Forces

• A vector has magnitude and direction
• A scalar has just magnitude.
• Generally, scalars cannot be negative, but vectors can be, as a certain direction is positive.
• Speed is a scalar
• Velocity is a vector
• Distance is scalar
• Displacement is vector
• Time is scalar
• Acceleration is vector
• Force is vector
• Mass is scalar
• Momentum is vector
• Energy is scalar
• Imagine a car travelling round a roundabout at constant sped. While its speed is constant, its direction is constantly changing - so its velocity is constantly changing therefore it is accelerating.
• Vectors can be represented by arrows, with their size/length representing the vector magnitude.
• A force is a push or pull that acts on an object due to the interaction with another object. All forces between objects are either contact or non-contact forces.
• Non-contact forces are forces that do not involve physical contact between two objects:

  • Electrostatic - the charges cause a force of attraction/repulsion.
  • Gravitational attraction - the mass creates a force of attraction.
• Contact forces are forces that act between two objects that are in contact with each other:

  • Normal contact force - the force is normal to the planes of contact.
  • Friction - the surfaces and their roughness cause friction when moved in contact.
• All matter has a gravitational field, and attracts all other matter. The larger the mass, the stronger the field, the greater the attraction.
• The force exerted on a mass by the gravitational field, in Newtons: weight = mass x gravitational field strength.
• Weight is measured by a force meter (calibrated spring-balance), weighing scales measures the force you exert, and then divided by 10 to give mass.
• Gravitational field strength on Earth: 9.8
• The weight of an object is considered to act as the object's centre mass.
• Resultant Force:

  • The is a single force representing the sum of all the forces acting on an object.
  • If more than one force act along a straight line, the resultant can be found by adding or subtracting them.
• Free body diagrams shows the forces acting on an object and the direction of the forces.
• Work Done: The work done by a force is the product of the force and the distance moved.
• Work Done = Force x Distance
• Work done against frictional forces causes a rise in temperature of the object.
• Springs:

  • To stretch, bend or compress an object, more than one forces has to be applied.
  • If a single force is applied to an object, it will just move in that direction - if it is pulled in opposite directions on either side of the object, it will stretch. If it is fixed at one point and stretched, a force is still being applied by the fixed point.
• Deformation: The change in shape of a material due to stress.
• Elastic deformation: A material that can be stretched or compressed without breaking.
• Plastic deformation: A type of deformation in which the material is permanently deformed but returns to its original shape after the stress is removed.
• Hooke's Law: The extension of an object is directly proportional to the force applied to it, provided that the limit of proportionality is not exceeded.
• Force = spring constant x extension of spring (F=kx)
• The point a line stops being linear on a Force/Extension graph, it is the limit of proportionality and then does not obey Hooke's Law.
• If a graph is just linear, with no non-linear end section, the material is brittle, so snaps instead of stretches after the elastic limit.
• Work Done = 1/2 x spring constant x spring extension^2
• When a force stretches/compresses a spring, the spring does work.

  • Elastic potential energy is stored in the spring.
  • Provided it does not inelastically deform: The work done on the spring = the elastic potential energy stored.
• Pressure: The force per unit area exerted by a fluid on a surface. Pressure = force divided by area.
• Distance is how far an object moves. Distance does not involve direction. Distance is a scalar quantity.