What is a force? (2)

An interaction between two objects involving a push or pull

A vector with direction in space

What are the four fundamental forces? (4)

Gravitational

Electromagnetic

Strong nuclear

Weak nuclear

What force acts on an object in water, allowing it to float?

Upthrust

What force acts on an object when it is pulled by a string?

Tension

What force acts between a surface and an object when they are in contact with each other?

Normal contact force

What is friction?

Particles colliding with a moving object, causing kinetic energy to be lost as thermal energy

What is a free body diagram?

A diagram using a dot to represent an object and arrows to scale to show the direction and magnitude of the forces acting on it

What is a plumb line?

A mass suspended by a length of string, allowing straight, vertical lines to be drawn

How is the centre of mass of a 2D object found? (4)

Choose a point on the object and allow it to swing freely from a pin before coming to rest

Using a plumb line, draw a vertical line through the pin

Repeat this twice more, creating a point where all three lines cross

The point where they cross is the centre of mass

What is drag?

The frictional force experienced by an object travelling through a fluid

What factors affect drag? (5)

Speed of the object Cross-sectional area Shape of the object Roughness/texture of the object Density of the fluid that is travelled through

How is drag force and speed linked?

Drag force is proportional to speed^2

What are the forces on an object in the instant that it falls? (2)

No drag

Weight (acceleration = g)

What happens as a falling object’s speed increases? (2)

Drag force increases

Instantaneous acceleration decreases (less than g)

What happens when an object reaches terminal velocity? (2)

Drag is equal and opposite to weight

Acceleration is zero so speed is constant

What is the moment of a force?

The turning effect of a force about a pivot

Moment = force x perpendicular distance from pivot

What is a couple?

A pair of equal but opposite forces acting parallel and on different lines

They cause a turning effect but no translational movement

What is the torque of a couple? (2)

The moment of a couple

Torque = one of the forces x perpendicular distance between forces

What is the principle of moments? (2)

When a body is in equilibrium, the net force acting on it is zero and its net moment is zero

The sum of clockwise moments = the sum of anti-clockwise moments

What is the centre of gravity?

The point at which the entire weight of the object can be said to act

What is the difference between a torque and a moment?

A moment can have a non-zero resultant force, thereby causing movement whereas a torque always has a resultant force of zero

What is density? (2)

Mass per unit volume

Density = mass / volume

What is pressure? (2)

Force per unit area (acting perpendicular to the surface)

Pressure = force / area

What is the difference between solid and liquid pressure? (2)

Solid pressure - the smaller the area, the higher the pressure

Liquid pressure - in a fluid at rest, pressure at any point acts equally in all directions and increases with depth

What is Archimedes’ Principle?

The upthrust that a floating or submerged object feels is equal to the weight of displaced fluid

How is the upthrust on an object in a fluid calculated?

Pressure = height of liquid above x density of liquid x acceleration due to gravity

How is the density of an object calculated? (2)

Use a Eureka can - volume of water displaced = volume of object

Use a balance to find the mass of the object