Section A P2


Weight, Mass and Gravity

  • Gravitational force is the force if attraction between all masses.
  • On Earth, the acceleration of gravity is m/s2.
  • Mass has the same value anywhere in the universe.
  • Weight is caused by the pull of the gravitational force. 
  • Weight is measured in Newtons.
  • Mass is measured in Kilograms.

weight = mass x gravitational field strength

e.g. What is the weight, in newtons, of 5kg mass?

W = 5 x 10 = 50N

Resultant forces

Resultant force is the is the overall force on a point or object.

The overall effect of these forces will determine the motion of the object, where it will accelerate, decelerate or stay at a steady speed.

e.g. stationary teapot 

  • The force of gravity (weight) is acting downwards.        
  • Thus causes a reaction force from the surface pushing up on the object.
  • This is how it balances.
  • Without a reaction force it would accelerate downwards due to the pull of gravity.
  • The resultant force of the teapot is 0: 10N - 10N = 0N

If there is a resulant force acting on an object, then it will change it's state of rest or motion.

Forces and acceleration

An object needs force to start moving.

If the resultant force on the stationary object is zero, the object will remain stationary. If there is no resultant force on a moving object, it will carry on moving at the same velocity. If there is a non-zero resultant force, then the object will accelerate in the direction of the force.

A non-zero resulatant force produces and acceleration. 

Formula for acceleration

F = ma     or a = F/m

a= m/s2 acceleration

F = N resultant force

Reaction forces are equal and opposite.

e.g. When skate A pushes on skater B, she feels and equal and opposite force from skater B. They both feel the same sized force but in opposite directions meaning that they accelerate away from eachother.

Frictional force and terminal velocity

  • If an object has no force propelling it along it will always slow down and stop because of friction
  • Friction always acts in the opposite direction to movement.
  • To tracel at a steady speed, the driving force needs to be equal to the frictional forces.
  • When an object passes through liquid it is called drag instead of friction.

Drag (or air restistance) increases as the speed increases.

  • When falling objects first set off, the force of gravity is much more than the frictional force that's slowing them down, so they accelerate.
  • As speed increases, the friction builds up.

Terminal velocity is the same as maximum speed.

The terminal velocity of any object is determined by it's air resistance in comparison to it's weight. Friction depends on the shape and area.

Stopping distance

Total stopping distance covered is the distance covered in the time between the driver spotting the hazard and coming to a complete stop. It's thinking distance + braking distance.

Thinking distance is…


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