Demonstrate understanding of
aspects of mechanics.
Science A.S. 1.1
2014
FORCES
Forces are pushes or pulls (a combination is a
twist).
Forces act in pairs.
Objects are stationary or travelling at a constant
speed when forces are balanced.
Unbalanced forces cause changes to objects
motion (speed or direction), or shape.
The unit of force is the Newton (N)
FORCE PAIRS
Forces act in pairs (e.g.
thrust and friction, Weight
and support).
Force diagrams show the
direction of forces acting on
an object and whether they
are balanced or
unbalanced.
Arrow size represents force
size if no measurements are
available.
Support
Thrust
Friction
Weight
Force pairs
• What are the
missing terms?
• Support
(Buoyancy)
• Friction(Drag)
• Thrust
• Weight
TYPES OF FORCES
If a force acts on a stationary object and causes motion, the object
has gained kinetic (movement) energy.
Friction will stop the object moving.
Types of force:
Gravity
Friction – the force that opposes motion
Magnetism
Tension – the force in rope, etc
Electrostatic
Support
Lift – in the air (planes/birds)
Bouyancy – in the water
CONTACT FORCES
Some forces only act on contact, others can act from a distance.
Which are which?
Gravity
Contact
Distance
Magnetism
Tension
Friction
Electrostatic
Support
Balanced and Unbalanced Forces
Label the forces
What do the size of the top and bottom arrows say about the forces and
how do the vertical arrows differ?
Support
Friction
Thrust
Weight
Net force is the overall force acting on the object
If the force of friction is 10N and Thrust is 22N what is the net force?
What is the vertical net force and what does this say about the motion
of the object?
BALANCED OR UNBALANCED?
Explain whether the forces in the following scenarios balanced
or unbalanced.
1. The international space station is orbiting Earth at about
28,000kmhr-1.
2. A can is being crushed.
3. A car is travelling at a constant speed.
4. A skydiver has just jumped from a plane.
5. A car stays at 50kmhr-1 as it turns a corner.
Motion of objects
• Balanced forces
– Net force is 0N
– Motion is stationary or travelling at a constant
speed
• Unbalanced forces
– Motion is accelerating or decelerating
• The rate of acceleration depends on the mass of
the object and force applied…
Force = mass × acceleration
F
(F = m × a).
If m is constant and a increases what
happens to F?
m
a
FORCE AND MOTION
What happens when you apply (using a Newton meter) a small
constant force to a trolley and time it over a set distance?
Small constant
force
Set distance
The trolley should accelerate because…
F
m
An unbalanced force causes acceleration.
a
FORCE AND MOTION
What happens when you apply (using a Newton meter) the
same small constant force to a trolley carrying a 1kg mass and
time it over a set distance?
Small constant
force
1Kg
Set distance
The trolley should accelerate but slower than previously
because…
The larger the mass the slower the
acceleration
F
m a
FORCES AND ACCELERATION
Given the formula F = ma try the following questions.
1. What are the names and units of F, m and a?
2. Complete the table….
3. The rider and cycle are 150kg:
150N
800N
F
m
A
a.
9kg
0.5ms-2
6N
0.2kg
b.
c.
800g
1.5ms-2
350N d.
e.
15ms-2
1200kg 0.015ms-2
a. What is the Nett force?
b. What is the cyclist’s acceleration?
4. A bike accelerates at 10ms-2 using a force of 900N. The
rider is 70kg. What is the mass of the bike?
Pressure
• Pressure is the measure of the
force that acts on an area.
• Pressure (Pa) = Force (N) / area (m2)
• 1Pa is the same as 1N/m2
• The larger the area the lower the pressure
F
P
a
Pressure Calculations
Pressure (Pa) = Force (N) / area (m2)
A skier weighs 500N, each of
her skis has an area
of 0.05m2.
Calculate the total pressure exerted by her skis
on the snow.
P=F/A
P =500/ (0.05 +0.05)
P= 5000Pa
Pressure Calculations
• If her weight increases to 600N, what area
would each ski have to exert to have the
same pressure as before?
