size of a body
Force (symbol F)




Is measured in newtons (symbol N)
Is measured with a force meter
Newton’s third law
Forces exist in pairs. “for every force there
is a reactive force”
If the forces acting on an object are
support




BALANCED
The object will be
STATIONARY
Or
Travelling at a
constant speed
gravity
support
Thrust/push
friction
gravity
If the forces are UNBALANCED

And the object is freely moving

It will change speed (or accelerate)
support
friction
Thrust/push
gravity
In the following examples the net
force is….
5000N
The net force of an object is the
overall force and direction being
applied by the two forces
5000N
10000 N
5000N
5000N
10 000 N
ZERO



If the forces are BALANCED
They are equal in strength but opposite in
direction
The NET force is ZERO
The net force
in
this
example
is
10 000 N
5000 N
20 000 N
10 000 N
15000 N to the left
Newton’s other laws





1st Law- law of inertia
Objects that are stationary stay staionary unless
an unbalanced force is applied.
An object that is moving (at a constant speed)
will stay this way unless an unbalanced force is
applied.
2nd law- Force = mass x acceleration
The size of a force is dependent on the mass of
the object and the rate it can accelerate.
Examples




Suggest what will happen in these scenarios.
1. If the mass of a car is halved. How will this
effect the rate of acceleration (if the force is
kept the same)?
2. If the object can accelerate faster and the
mass is the same. What size must the force be
in comparison?
3. If less force is applied to an object how will
this effect acceleration (if the mass is kept the
same)?
Force and acceleration
12/01/2017
If the forces acting on an object
are unbalanced then the object will
accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
a
Force, mass and acceleration
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1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
F
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
much does it accelerate by?
M
A
MASS
Mass is the amount of matter in an
object.
Mass is measured in kilograms (kg).
Mass is constant anywhere in the
universe.
For the mass of an object to change it
must lose or gain matter.
weight
Weight is the force of gravity acting on
a mass.
Weight is measured in Newtons (N).
Larger gravity means larger weight
force.
On earth a one kg mass has a weight of
10N.
How to calculate Weight
• This can be calculated by:
• F gravity= mass X gravity
» Where gravity is 10N Kg -1
» What does that mean?
» Multiply any mass by 10 to convert into
weight!
12/01/2017
Alien Gravity
Use the boxes to help you work out the weight of the alien.
My mass is 50kg. If
gravity on Earth is
10m/s2, what is my
weight?
Weight = mass x gravity
= _______kg x ______m/s2
= ________ N
Weight = mass x gravity
= 50 kg x 10 m/s2
= 500 N
My mass is
still 50kg. But
how much do I
weigh here?
Gravity = 30N/kg
= 30Nkg-1
Weight = mass x gravity
= _______kg x _____N/kg
= ________ N
Weight = mass x gravity
= 50 kg x 30 N/kg
= 1500 N
More Alien Gravity
On this small
moon I only
weigh 75N. I
wonder what
the gravity
is?
g = 12 m/s2
Weight = mass x gravity
= 50 kg x 12 m/s2
= 600 N
Gravity
= _______
/ ________
Gravity
=
weight
/ mass
= _______N / ______kg
2
= 75
/ 50 kg
= ________
m/sN
= 1.5 m/s2
More Alien Gravity
On this planet,
I weigh 750N.
My brother
weighs 600N.
What’s my
mass?
Gravity = weight / mass
= 750 N / 50 kg
= 15 m/s2
Mass = weight / gravity
= 600 N / 15 m/s2
= 40kg
Hint :- Use the weight and mass of the
first alien to find the gravity of the
planet. Then use that to find the mass
of his brother.
Pressure
• Pressure is a measure of how
much force is pushing on a
certain area.
P
• It can be calculated by:
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F
Pressure (in Pa) = Force (in N) / Area (in m2)
• P=F/A
• The unit for pressure is
Pascals (Pa), but the
measurements are often large
so kPa are common (KiloPascal)
A
Worked example
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• Calculate the pressure acting on the
tyres of a car if the area per tyre is
0.12m2. The mass of the car is
1700Kg.
Explaining pressure
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• Use the variables in the formula for
pressure to explain what is happening.
• E.g if a person stands on two feet and tries
