Forces Introduction

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Forces Introduction

• Intersections
• smart car crash
• stopping distance
• Crash investigation

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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
Effort
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Class 1

• The force you apply is on the opposite side of
  the fulcrum to the force you produce.

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Class 2

• The fulcrum is at one end.
• You apply force at the other end and the force
  you produce is in the middle.

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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.

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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

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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.

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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.

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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.

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FORCES
Forces are pushes or pulls (a combination is a
twist).   Objects are stationary when forces
are balanced gravity is always acting but we
dont keep falling due to a support force
Forces can be measured using a Newton meter.
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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
Tension the force in rope, etc
Magnetism
Electrostatic
Support
Lift in the air (planes/birds)
Bouyancy in the water
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CONTACT FORCES
Some forces only act on contact, others can act
from a distance. Which are which?
Gravity
Magnetism
Tension
Friction
Electrostatic
Support
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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.
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Force pairs

• What are the missing terms?
• Buoyancy
• Drag
• Thrust
• Weight

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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.
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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).
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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?
Set distance
The trolley should accelerate because
An unbalanced force causes acceleration.
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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?
1Kg
Set distance
The trolley should accelerate but slower than
previously because
The larger the mass the slower the acceleration
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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 a. What is the Nett force? b.
What is the cyclists acceleration? 4. A bike
accelerates at 10ms-2 using a force of 6000N. The
rider is 70kg. What is the mass of the bike?
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WEIGHT FORCE
Weight is a force. It is therefore measured
in An objects weight depends on two things
Newtons (N)

• 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

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Weight Loss
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MASS AND WEIGHT

• a. What is mass?
• b. What is its unit?
• 2. a. What is Weight?
• b. What is its unit?
• 3. ON EARTH 1N kg 1kg N
• 4. How would your mass and weight change if you
  went to Jupiter?

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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
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Reaction Times

• http//www.bbc.co.uk/science/humanbody/sleep/sheep
  /reaction_version5.swf

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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.
A cyclist travels 25 km in ½ an hour. What is
their speed - in kmhr-1 - in ms-1
25km/0.5hr 50km/hr 25000m/1800s 13?m/s
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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.
?d 1km
Steepest section is fastest

• Using the distance/time graph
• What is the total distance traveled?
• 2. In what part of the trip is the car going the
  fastest?
• 3. What is the fastest speed?

1.5 km
?t 1min
part 3
v ?d / ?t v 1km/1min v 1000m/60s v 16?m/s
In a distance/time graph the slope of the line
the speed of the object.
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SPEED QUESTIONS
What would these look like on a distance/time
graph? 1. stopped 2. slow 3. fast 4.
accelerating
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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? What is 14m/s in km/hr?
a ?v/?t a 14m/s / 20s a 0.7ms-2 14 60s
60min ? 1000m 50.4km/hr
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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? What is
the acceleration in part 4?
part 5
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
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SPEED/TIME GRAPHS
In a speed/time graph the distance covered the
area under the graph.
Part 2
Part 3
What distance is covered in part 1? What is
the total distance covered?
d v t d 4m/s 10s d 40 m
Part 4
Part 1
Part 1 40m 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
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ACCELERATION QUESTIONS
What would these look like on a speed/time
graph? 1. stopped 2. slow 3. fast 4.
accelerating
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Crumple Zone

• Absorbs the force of impact by
• Increasing the time of impact
• Which decreases the force of impact

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Stopping Distances