A = F/P
A = 600/5000
A = 0.12m2 in total
• Therefore 0.12/2 = each ski needs an area
of 0.06m2
Test Yourself
Explain using pressure why trying to use the sharp
end to push the round end into the board is not a
good idea.
P=F/A. The force applied remains constant but the
round end has a larger area so applies a low pressure
to your thumb, but the sharp end has a smaller area
and applies a high pressure to the notice board, so it
pushes in.
WEIGHT FORCE
Weight is a force. It is therefore measured in… Newtons (N)
An object’s weight depends on two things…
Gravity
• varies depending where you are
• 10ms-2 or 10N/Kg on Earth
Mass
• does not vary
• measured in Kg
•A man with mass of 75Kg on earth weighs 750N
BUT on the moon he weighs 125N
Weight Loss
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Mass relative
to Earth
0.06
0.82
1
0.1
318
95
14
17
0.002
Radius
2400
6000
6400
3400
71500
60300
25600
24800
1200
MASS AND WEIGHT
1. a. What is mass?
b. What is it’s unit?
2. a. What is Weight?
b. What is it’s unit?
3. ON EARTH: 1N =
kg
1kg =
N
4. How would your mass and weight change if you went to
Jupiter?
FRICTION
Friction is a contact force that opposes motion, it
causes heat, damage, wear and slowing
Friction can be reduced by…
lubrication,
streamlining (aerodynamics),
slowing down,
smoothing surfaces
ENERGY
There are 2 main energy forms…
Moving objects have Kinetic Energy
Stored energy is called Potential Energy
Some different types are:
Potential
Heat
Sound
Chemical
Elastic
Electrical
Light
Kinetic
Gravitational Nuclear
ENERGY CHANGES
Energy cannot be created or destroyed. It can
only be converted from one form into another.
Energy changes occur all of the time…
A light bulb glowing
Electrical

Light
+
Heat
A person climbing a ladder
Chemical

Gravitational +
Heat (body)
Something that is energy efficient converts from one form to
another very well; it loses very little energy.
Energy Transformations
• Energy can neither be created or destroyed
but it is converted from one form to another.
• What transformations are involved when a
long jumper competes
• Chemical
kinetic
gravitational potential
kinetic
sound/heat
What about driving and turning on a light?
ENERGY CALCULATIONS
Energy possessed by objects can be calculated. It is measured in
joules.
Ek = ½ mv2
Kinetic Energy (J)
Mass (Kg)
Velocity (ms-1)
Calculate Ek of a 500Kg car travelling at 10m/s
Ek = ½ mv2
Ek = ½ x 500 x 100
Ek = 25000j
Write Formula
Substitute numbers
Answer with units
ENERGY CALCULATIONS
Ep = mgh
Gravitational
potential
Energy (J)
Mass (Kg)
10ms-1 or
10NKg-1
Height (m)
Calculate Ep of a 500Kg car being lifted 1.5m onto a tow
truck
Ep = mgh
Write Formula
Ep = 500 x 10 x 1.5
Substitute numbers
Ep = 7500j
Answer with units
Test Yourself
• A 70kg climber is 50m up a wall when he
slips. He falls 30m in 3 sec and remains
stationary hanging from his rope.
• Discuss the relative size of the forces
acting on the climber as he falls.
• State whether they are balanced or
unbalanced.
• Sketch a speed v time graph for his
motion during the 4 secs following his
slip.
Test yourself (cont.)
• State the energy transformations involved
as he falls.
• Calculate the gravitational
potential energy before
and after his fall and why
there is a difference in
these values.
WORK
Work is done when a force moves a distance.
When work is done, energy is
converted into different forms.
When our skydivers fall energy is
converted from…
Gravitational Potential energy into…
Kinetic Energy (and heat).
Work = Force  distance
W
W = Fd
In Joules (J)
F
d
Work Calculations
Work (J) = Force (N)  distance (m)
• How much work is done when a crane lifts
1000Kg of concrete 3m?
