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Car Crashes - Science

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					            Forces Introduction
•   Intersections
•   smart car crash
•   stopping distance
•   Crash investigation
                    Levers
• You can use a           Load =
  mechanism to move
  something more
  easily.
• Force Multiplier
• force you produce is
  bigger than the force
  you apply
• Mechanical
  Advantage                        = Effort
• 3 types
                   Class 1
• The force you apply is
  on the opposite side
  of the fulcrum to the
  force you produce.
                   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
          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      1Kg
force


   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….
                                           F    m       A
3. The rider and cycle are 150kg:
                                           a.   9kg     0.5ms-2
                                           6N   0.2kg   b.
    150N                   800N            c.   800g    1.5ms-2
                                           350N d.      15ms-2
                                           e.   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 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
                    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
                            SPEED
 Speed is the distance that an object travels in a period of time.


Units are meters and seconds
(and therefore meters per                               d
second). However, sometimes
km/hr is more sensible.
                                                    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.
                                          1.5
Plot this on a distance/time graph.
                                                  Δd =      Steepest
                                Distance
Using the distance/time                    1      1km       section is
graph:                            (km)                      fastest

1.What is the total distance        0.5
traveled?                  = 1.5 km                               Δt =
                                                                  1min
2. In what part of the trip is
                               = part 3
the car going the fastest?                           1          2        3
                                                         Time (min)
3. What is the fastest    v = Δd / Δt
speed?                                          In a distance/time graph
                          v = 1km/1min
                                                the slope of the line =
                          v = 1000m/60s         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
              ACCELERATION
Acceleration is the change in speed in an object in a period of
time.


Units ms-2                                           Δv


                                                 a        Δt

It takes a cyclist 20 seconds to go from
a standing start to 14m/s.


What is their acceleration?   a = Δv/Δt a = 14m/s / 20s    a = 0.7ms-2
What is 14m/s in km/hr?       = 14 × 60s × 60min  1000m
                                                     = 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.
                                   Speed
 Using the speed/time                      8
 graph:                            (m/s)

 In what part of the trip                  4
 is the runner going the
 fastest?
     = part 5                                       10        20      30
                                                         Time (sec)
 What is the
 acceleration in part 4?
     a = Δv/Δt                         In a speed/time graph the
                                       slope of the line = the
     a = 8m/s/5s                       acceleration of the object.
     a = 1.6ms-2
              SPEED/TIME GRAPHS
In a speed/time graph the distance covered = the 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
     d = 40 m                                 4
                                                  Part                  Part
                                                  1                     4
What is the total distance covered?
     Part 1 = 40m                                        10        20          30
                                                              Time (sec)
     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
    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

				
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