to balance on one foot how would the
pressure on the ground (or one foot)
change?
• 1. Has the area changed?
• 2. Has the force changed?
• 3. Has the pressure changed as a result?
Big Picture Science
• To move an object a _________
must be ________
• This may cause an object to:
• ______, ________ or _____
• If the ________ are balanced the
object may…_______
• To apply a _______, ______ is
required.
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Energy
Energy is the ability to make
something happen.
If a force moves an object energy
must have been applied
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The ULTIMATE energy source
The sun is the ultimate source of all our energy.
For example, we often get energy from beef:
BEEF comes from COWS,
COWS eat GRASS,
GRASS gets energy from the sun by PHOTOSYNTHESIS
The 9 types of energy
Type
Heat
Kinetic (movement)
Nuclear
Sound
Light
Chemical
Electrical
Gravitational potential
Elastic potential
3 example sources
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Energy changes
12/01/2017
To describe an energy change for a
light bulb we need to do 3 steps:
1) Write down the
starting energy:
3) Write down
what energy types
are given out:
2) Draw an arrow
Electricity
Light + heat
What are the energy changes for the following…?
1) An electric heater
2) A rock that is dropped
3) An arrow that is fired
Conservation of Energy
12/01/2017
Energy CANNOT be created or destroyed but passed on
from one form to another.
e.g. a light bulb:
Electricity
Light
+
heat
In this example HEAT is wasted – it’s given to the
surroundings. Waste energy is virtually always
heat and it’s always given to the surroundings.
Download and play Hill climb
racing
12/01/2017
Summary questions:
12/01/2017
1. Explain how Ep and Ek change as you
travel up and down the hill.
2. Draw in energy change diagram
3. When will Ek be the highest?
4. When will Ep be the highest?
5. Will Ep always be fully conserved into Ek?
Why?
6. Write a paragraph to explain how energy
forces and motion are related when the
bike climbs the hill from the bottom.
The unit for energy are Joules
12/01/2017
What is a Joule?
• 1 Joule= the energy required to movie a
1kg object 1m
• E.g. A big mac has 2000 KJ of energy.
• = 2000000J
• A person with a 50kg mass would need to
move 40000m or 40km to use this energy
up!!!
Feedback
12/01/2017
• 1. I liked the NCEA research assessment on
microbes last year. Agree/disagree
• 2. The assessment prepared me for the
geology assessment this year.
Agree/disagree
• 3. The NCEA assessment prepared me well
for this year. Agree/disagree
• 4. I would offer the assessment again this
year. Agree/disagree
Gravitational Potential Energy
12/01/2017
This is how much stored energy in an object
Potential energy (Ep)= Mass (m) x Gravity (g) x Height (h)
g on earth is about 10ms-2 and h = the height you climb
OR Ep=mgh
E = Energy (in joules/J)
m = mass (in kilograms)
Ep
m
g
h = height (metres)
h
g = acceleration (ms-2)
Worked example
12/01/2017
A rock with a mass of 120Kg sits at the top of a cliff
15m in height.
Calculate the amount of potential energy in the rock.
Ep= mgh
= 120x10x15
= 18000J
Experiment 1
12/01/2017
• You are going to investigate energy changes in a vertical catapult.
Method
o Stretch an elastic band across the top of a tripod to make a vertical
catapult.
o Measure the mass of your projectile & then put it into the elastic band
catapult.
o Fire the catapult upwards. SAFETY – WEAR GOGGLES WHEN
LAUNCHING THE PROJECTILE!!!
Kinetic Energy (Ek)
• A form of energy that a body in motion
possess.
• A body at rest will not possess any Ek.
• Examples:
– Bullet shot out from pistol
– Helicopter flying at 120km/h
41
Kinetic Energy (Ek)
• The amount of Ek of a moving body
depends on its:
– Mass (kg) and
– Velocity (ms-1)
• When either mass or velocity of moving
body is increased, Ek will also increase.
42
Kinetic Energy
12/01/2017
E = Energy (in joules)
EK
m = mass (in kilograms)
v = speed (metre per second)
½m
v2
1 Joule= the energy required to
movie a 1kg object 1m
Kinetic Energy (KE)
KE = ½  m  v2
Velocity, v
Mass = m
44
How much energy is lost
between Ep and Ek?
• 1.Set up the ramp at the desired height and measure.
• 2. Find the mass of a marble and calculate the Ep.
