2. Forces in Action

What do mass, weight + newton mean?

• Mass - amount of stuff/matter in an object

• Weight - force experienced by an object due to a gravitational field

• Newton - the force that will give a mass of 1kg an acceleration of 1ms^-2

What is the equation for weight?

W = mg (weight = mass x gravitational field strength)

What does centre of mass mean?

The single point that you can consider an object’s whole weight to act through

How to find the centre of mass of an object?

1. Hang the object from a single point on a nail attached to a clamp stand

2. Put a plumb line on the nail

3. Draw the line of the plumb line

4. Repeat for point on the object

5. The point where the two lines intersect is the COM

What does reaction force mean?

Reaction / normal contact force - equal + opposite force exerted by the surface on the object, perpendicular to the surface

What is the equation for resultant force?

F = ma (resultant force = mass x acceleration)

How to check if an object is in equilbrium using a diagram + equations?

• Diagram - if forces form a closed shape when drawn tip-to-tail, the object is in equilbrium

• Equations - forces cancel each other (resultant force is 0)

What does moment mean + equation?

• Moment - turning effect of a force around a turning point

• M = Fs (Moment = force x perpendicular distance)

• M = Fs sinθ

What is the principle of moments?

The sum of the clockwise moments = the sum of the anticlockwise moments about the same point

When will an object topple over?

If its COM falls outside of its base area

How to find forces when multiple unknown?

Put the pivot at one of the forces to remove its effect

What is a couple + torque?

• Couple - equal + opposite forces on either side of the pivot

• Torque - rotational effect produce by the couple (but no movement)

What is friction + drag?

• Friction - force that opposes motion (between solid surfaces)

• Drag / air resistance - friction in fluids

What four factors affect drag?

• Fluid density

• Object’s shape

• Object’s cross-sectional area

• Object’s velocity

What does terminal velocity mean?

Terminal velocity - frictional forces = driving force → no resultant force → max velocity reached

What does density mean + equation?

• Density - compactness of a substance

• 𝜌 = m / v (density = mass / volume)

What does pressure mean + equation?

• Pressure - amount of force applied per unit area

• P = F / A (pressure = force / area)

What is the equation for pressure in a fluid?

P = 𝜌gh (pressure = density x gravitational field strength x height)

What determines if an object floats or sinks?

• Floats → object is less dense than fluid

• Sinks → object is more dense than fluid

What is Archimedes’ principle?

• Upthrust on an object = the weight of fluid displaced by the object

• U = 𝜌_fVg (Upthrust = density of fluid x volume x gravitational field strength)