forces and their interactions

• scalar quantities have magnitude only
• vector quantities have magnitude and a direction
• a vector quantity may be represented by an arrow. The length of the arrow represents the magnitude, and the direction of the arrow the direction of the vector quantity.
• a force is a push or pull that acts on an object due to the interaction with another object.
• • contact forces – the objects are physically touching
• • non-contact forces – the objects are physically separated
• contact forces include friction, air resistance, tension and normal contact force
• non-contact forces are gravitational force, electrostatic force and magnetic force
• force is a vector quantity
• weight is the force acting on an object due to gravity
• the force of gravity close to the Earth is due to the gravitational field around the Earth
• the weight of an object depends on the gravitational field strength at the point where the object is
• weight =mass ×gravitational field strength


  W = m g 
• weight, W , in newtons, N
• mass, m , in kilograms, kg
• gravitational field strength, g , in newtons per kilogram, N/kg
• the weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’.
• the weight of an object and the mass of an object are directly proportional
• weight is measured using a calibrated spring-balance (a newtonmeter)
• a number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together
• this single force is called the resultant force