Newton's laws

• Balanced forces


  Cancel each other out
• Unbalanced forces


  Cause a resultant force to act on an object
• Resultant force = 5N
• If the forces are not balanced then the motion of the object will change
• Unbalanced forces


  This van will accelerate
• Unbalanced forces


  This van will slow
• Newton's second law


  Can be used to calculate the acceleration or deceleration
• Velocity = 12m/s
• Resultant force = 200N
• Newton's third law


  If a body A exerts a force on body B, then body B exerts an equal and opposite force on body A
• Newton's third law


  • A rocket pushes the exhaust gases backwards, so the gases will push the rocket forwards with the same force but opposite direction
  • The propeller of this boat pushes the water backwards, so the water pushes the boat forwards with a force the same size but opposite direction
• Weight


  The force of gravity acting on an object
• Mass


  The amount of matter in an object
• Mass does not change when you go into space but your weight will
• On Earth the gravitational field strength is 10N/kg but this is different on different planets
• Newton's first law


  An object will remain at rest or at a constant velocity unless it is acted upon by an external resultant force
• This object will remain stationary
• This object will continue moving at 12m/s in a straight line
• Terminal speed


  1. Initially the air resistance is smaller than the weight, this causes the sky diver to accelerate

  2. As they accelerate, the air resistance increases

  3. This continues until the air resistance is equal and opposite to the weight. The forces are balanced so the sky diver continues to fall at a constant speed. This is known as their terminal speed

  4. When the sky diver opens their parachute, the forces become unbalanced. The air resistance increases to be larger than the weight, this slows the sky diver

  5. As the sky diver slows, the air resistance decreases to become equal to the weight again. The forces balance and a new lower terminal speed is reached
• Terminal speeds can also be achieved with vehicles travelling horizontally, where air resistance and friction backwards increase to be equal to the engine's force forward
• Weight


  Force of gravity acting downwards
• Air resistance


  Force acting upwards