• 3. Release the marble and accurately record the speed
that the marble reaches after it leaves the ramp. Think
about how you can make this as accurate as possible.
• 4. Collect at least 3 quality average times and then
average these to find the speed above.
• 5. Calculate Ek
• 6. What is the difference between Ep and Ek? Why?
45
Summary questions
• 1. Identify two variables you controlled and
how you controlled them
• 2. Why did you need to think about the
distance you recoded the speed over?
• 3. Why did you have to do multiple repeats
(two concepts to think about here)?
46
Kinetic Energy ( Ek)
• Formula: = Ek ½ mv2
• From the formula, what can you infer
about the change in Ek when…
– Mass doubles
Ek doubles
– Velocity doubles Ek increases by FOUR times
– Because velocity is squared
47
KINETIC ENERGY- notes
• Kinetic energy is the energy associated with a
_______ object.
• The symbol for kinetic energy is ___.
• Formula: Ek = ½ mv2 (where energy is
measured in_______, mass is measured in
__________and _______is measured in
ms-1)
• Words: Ek, joules , kilograms, moving, speed
48
Examples of KE
• Find the Ek of an empty van of mass 1000kg moving at 2ms-1.
Ek
= ½ x 1000 x (2)2
= 2000 J = 2 kJ
• Find the Ek of the van when it is loaded with goods to give a
total mass of 2000kg, and moving at 2ms-1.
Ek
= ½ x 2000 x (2) 2
= 4000 J = 4 kJ
• Find the Ek of an unloaded van when it speeds up to 4ms-1.
Ek
= ½ x 1000 x (4) 2
= 8000 J = 8 kJ
49
2006 NCEA question
• The speed of Toni and the bike during
Section B is 8.3 m s–1.
• The combined mass of Toni and the bike
is 70 kg.
• Calculate the kinetic energy of Toni and
the bike during section B as she travels at
that constant speed.
50
answer
• The speed of Toni and the bike during Section B is 8.3
m s–1.
• The combined mass of Toni and the bike is 70 kg.
• Calculate the kinetic energy of Toni and the bike during
section B as she travels at that constant speed.
• Ek = ½ mv 2
• = ½ x 70 x 8.3 x8.3
• =2411.15J
• =2411.2 J (with sensible rounding)
51
Sensible rounding is needed
for excellence in physics
• Do the calculation (0.5 X 75.3x 4.7x4.7)
• Write down the answer your calculator to
four decimal places (if present)
• Eg 831.6885
• Then round to the number of decimal
places in given in the question
• Eg 831.7
52
Excellence Examples of Ek
• A 200 kg motorcycle accelerates at 2ms-2 from
rest for 5s. Find the Ek of a motorcycle after 5 s.
Mass of motorcycle is 200 kg.
53
Eg. of Conversion of Energy
• A car of 800 kg is moving at an average speed of 5 ms-1.
The traffic light changed to red and so the driver stepped
on the brakes to bring the car to a quick, sudden and
screeching halt.
• Find the energy of the moving car and what form of
energy is this?
– Ek = ½ mv2 = ½ x 800 x 52 = 10,000 J.
• What energy does the car possess when it stops?
– None.
• What happened to the original energy of the moving car?
54
– Ek has changed to Sound and Heat Energy.
Rearranging Ep and Ek
• Ep= mgh
Ek=1/2 mv2
• m= Ep/gh
m= 2Ek/v2
• h= Ep/mg
v= square root
»
2Ek/m
55
NCEA E questions 1.
• A girl of mass 60 kg
uses 5 100 J of
energy when she
climbs a vertical rope.
• a. EP = mgh
• (a) Calculate the
maximum height it
would be possible for
the girl to reach.
• b. Ep= Ek
• (b) Calculate the
speed that the girl
would hit the ground
• Ek= ½ mv2
• = square root 2x
5100/ 60
• v= 13.04ms-1
56
2.
• A crane lifts a container with a
mass of 1200kg up 13m.
• A) Ep= mgh
• A) calculate the amount of Ep
stored in the container at the
highest point.
• Ep= 156000j
• B) calculate the speed that the
container would hit the ground
if it fell
• C) why would the container not
actually reach this speed?
• Ep= 1200x10x13
• B)Ep= Ek
•
Ek=1/2 mv2
• =
• V= 16.12 ms-1
• C) the force of air
resistance would cause
some of the energy to be
lost as heat (only a little).57
Work
• Define work?
• Are you doing WORK when…
– Lifting weights?
– Walking with a big bag of grocery in your
hand?
– Completing your homework assignment?
– Writing an essay?
58
Physics concept of WORK
• WORK is the amount of energy
transferred to an object when a
constant force acts on an object. This
causes the object to move in the same
direction as the force applied.
59
Physics concept of WORK
• What IS considered as work done in
Physics:
– You push a heavy shopping trolley for 10 m
