SP2 Motion and Forces


SP2a Resultant Forces

  • Scalar quantity - only has magnitude.
  • Vector quantity - has both magnitude and direction.
  • Resultant forces are the overall force on an object.
  • If the resultant force is zero, the object is balanced.
  • If there is a non-zero resultant force, the forces are unbalanced.

To calculate the resulant force:

  • acting in the same direction - add forces together 
  • acting in opposite direction - subtract one force from another.
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SP2b Newton's First Law

  • Newton's 1st Law - a object will remain unchanged unless a resultant force acts on it.
  • Balanced forces - doesn't change velocity.
  • Unbalanced forces - changes direction and/or speed.
  • Object moving in a circle is constantly changing velocity, but speed remains the same. 
  • Centripetal force - resultant force that causes a change in direction of an object and acts towards the centre of the circle. e.g. friction, gravity and tension.
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SP2c Mass and Weight

  • Mass - how much matter there is in an object, (kg).
  • Weight - measure of the pull of gravity on an object, (N).
  • Measured using a force meter
  • Gravitational Field Strength - 10N/kg
  • weight (N) = mass (kg) x gravity (N/kg)

Forces on falling bodies 

  • Air resistance increase with speed - resistance is smaller than weight & large resultant force - accelerate downwards 
  • Air resistance larger but weight is the same & resultant force smaller but accelerating (not as much).
  • Moving fast - air resistance balances weight & falls at same speed. 
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SP2d Newton's 2nd Law

  • Newtons's 2nd Law - f=ma/acceleration depends on:
  • Size of force (same mass, the bigger the force, bigger the acceleration).
  • Mass of object (same force, the bigger the object, smaller the acceleration).
  • force (N) = mass (kg) x acceleration (m/s²).
  • inertial mass = the force on it divided by the acceleration that force produces.  
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SP2d Core Practical

  • Investigate the relationship between force, mass and acceleration by varying the masses added to trolleys.

1. Prop up one end of the ramp so that the trolley just starts to move on its own. Set up the light gates and the pulley and string, so the masses are hanging off the table.
2. Release the trolley and calculate the speed from the data logger.                                                3.Put a mass ontop of the trolley and repeat steps.

  • The investigation was how the mass of the trolley affects the acceleration 
  • To investigate the effect of force on acceleration, keep the mass the same and change the masses at the end of the string. 


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SP2e Newton's Third Law

  • Newton's 3rd Law - for every action, there is an equal and opposite reaction. They are also the same type of force.
  • Equilibrium situation - nothing is moving.
  • E.g The weight of a dog on ground = the force pushing up on the dog from ground & force from dog on rope = force from rope on dog. 
  • Action-reaction pairs act on different objects 
  • Balanced forces all acto on the same object
  • E.g. Force of ground on dog = gravity on dog & force of rope on dog = force of friction on dog.
  • In a collision, action-reaction forces are the same size but don't have the same affect due to the objects having different masses
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SP2f Momentum

  • Momentum - tendency of an object to keep moving and how hard to stop moving.
  • Depends on mass and velocity, and is a vector.
  • Momentum (kg m/s) = mass (kg) x velocity (m/s) or p= m x v
  • Force = change in momentum ÷ time or (mv-mu)÷ t 
  • Conservation of momentum - total momentum before = total momentum after.
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SP2g Stopping Distances

  • Thinking distance - distance travelled while driver reacts to situation 
  • Braking distance - distance travelled while brake works to bring car to a stop.
  • Stopping time = thinking distance + braking distance 
  • Reaction time - time between person detecting stimulus and their response 
  • Typical reaction time - 0.25 seconds
  • If a person is ill, taking drugs, drinking, using mobile phone, all increases reaction times 
  • If road is wet, brakes are worn, loose gravel on road, tyres worn - less friction, braking distance increases
  • Vehicle with more mass - more force needed to deacelerate
  • Same friction used to stop a vehicle - heavier vehicle will travel futher (greater braking distance)
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SP2h Braking Distances and Energy

  • work done - energy transferred by a force acting over a distance 
  • work done (J) = force (N) x distance in direction of force (m)
  • kinetic energy - energy stored in a moving object
  • kinetic energy (J) = 0.5 x mass (kg) x (speed)² (m/s)²
  • kinetic energy = energy transferred to accelerate
  • when vehicle stops - kinetic energy transfers to other energy stores by braking force 
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SP2i Crash Hazards

  • Crumple zones (front and back) - takes time to crumple, deacceleration is less, reduces force on passengers and force is less than car with solid front.
  • Seat belts - hold passengers into car. 
  • Air bags - increase time for a head to stop in collision. 
  • Force in collision depends on change in momentum 
  • F = mv-mu ÷ t 
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