Forces

• A vector has magnitude and direction
• A scalar just has magnitude
• Scalars cannot be negative, but vectors can be, as a certain direction is positive
• Scalar examples


  Speed, distance, time, mass and energy.
• Vector examples


  Velocity, displacement, acceleration, force and momentum
• Imagine a car travelling round a roundabout at constant speed. While its speed is constant, its direction is constantly changing - so its velocity is constantly changing therefore it is accelerating.
• Imagine a ball thrown off a cliff, displacement is 0 at height of cliff, above the cliff the ball has positive displacement, and below the clifftop the ball has negative displacement.
• Vectors can be represented by arrows, with their size/length representing the vector magnitude
• A force is a push or pull on an object that causes it to move or change its shape, due to the interaction of another object
• 2 forces of an object 


  Contact and non-contact
• Non-contact force


  • The objects are physically separated e.g. electrostatic (the charges cause a force of attraction/repulsion) and gravitational (the mass creates a force of attraction)
• Contact force


  • The objects are physically touching e.g. 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 (W) in newtons (N) and mass (M) in kilograms (Kg)
• Weight is measured by a force meter (also known as calibrated spring balance)
• Weighing scale measures the force you exert, and then divides by 10 to give mass
• Gravity on earth = 9.8
• On two different planets a persons mass is the same
• On two different planets the

  gravitational field strength, g, at the two planets will be different so their weight will be different on both.
• Acceleration in free fall is due to gravity, and is the same as g.
• The weight of an object is considered to act at the object's centre of mass
• Resultant force is a single force representing the sum of all the forces acting on an object
• If more than on force acts along a straight line, the resultant can be found by adding or subtracting them
• Work done is when energy is transferred from the object doing the work to another form
• One joule of work is done when a force of one newton causes a displacement of 1 metre.
• Work done against frictional forces causes a rise in temperature of the object
• To stretch, bend or compress an object, more than one force has to be applied
• If a single force is applied to an object, it will just move in that direction
• If something is pulled in opposite directions on either side, it will stretch
• If something is fixed at one point and stretched, a force is still being applied by the fixed point
• Deformation = changing shape
• Elastic deformation:

  • The object returns to its original shape when the load has been removed
  • Elastic band
• Plastic deformation:

  • The object does not return to its original shape when the load has been removed.
  • A spring when pulled too far.
• Hooke's law - The extension of an elastic object, such as a spring, is directly proportional to the force applied, provide that the limit of proportionality is not exceeded
• Linear line for force / extension graph:

  • This is elastic region
  • It is following Hooke's Law
  • Gradient is k
• The point the line for a force / extension graph stops being linear is the limit of proportionality:

  • From then on, it does not obey Hooke's Law
• Non linear line for force / extension graph:

  • There is plastic behaviour here
  • It is not following Hooke's Law
  • If shallow = lots of extension for not a lot of force, easy to stretch
• If a force / extension graph is just linear with no non-linear section, the material is brittle, so snaps instead of stretches after the elastic limit