P2a Forces in Action

All Formulas:

  • Density = Mass / Volume

  • P1V1 = P2V2

  • Energy = Mass x Specific heat capacity x Temperature change

  • Energy = Mass x Specific latent heat

  • Energy transferred in stretching = ½ x spring constant x extension²

  • Moment = Force x Perpendicular distance from the pivot (fulcrum)

  • Mechanical advantage = Load / Effort

  • Mechanical Advantage = Diameter of a driven gear / Diameter of driver gear

  • Mechanical Advantage = Number of teeth on driven gear / Number of teeth on the driver gear

  • Pressure = Force / Area

  • Pressure = Height x Density x g

  • Force exerted by a spring = spring constant x extension

  • Weight = Mass x g

g = gravitational field strength on Earth is 10N/kg

Contact Forces:

Friction - Occurs when rough surfaces slide over each other due to the atoms in each surface interacting

Normal contact force - A force that is exerted by a solid surface on an object

Push - A force which acts to move an object away from something

Upthrust - The force on an object in a fluid due to the difference in pressure over the area of the object

Non-Contact Forces:

Magnetism - Force on a magnetic object in a magnetic field. For two magnets, same poles = repel. Opposite poles = attract

Gravitational Force (weight) - Force on a mass in a gravitational field

Electrostatic - Force on a charged object in an electric field. Same charges = repel. Opposite charges = attract

Newton’s First Law of Motion: An object will remain at rest, or move at a constant speed in a straight line, unless acted on by a resultant force

If an object does have a resultant force acting on it will change its motion- it can accelerate, decelerate or change direction.

Hooke’s Law: The extension of a spring is directly proportional to the force applied

The Principle of Moments: For an object to balance, the sum of the clockwise moments must be equal to the sum of the anticlockwise moments about a pivot

Archimedes Principle: The upthrust exerted on a body immersed in a fluid, is equal to the weight of the fluid that the body displaces

Keywords:

Effort - The force you apply to move or lift the object

Elastic deformation - A change in shape or length where the material returns to its original shape or length when the stretching force is removed

Free Body Diagram - A diagram that shows all the forces acting on an object

Fulcrum - Another word for a pivot point

Gear - Wheels with toothed edges that rotate on an axe. The teeth of one gear fit into the teeth of another gear

Lever - An object that won't bend and has a pivot. A force at one end produces a force in the opposite direction at the other

Linear - A straight line graph

Load - The force or object that you are trying to move

Moment - The turning effect of a force around a pivot point

Plastic deformation - A permanent change in shape where the material remains distorted after the force is removed. This happens when the force applied goes beyond the material’s elastic limit

Pressure - The force per unit area acting perpendicular to a surface

Spring Constant - The ratio of force extension. A measure of the stiffness of a material

Upthrust - The upward force on an object in a fluid caused by differences in pressure acting on different parts of the object

Resultant Force - The overall force you get when you combine all the forces acting on an object

Brittle Materials - Materials that fracture or shatter easily under tension without significant deformation

Ductile Materials - Materials that are capable of undergoing significant plastic deformation before fracture

Levers- A small force applied at a large distance from the pivot will create a large turning effect (a large moment)

Examples of levers: Crowbar, floorboards, taps, opening drink bottles

Examples of Brittle Materials: Glass, graphite, concrete

Examples of Ductile Materials: Chewing gum, copper, steel, iron, silver, zinc, aluminium

In hydraulics the particles in fluids move randomly and in all directions. The pressure is created due to collisions with the walls of a container. Fluids trapped in containers create a net force at right angles to all surfaces.

Key point: The pressure in a trapped liquid will be the same everywhere.

Liquid is used in hydraulic systems because it cannot be compressed.