– You lift your school bags upwards by 1 m
60
Physics concept of WORK
• What is NOT considered as work done:
– You push against a wall
– Jumping continuously on the same spot
– Holding a chair and walking around the
classroom
61
Physics concept of WORK
Work done = Constant x
force (N)
Distance moved
in the direction
of force (m)
W =F x d
Units: [J]
[N]
[m]
Unit for Work is JOULE (J)
62
Energy and Big Macs
12/01/2017
• 1. Calculate the amount of Ep a 20kg box would have if you
lifted it onto a table 1m high?
• 2. What energy transformation occurred for you to lift the
box onto the table?
• 3. How much kinetic energy must you have used to lift the
box?
• 4. If you place the box on the ground how much Ek have you
used in one repetition?
• 5. If you did this 10 times how many Joules of energy would
you have used?
• 6. A Big Mac has 2 200 000J of energy. How many times
would you need to lift and place the box to burn up this
amount of energy?
More Examples of WORK
• You are helping to push your mother’s heavy
shopping cart with a force of 50 N for 200 m.
What is amount of work done?
Work done,
W= F x d
= 50 x 200
= 10,000 J
or
10 kJ (kilo-Joules)
64
More Examples of WORK:
• Jack put on his bag-pack of weight 120 N. He
then starts running on level ground for 100 m
before he started to climb up a ladder up a
height of 10 m. How much work was done?
From Physics point of view, no work is done on pack at
level ground. Reason: Lift is perpendicular to movement.
Work is done on pack only when Jack climbs up the ladder.
Work done, W = F x d
= 120 x 10
= 1200 J or 1.2 kJ
65
Work done to lift an object=
gravitational potential energy
gained
• Force to lift an object = force of
gravity = weight
• Work = force x distance
= force of gravity x distance
= weight x distance
= mass x gravity x distance
= mass x gravity x height
= gravitational potential energy
66
Complete the work and energy
worksheet
67
power
• Power is the rate at which work is done or
energy is gained or lost
W
• Power = Energy = work
•
Time
time
• P = W/t
P
t
• Power is measured in watts (W)
• 1 watt = 1 joule per second
68
How powerful are you?
• 1. Find the mass of each Brick and add
this together (in Kg’s)
• 2. Measure the distance from the floor to
the side bench (in meters)
• 3. Time how long it takes to lift the bricks
(in seconds)
69
How powerful are you?
• During a push up (and down)?
• 1. find your body mass and calculate 68%
and 75% of this mass
• 2. find the height you push up to from the
ground. (measure in meters)
• 3. Time (in seconds) how long it takes to
complete 5 push ups (can increase or
decrease if you want).
70
Practice
• A girl climbs up a rope in 5 seconds and
gains 3500 J of energy
• A) What power does she use?
• B) If the rope is 5 metres high, what is her
mass?
• C) If she let go of the rope and fell to the
ground what speed would she be going
just before she hit the ground
71
Examples
• You lifted a set of books of mass 3kg, for 2m
in 4 seconds.
• What is work did you do?
• What was the power?
72
Power
12/01/2017
The POWER RATING of an appliance is simply how much work
it does every second (i.e. how much energy it produces every
second).
Definition: 1 Watt = 1 Joule per second
E = Energy (in joules)
E
P = Power (in watts)
T = Time (in seconds)
P
T
Power
12/01/2017
Now you are going to find your power when speeding up on flat
ground and compare that with the power while climbing stairs.
You have already measured your Energy in each case, so now
you only need to find the time it takes to reach top running
speed and time to climb the stairs.
Make a prediction,
which are you best at?
E
Make a table
Collect the data
P
T
Do the calculations
Compare with your class
mates.
Example questions
12/01/2017
1. Shane does 20J of work in 5s. What is his power rating?
2. Danni runs up some stairs with a power rating of 500W. If
it takes her 20s how high did she climb if she is 40kg?
3. Sam lifts a book 2m into the air with a force of 3N. He
does this in 3s. What was his power rating?
4. Alexandra runs up a hill with a power rating of 300W. If
her mass is 60kg and she ran up the hill in 5s how high was
the hill?
5. Seb’s car produces a driving force of 3000N and he drives
the car for 100m. If the power rating of the car is 60kW
how long did the journey take?