P5 - Forces

P5 - Forces

1. Scalar and Vector Quantities

   • Scalar quantities only have a magnitude or size.
   • Vector quantities have both magnitude and an associated direction.
   • The resultant force is a single force that has the same effect as two or more forces acting together.

2. Contact and non-contact forces

   • Contact forces are forces that act between two objects that are physically touching each other.
     • An object at rest on a surface experiences reaction force. For example, a book on a table.
     • An object that is being stretched experiences a tension force. For example, a cable holding a ceiling lamp.
     • Two objects sliding past each other experience friction forces. For example, a box sliding down a slope.
     • An object moving through the air experiences air resistance. For example, a skydiver falling through the air.
   • Non-contact forces are forces that act between two objects that are not physically touching each other.
     • A magnetic force is experienced by any magnetic material in a magnetic field.
     • An electrostatic force is experienced by any charged particle in an electric field.
     • A gravitational force is experienced by any mass in a gravitational field.

3. Gravity

   • All objects with mass produce a gravitational field. The more mass an object has, the greater its gravitational field will be.
   • Weight is the force acting on an object due to gravity - it has the unit newtons (N) and acts towards the centre of a gravitational field.
   • The weight of an object can be measured using a calibrated spring-balance, often called a Newton meter.
   • weight = mass × gravitational field strength weight is newtons, mass is kilograms, gravitational field strength is newtons per kilogram (N/kg)
   • Work is done when energy is transferred from one store to another. Work is also done when a force causes an object to move.
   • work done = force × distance, work done is joules, force is newtons, distance is meters

4. Forces and elasticity

   • When a force acts on an object, the object may change shape. owever, there must be more than one force acting to change the shape of a stationary object A change in shape is called deformation:
     • elastic deformation is reversed when the force is removed
     • inelastic deformation is not fully reversed when the force is removed - there is a permanent change in shape
   • Extension happens when an object increases in length, and compression happens when it decreases in length. The extension of an elastic object, such as a spring, is described by Hooke's law
     • force = spring constant × extension force (F) is measured in newtons (N) spring constant (k) is measured in newtons per metre (N/m) extension (e), is measured in metres (m)
   • limit of proportionality refers to the elastic limit of a material that is at the furthest point it can be stretched or deformed while being able to return to its previous shape.
   • Work is done when a spring is extended or compressed. Elastic potential energy is stored in the spring.
     • elastic potential energy = 0.5 × spring constant × (extension)2 elastic potential energy (Ee) is measured in joules (J) spring constant (k) is measured in newtons per metre (N/m) extension (e), is measured in metres (m)

5. Describing Motion