Forces

Forces can

  • Change the shape of an object (deform)

  • Make an object accelerate

  • (Forces are not required to keep things moving)

Forces

  • Gravitational force (weight)

  • Normal contact force

  • Upthrust

  • Drag

  • Tension

  • Friction

  • Magnetic force

  • Electrostatic force

  • Lift

  • Nuclear force

Force diagram rule

  • Use a pencil

  • Start all forces in the middle of the object

  • Length of arrow = the size of the force = direction of the force

  • Label the forces

Newton’s first law

  • If an object is moving at a constant velocity (or stationary), there is no unbalanced force (resultant force).

Resultant force

  • A total force acting on an object after all the forces are added up.

  • When two forces are acting in opposite direction the forces are subtracted from each other.

Newton’s second law

  • If there is a resultant force acting on an object, the object will accelerate in the direction of the resultant force, with acceleration proportional to the resultant force.

  • F=ma

    • F = Resultant force [N]

    • m = mass [kg]

    • a = acceleration [m/s2]

  • When the force increases the acceleration increases.

Friction

  • It is a force which opposes motion. It arises when 2 solid objects are in close contact. It is caused by surfaces that have bumps at a microscopic level.

  • Static - is when an object is stationary and a force is applied

  • Dynamic - when an object is moving

  • Static friction is larger than dynamic friction

What does it depend on?

  • Smooth surface - less friction

  • Lubricant - reduces friction

  • Larger weight - larger friction

  • Surface area - does not matter

Useful

  • When stopping or turning in a car.

  • When rubbing hands to keep warm.

  • Matches to start a fire.

  • When sanding wood.

Nuisance

  • Wears things out so they need to be replaced (engines, tires).

  • When driving straight it leads to petrol wastage.

  • Friction/carpet burns

Mass: measured in kg

  • Related to how many and what particles something is made out of.

  • Stays constant, unless more matter is added, or it is split

  • Related to inertia - which is a measure of how difficult it is to change its motion.

Weight: measured in N

  • It is gravitational force, when an object experiences gravitational force on a planet we call it weight.

  • Its size depends on the mass of the planet that it is on.

W = m x g

  • W = Weight [N]

  • m = mass [kg]

  • g = gravitational field strength (on earth it is 10)