• Weight of concrete
= 1000kg x 10N/Kg
= 10000N
• Work = 10000N x 3m
• Work = 30000j
W
F
d
Test Yourself
• A 1200kg car drives up a 25m slope using a force
of 5000N. Calculate the work done.
25m
•
•
•
•
10m
W=Fxd
W = 5000N x 25m
W = 125000 j
Calculate the difference between energy gained
and work done and explain what happened to the
energy?
• Describe the energy transformation which takes
place during the journey.
POWER
How Powerful something is depends on 2 things…
How much Work is done (= energy converted)
and the Time it takes.
Power = Work ÷ time
Power is
measured
in…
Watts
W
P = W/t
or
P = E/t
P
t
How come?
A weightlifter raises 50Kg by 60cm in 1.2s. Calculate his
power.
SPEED
Speed is the distance that an object travels in a period of time.
Units are meters and seconds
(and therefore meters per
second). However, sometimes
km/hr is more sensible.
d
t
A cyclist travels 25 km in ½ an hour.
V=d/t
What is their speed - in kmh-1
V= 25km/0.5hr
V= 50km/h
- in ms-1
V=d/t
V= 25000m/1800s
V= 13.89m/s
v
DISTANCE/TIME GRAPHS
A car takes 1.5 minutes to travel 500m down a busy road. It stops
at lights for 30 seconds, then continues on for 1 minute as it goes
another 1km.
Plot this on a distance/time graph.
Using the distance/time
graph:
1.5
Distance
1
Δd =
1km
(km)
1.What is the total distance
0.5
traveled?
= 1.5 km
2. In what part of the trip is
= part 3
the car going the fastest?
3. What is the fastest
speed?
v = Δd / Δt
v = 1km/1min
v = 1000m/60s
v = 16.67m/s
Steepest
section is
fastest
Δt =
1min
1
2
Time (min)
3
In a distance/time graph
the slope of the line =
the speed of the object.
SPEED QUESTIONS
What would these look like on a distance/time graph?
1. stopped
2. slow
3. fast
4. accelerating
ACCELERATION
Acceleration is the change in speed in an object in a period of
time.
Units ms-2
It takes a cyclist 20 seconds to go from
a standing start to 14m/s.
What is their acceleration?
Δv
a = Δv/Δt
a = 14m/s / 20s
a
Δt
a = 0.7ms-2
What is 14m/s in km/hr?
= 14 × 60s × 60min  1000m
= 50.4km/h
Convert m to Km divide by 1000
Convert Km to m multiply by 1000
Convert hours to seconds multiply by 3600 (60 minx 60 sec)
Convert seconds to hours divide by 3600
SPEED/TIME GRAPHS
A runner travels at 4m/s for 10 seconds, then stops suddenly for 5
seconds, then accelerates for 5 seconds to get to 8m/s and
continues for 10 seconds.
Plot this on a speed/time graph.
Using the speed/time
graph:
In what part of the trip
is the runner going the
fastest?
= part 5
What is the
acceleration in part 4?
a = Δv/Δt
a = 8m/s/5s
a = 1.6ms-2
Speed
8
(m/s)
4
10
20
Time (sec)
In a speed/time graph the
slope of the line = the
acceleration of the object.
30
SPEED/TIME GRAPHS
In a speed/time graph the distance covered = area under the graph.
Part Part
2
3
What distance is covered in part 1?
Speed
d=v×t
8
(m/s)
d = 4m/s × 10s
4
d = 40 m
What is the total distance covered?
Part 1 = 40m
Part
4
Part
1
10
Part 2 = 0m
Part 3 = v × t × 
= 8m/s × 5s × 
= 20m
Part 4 = v × t
= 8m/s × 10 s
= 80m
Total
= 40 + 20 + 80
=140m
20
Time (sec)
Honda
30
ACCELERATION QUESTIONS
What would these look like on a speed/time graph?
1. stopped
2. slow
3. fast
4. accelerating