Forces

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  • Created by: Katy **
  • Created on: 03-01-14 14:49

Newton's First Law

  • The resultant force can either be positive for negative.

5N←o→7N (7+5 = 12N) 12N

5N←O→7N (7-5 = 2N) 2N

  • Resultant force = 0no change in velocity (direction & magnitude).
  • Resultant force = not 0 → change in velocity (direction & magnitude).
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Momentum

  • Momentum is the product of an objects mass & velocity.
  • Momentum = mass x velocity
  • kgms-1 = kg x m/s
  • p = m x v
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Inertia

  • Inertia is the reluctance to change:

-stationary → to move

-moving → change velocity


  • Mass is the measure of an objects inertia.
  • Momentum is the expression of a moving objects ineria.
  • Greater mass = greater inertia
  • Greater momentum = greater inertia
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Newton's Second Law

  • Acceleration is proportional to the force (constant mass).
  • Acceleration is inversly propotraional to the mass.

Force = mass x acceleration


Resultant force = mass x acceleration


Resultant force = change in momentum/time

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Weight (force)

  • Weight is a result of gravity fom a planet pulling on an objects mass.
  • On Earth - every kg of mass experiences a graviational pull of 9.81N.

Weight = mass x gravity

W = m x g


  • Gravity depends on the mass of the planet (proportional).
  • The further the object is from the centre of the planet (core), the smaller the gravitational pull is.
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Air Resistance

  • Resistance is friction.
  • Resistance increase with speed (speed x2 = friction x4).
  • As the speed of a falling object increase = air resistance increases


Resistance factors:

-resistance & friction act opposite to movement

-resistance increases as speed2 increases

-greater surface area = greater friction / air resistance

-fluid resistance (air / liquid) depends on collision of paricles with moving object

-denser fluid = greater resistance (more paricles per volume).

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Falling Vertically (1)

  • Resultant force = weight - air resistance

Speed increases:

-air resistance increases

-weight stays the same

1. At start - not moving

  • air resistance = 0
  • resultant force = max.
  • acceleration = max.

2. Moving / Falling / Speeding up

  • air resistance = increases
  • resultant force = decreases
  • acceleration = decreases
  • weight = unchanged
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Falling Vertically (2)

3. Resultant is 0

  • air resistance increase until it = to weight
  • acceleration = 0
  • weight = air resistance } terminal velocity
  • velocity = constant } terminal velociy

4. Parachute opens

  • air resistance = increases / greater than weight
  • resultant force = negative → upwards
  • surface area = increases
  • deccelerating force - acceleration = negative → slows down
  • air resistance = max.
  • resultant force = max. negative value

 

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Falling Vertically (3)

5. Slowing down

  • air resistance = decreases
  • resultant force = less negative (increasing)
  • acceleration = less negative (increasing)
  • weight = stays the same
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Falling Horizontally

reaction force ↑

weight ↓

drive ←

air resistance / drag →

Resultant force:

-drive - drag

-mass x acceleration

-positive → accelerating

-0 → velocity constant

-negative → deccelerating

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Velocity & particle collision

As velocity increases:

  • collisions with air resistance increases
  • there is a greater rate of collisions
  • there is greater momentum
  • there is greater air resistance
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Falling with negligible air resistance

  • The moon has no air resistance.
  • A brick is a dense object.
  • In the absense of air resistance, all objects fall with at the same rate.
  • Acceleration is due to gravity (9.81 m/s/s).
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