Force & Acceleration (OCR A Level Physics)

Force & Acceleration

• When a resultant force, F, acts on a body with mass, m, the body will speed up with acceleration, a

• The resultant force is related to mass and acceleration by the formula:

• This relationship means that objects will accelerate if there is a resultant force acting upon them

• An unbalanced force on a body means it experiences a resultant force

  • If the resultant force is along the direction of motion, it will speed up (accelerate) or slow down (decelerate) the body

  • If the resultant force is at an angle, it will change the direction of the body

The Newton

• The Newton, N, is commonly used to measure force

• The SI unit for force is kg m s^–2

• One Newton is defined as:

  The force that will give a mass of 1 kg an acceleration of 1 m s^–2

Resultant Force

• Since force is a vector, every force on a body has a magnitude and direction

• The resultant force is, therefore, the vector sum of all the forces acting on the body

• The direction is given by either the positive or negative direction as shown in the examples below

Resultant forces on a body can be positive or negative depending on their direction

• The resultant force could also be at an angle, in which case addition of vectors with calculation or scale drawing is used to find the magnitude and direction of the resultant force

Acceleration

• Since acceleration is a vector, it can be either positive or negative depending on the direction of the resultant force

  • If the resultant force is in the same direction as the motion of an object, the acceleration is positive

  • If the resultant force is in the opposite direction to the motion of an object, the acceleration is negative

• An object may continue in the same direction however with a resultant force in the opposite direction to its motion, it will slow down and eventually come to a stop

• If drag forces are ignored, or severely reduced, the acceleration is independent of the mass of an object

  • This has been shown in experiments by astronauts who have dropped a feather and a hammer on the Moon from the same height

  • Both the hammer and feather drop to the Moon's surface at the same time