Forces

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Newton's Laws

1st:

  • An object remains at rest or continues at a constant velocity unless an unbalanced force acts
  • No force = No change

2nd:

  • F= ma
  • Resultant Force = mass x acceleration (See Equations)

3rd:

  • Forces exist in pairs
  • Every force has an equal, opposite force
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Equations

F=ma:

  • Resultant force (N) = mass (kg) x acceleration(m/s^2)

W=mg:

  • Weight (N) = mass (kg) x grav. field strength (N/kg)

a=(v-u)/t:

  • Acceleration(m/s^2) = (change in velocity (End velocity - start velocity)(m/s))/ time taken (secs)

F=ke:

Force (N) = spring constant (N/m) x extension (m)

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Stopping & Starting

  • When a car is moving at a constant velocity on a flat road, the force of the engine is qual to the resistive forces on the car
  • When a car is decelerating on a flat road, the force of the engine is less than the resistive forces on the car
  • When a car is accelerating on a flat road, the force of the engine is greater than the resistive forces on the car
  • Thinking distance = the distance travelled in the time it takes the driver to react
  • Braking distance = the distance travelled by the vehicle in the time in which the driver is pressing the brakes
  • Factors:
    • Greater speed
    • Alcohol/ Drugs
    • Poor road/ brakes condition
    • Distrations
  • Thinking distance = speed
  • Braking distance = speed^2
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Resultant forces // Falling Objects (Moon)

  • The leftover force in any particular direction
  • This can cause a change in motion (Start moving or accelerate)
  • No resultant force means that a stationary object remains stationary or a moving object remains moving at the same speed in the same direction
  • The greater the mass of an object, the smaller its acceleration for a given force

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  • Weight (kg) = The force of the gravity on the object
  • Mass (N) = Quantity of the matter (Never changes)
  • Terminal velocity = When the resultant force acting on a falling object reaches 0, so the object moves at a constant speed
  • If two objects fall without air resistance, both object will hit the ground at the same time as they have the same acceleration. 
  • Although one has a bigger weight, it has to accelerate a bigger mass, so it cancels out
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Falling objects (Earth)

Parachutist:

1.

  • Parachutist pulled down by weight
  • No speed, so no drag
  • Resultant force downwards causes acceleration

2.

  • Drag increases
  • Still resultant downwards, but smaller
  • Smaller acceleration downwards

3.

  • Drag increases till equal to weight
  • No resultant, no acceleration
  • Terminal velocity
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Falling objects (Earth) Part 2

4.

  • Parachute opened
  • More surface area to cause more drag
  • Resultant force upwards causes deceleration
  • Drag is smaller

5.

  • Drag reduces till the forces are balanced
  • No resultant, no acceleration
  • Terminal velocity
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Spring Constant (k)

  • The amount of force needed to produce a certain extension (usually 1m)
  • eg. Apply 20N to a spring which stretches it 37cm
    • k = F/ e
    • k = (20/37) X 100
    • k = 54 N/m
  • The spring constant indicates the stiffness of the spring
  • A higher 'k' means stiffer spring
  • eg. Apply 12N to a spring stretches it 15cm
    • k = 12/0.15
    • k = 80 N/m
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