P2.3 - Forces in Action

• Plastic deformation is when an object is distorted and cannot revert back to its original shape when forces are removed.
• Elastic deformation is when an object is distorted but can revert back to its original shape once forces are removed.
• Hooke's law explains that force and extension have a linear relationship up to a certain point. This point is known as the limit of proportionality.
• Below an object's elastic limit, it can revert back to its original shape once a force is removed. Above this limit, and it cannot.
• Force on a spring (N) = spring constant (N/m) * extension (m)
• The gradient of a linear graph of force against extension is the spring constant.
• The spring constant tells you how stiff the spring is, or how difficult it is to to stretch.
• Energy transferred in stretching (J) = 0.5 * spring constant (N/m) * extension$^2$
• An elastic band has a non-linear relationship between a force and its extension.
• As you blow up a balloon, at first it is difficult. But as the balloon stretches more to hold more air, it becomes easier. Eventually, it becomes stiff again.

  
• A gravitational field is a region where a mass experiences an attractive force.
• Gravity force (N) = mass (kg) * gravitational field strength (N/kg)
• Gravitational field strength (or gravity constant) is a measure of force on 1 kg of mass when it is in a gravitational field due to another mass.
• Mass does not change, but the force of the mass relies on the field.
• The force between a mass in a gravitational field and the mass with the field can be bigger if:

  • The mass of one or both objects are big.
  • The distance is small.
• Weight (N) = mass (kg) * gravitational field strength (N/kg)
• The gravitational field strength of the earth is 10 N/kg.
• Resultant force (N) = mass (kg) * acceleration due to gravity (m/s$^2$)
• Gravitational potential energy is the energy needed to lift an object in a gravitational field.
• gravitational potential energy (J) = mass (kg) * height (m) * gravitational field strength (N/kg)
• A lever is a force multiplier that transmits forces by rotating around a pivot.
• The effort is the force exerted on a lever.
• The load is the force a lever exerts onto an object.
• If the pivot is close to the load, little effort is required. Pushing down on the lever produces a turning force, but your hand moves further than the load.
• The ratio of the load to the effort is known as the mechanical advantage:

  • mechanical advantage = load / effort
• Gears are like levers that rotate. A small cog can exert a force onto a larger cog but it has to move further than if the larger cog exerted the force onto the smaller cog. In this case, the larger cog would use a bigger force to move the smaller cog.
• The ratio between the diameters of cogs tells you the ratio of effort to load.
• Gears can be used to change direction in which the rotating force acts, or the speed at which it rotates.
• Inclined planes reduces the force needed to lift certain objects.

  
• Pressure produces forces at right angles to any surface.
• Pressure (Pa) = force normal on the surface (N) / surface area (m$^2$)
• Hydraulic machines use liquids to exert forces as it is almost impossible to compress them.
• A simple hydraulic machine is made of two pistons and a pipe with a liquid inside it. When one piston moves, it moves a liquid inside the pipe which pushes the other piston. A small force is required to push the other piston, this makes hydraulic machines force multipliers.