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Forces Introduction
•
•
•
•
Intersections
smart car crash
stopping distance
Crash investigation
Levers
• You can use a
mechanism to move
something more
easily.
• Force Multiplier
• force you produce is
bigger than the force
you apply
• Mechanical
Advantage
• 3 types
Load =
Class 1
• The force you apply is
on the opposite side
of the fulcrum to the
force you produce.
= Effort
Class 2
• The fulcrum is at one
end.
• You apply force at the
other end and the
force you produce is
in the middle.
Class 3
• apply the force in the
middle and the force
you produce is at the
opposite end.
• They reduce the force
you apply, giving you
much greater control.
Inclined plane -ramp
• You use less
force, but you
need to pull/push
a longer distance
• you use the
same amount of
energy in each
case
Pulleys
• Two or more wheels and a loop of rope
around them creates a lifting machine.
• Each time the rope wraps around the
wheels, you create more lifting power or
mechanical advantage.
Pulleys
• Pulleys transfer
rotation from one
shaft to another.
• Same diameter =
same speed rotation.
• Large drive pulley
makes a smaller
pulley rotate faster.
• If the belt is crossed
rotation is in the
opposite direction.
Wheel
• Wheels can multiply
speed/ distance or
force.
• The axle turns a short
distance (blue arrow)
• leverage of the wheel
means the outer rim
turns much further
(red arrow) in the
same time.
FORCES
Forces are pushes or pulls (a combination is a
twist).
Objects are stationary when forces are
balanced
gravity is always acting but we don’t keep
falling due to a support force
Forces can be measured using a Newton meter.
BALANCED FORCES
An unbalanced forces cause changes to objects motion (speed or
direction), or shape.
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
FORCE PAIRS
Forces act in pairs (e.g.
thrust and friction, gravity
and support).
Force diagrams show the
forces acting on an object
and whether they are
balanced or unbalanced.
Arrow size represents force
size if no measurements are
available.
Force pairs
• What are the
missing terms?
• Buoyancy
• Drag
• Thrust
• Weight
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.
UNBALANCED FORCES
An unbalanced force (a net force) results in acceleration.
The rate of acceleration depends on the mass of the object and
force applied…
Force = mass × acceleration
(F = m × a).
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…
An unbalanced force causes acceleration.
FORCE AND MOTION
What happens when you apply (using a Newton meter) a 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
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.
15ms-2
1200kg 0.015ms-2
e.
a. What is the Nett force?
b. What is the cyclist’s acceleration?
4. A bike accelerates at 10ms-2 using a force of 6000N. The
rider is 70kg. What is the mass of the bike?
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
Reaction Times
• http://www.bbc.co.uk/science/humanbody/
sleep/sheep/reaction_version5.swf
• Click Above for the Reaction Time Game
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
v
A cyclist travels 25 km in ½ an hour.
What is their speed
- in kmhr-1
= 25km/0.5hr
= 50km/hr
- in ms-1
= 25000m/1800s
= 13m/s
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
∆d =
1km
Steepest
section is
fastest
(km)
1.What is the total distance
0.5
traveled?
= 1.5 km
2. In what part of the trip is
the car going the fastest? = part 3
3. What is the fastest
speed?
v = ∆d / ∆t
v = 1km/1min
v = 1000m/60s
∆t =
1min
1
2
Time (min)
In a distance/time graph
the slope of the line =
the speed of the object.
v = 16m/s
SPEED QUESTIONS
What would these look like on a distance/time graph?
1. stopped
2. slow
3. fast
4. accelerating
3
ACCELERATION
Acceleration is the change in speed in an object in a period of
time.
Units ms-2
∆v
a
∆t
It takes a cyclist 20 seconds to go from
a standing start to 14m/s.
What is their acceleration?
a = ∆v/∆t a = 14m/s / 20s
What is 14m/s in km/hr?
= 14 × 60s × 60min ÷ 1000m
a = 0.7ms-2
= 50.4km/hr
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:
Speed
8
(m/s)
In what part of the trip
is the runner going the
fastest?
4
= part 5
10
20
Time (sec)
What is the
acceleration in part 4?
30
In a speed/time graph the
slope of the line = the
acceleration of the object.
a = ∆v/∆t
a = 8m/s/5s
a = 1.6ms-2
SPEED/TIME GRAPHS
In a speed/time graph the distance covered = the area under the graph.
Part
2
What distance is covered in part 1?
Speed
d=v×t
Part
3
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)
30
ACCELERATION QUESTIONS
What would these look like on a speed/time graph?
1. stopped
2. slow
3. fast
4. accelerating
Crumple Zone
• Absorbs the force of
impact by
• Increasing the time of
impact
• Which decreases the
force of impact
Stopping Distances