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Forces at Work

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Speed is the distance travelled in a particular time interval. ... Common examples of first-class levers include crowbars, scissors, pliers, tin snips and seesaws ... – PowerPoint PPT presentation

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Title: Forces at Work


1
Forces at Work
  • Speed and Acceleration

2
Average Speed
  • Speed is the distance travelled in a particular
    time interval.
  • The instantaneous speed is the speed at a
    specific instant. The cars speedometer records
    instantaneous speed.

Australian Mark Webber At the Imola
GP http//www.markwebber.com
3
An example How long does it take for a car
moving at 90 km/h to travel 230 km
Another way to use the formula is to use a
triangle approach
230
TIME
90
DIST.
  • ? Time 230 ? 90
  • Time 2.56 hours
  • Or 2 hours 33.6 minutes

TIME
SPEED
4
Ticker Timers
Source www.physicsclassroom.com
5
Average Speed Vs Instantaneous Speed
6
Acceleration
  • Example Suppose a car accelerates from 60 km/h
    to 100 km/h in 5 seconds. Find it acceleration.
  • Data
  • Initial speed 60km/h
  • Final speed 100 km/h
  • ?change in speed 40 km/h
  • Time 5 s.
  • Acceleration 40/5
  • Acceleration 8 km/h per s.
  • Acceleration is the rate at which the moving
    object changes speed.

7
An example How long does it take a car to
accelerate from 4 m/s to 10 m/s if the
acceleration was 3 m/s2
Another way to use the formula is to use a
triangle approach
10-4 6m/s
TIME
Change In speed.
3
  • ? Time (10-4) ? 3
  • Time 2 s

TIME
Accel.
8
Forces on a moving car
Normal Force the force the road exerts On the
Car.
Force of Friction
Thrust
Weight of the car on the road
9
Newtons First Law The Law of Inertia.
  • The Law of Inertia.
  • An object with no force acting on it remains at
    rest or moves with constant velocity in a
    straight line.

10
Newtons Second Law The Force Law.
  • The Force Law.
  • The acceleration of an object is directly
    proportional to the net force on it and inversely
    proportional to its mass.
  • Fma

11
Newtons Third Law Action-Reaction Law.
  • Action-Reaction Law.
  • When one object exerts a force on a second
    object, the second exerts a force on the first
    that is equal in magnitude but opposite in
    direction.

12
Getting Down to Work
  • Work is a product of a force and the displacement
    of an object in the direction of the force.
  • W Fd
  • Work has the units of Joules
  • 1 Joule 1 Newton metre (Nm)
  • Work is done on an object only if it moves in the
    direction of the force.

13
Work and Potential Energy
  • Potential energy is energy stored in an object
    because of its state or position.
  • The change in potential energy associated with a
    particular force when an object is moved form one
    point to another to be the work required by some
    other (external) force to move the object from
    the first point to the second point without
    accelerating it.

14
Gravitational Potential Energy
  • Gravitational Potential Energy is the Energy of
    an object because of its position. For example,
    a person at the top of a waterslide.
  • GPE mgh
  • GPE has units of Joules.

15
Work - Energy Theorem
  • Doing work on an object can increase its kinetic
    energy.
  • The work-energy theorem states that The net work
    done on an object equals to its change in kinetic
    energy.
  • Net Work done ?KE

16
Simple Machines
  • All machines, no matter how complex, are made up
    of one or more of the six simple machines. The
    six simple machines are
  • Lever
  • Wheel and Axle
  • Pulley
  • Inclined Plane
  • Wedge
  • Screw

17
First Class Levers
  • In a first-class lever the fulcrum is located at
    some point between the effort and resistance
    forces. Common examples of first-class levers
    include crowbars, scissors, pliers, tin snips and
    seesaws

Source http//www.uark.edu/depts/aeedhp/agscience
/simpmach.htm
18
Second Class Levers
  • A second-class lever does not change the
    direction of force. When the fulcrum is located
    closer to the resistance than to the force, an
    increase in force (mechanical advantage) results.

Source http//www.uark.edu/depts/aeedhp/agscience
/simpmach.htm
19
Pulley Systems
  • A pulley is said to be a fixed pulley if it does
    not rise or fall with the load being moved. A
    fixed pulley changes the direction of a force
    however, it does not create a mechanical
    advantage.

http//en.wikipedia.org/wiki/Pulleys
20
Movable Pulleys
  • A moveable pulley rises and falls with the load
    that is being moved. A single moveable pulley
    creates a mechanical advantage however, it does
    not change the direction of a force

http//en.wikipedia.org/wiki/Pulleys
21
More Complex Pulley Systems
http//en.wikipedia.org/wiki/Pulleys
22
Incline Planes
  • The inclined plane makes it easier to move a
    weight from a lower to higher elevation.

23
Simple Machine and Mechanical Advantage
24
Efficiency of Simple Machines
  • The efficiency of a machine is defined as the
    ration of output work to input work. The ideal
    machine has equal output and input work,
    therefore it has 100 efficient.

25
Gears
  • Work equals the product of force and distance,
    therefore the small gear is required to run a
    longer distance and in the process is able to
    exert a larger twisting force or torque, than
    would have been the case if the gears were the
    same size

Source - http//en.wikipedia.org/wiki/Gears
26
Gear Ratios
  • The gear ratio is the relationship between the
    number of teeth on two gears that are meshed or
    two sprockets connected with a common chain

The smaller gear has thirteen teeth, while the
second, larger gear has twenty-one teeth. The
gear ratio is therefore 13/21 or 1/1.61
27
Bicycles Fixed Gears
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