Force and motion

10.1 Force and acceleration

• resultant force (N) = mass (kg) x acceleration (m/s^2)
• Newton's 2nd Law of Motion says an object's acceleration is directly proportional to its resultant force and inversely proportional to its mass. Velocity increases if the resultant force is in its direction, but if the resultant force is in the opposite direction of velocity, it decreases.
• Inertia is the tendency to stay at rest or in uniform motion, a measure of how difficult it is to change the velocity.
• Inertial mass = force/acceleration

Acceleration (required practical)

• Use the Newton meter to pull the trolley along at constant force within a given distance. The acceleration will increase if force increases, but will decrease if mass increases. Double or treble the total moving mass by using double- decked or treble-decked trolleys. For safety, protect yourself from falling trolleys.

10.2 Weight and terminal velocity 

• Weight is caused by the gravitational attraction between you and the Earth, and mass depends on the quantity of matter. Gravitational field strength on Earth is 9.8N/kg - this value won't be given in the exam.
• Weight (N) = mass (kg) x gravitational field strength (N/kg)
• An object's acceleration increases as it falls down, because friction increases as it speeds up. The acceleration and resultant force then decrease. Then a constant velocity where friction will become equal and opposite to the object's weight will be reached, and there will be no acceleration - terminal velocity.

10.3 Forces and braking

• For cars travelling at a constant speed, their Rf is 0 because the driving force = resistive forces
• Braking force is needed to stop a vehicle, and it depends on the speed when the brakes are applied and mass. The greater the speed, the greater the deceleration needed to stop the car in a given distance. So the braking force must be bigger than braking force of a car w/ smalled speed. The bigger the mass, the greater the braking force needed for a given decceleration.
• Stopping distance = thinking distance (the distance travelled during the driver's reaction time) + braking distance (distance travelled when braking)
• Stopping distance will be impacted by:
  • poorly maintained tyres, icy roads or weather -- increase braking time
  • tiredness, alcohol, drugs, music,…