Forces - GCSE Physics AQA

What is a force?
How are they measured
Quantity

A push or pull that acts on an object due to its interaction with another object (Newtons)

Vector Quantity

What is the difference between a vector quantity and a scalar quantity?

Vector quantities have magnitude and an associated direction whereas Scalar quantities have magnitude only

Examples of Vector and Scalar quantities

Scalar - Distance, mass, temperature, time
Vectors - Velocity, Displacement, Acceleration, Force, Momentum

What are the 2 categories of forces?

Contact Forces - The objects are physically touching
Non-contact forces - The objects are physically separated

Examples of Contact and Non-contact forces

Contact forces: Friction, Air resistance, tension and normal contact force
Non-contact forces - gravitational force, electrostatic force and magnetic force

What is a normal contact force?
Example

A rock on a table has mass therefore weight is being exerted downwards.
The table exerts an equal and opposite force. This is the normal contact force

How are vector quantities represented?

Arrow - The length represents the magnitude and the direction of arrow represents direction

What is a resultant force?

The overall force of the object
Some force cancel each other out

What is a balanced force/equilibrium of an object?

When there is no resultant force acting upon it

What is weight and what does it depend on?

Weight is the force acting on an object due to gravity
The weight of an object depends on the gravitational field strength at the point where the object is

What is deformation?
2 Types

When an object changes shape it is deformed
Elastic deformation and Inelastic deformation

Difference between Elastic and Inelastic deformation

Elastic deformation is when an object returns to its original shape after forces are removed whereas inelastic deformation is when an object stays deformed after the forces are removed

What is extension?
Spring with mass added onto it

Increase in length of a spring when stretched
As force is increased on the spring (in the form of extra mass added), the extension increases proportionally
F∝e

What does an object’s extension depend on?

Spring constant (k) - How many newtons it would take to stretch the object by 1 Metre

Equation linking force, extension and spring constant

F = k e
(N) (N/m) (m)

Describe a force-extension Graph

As the force increase, so does the extension
Straight line that passes through the origin so force and extension are directly proportional
All elastic deformation

What happens when force-extension line starts to curve?

Object has reached its elastic limit/limit of proportionality
Hooke’s law no longer applies
It will have inelastically deformed

Force-extension graph information before it curves

Gradient of line = Spring constant (k)

Area under curve/line = Energy transferred to spring (EPE)

What is elastic potential energy?

Energy transferred to an object as it is stretched

Elastic potential energy equation

EPE = ½ k e²

(J) (N/m) (m)

What is a moment?
Example

The rotational or turning effect of a force
Applying a force down at the end of a spanner - it will turn around the central point (pivot) - turning effect is the moment

Moment Equation
2 moments on one object

M - F d
Moment (Nm) = Force (N) x Perpendicular distance between pivot and place where force is being applied (m)
Clockwise or anticlockwise
If two moments are on one object, the leftover moment is the overall moment

What is the perpendicular distance?

Perpendicular distance from the line of action of the force to the axis of rotation

What do levers do?
Input and output forces and where they are in relation to the pivot

Transmit the turning effect of a force

If the input and output forces are on different sides of the pivot, they act in different directions

If the forces are on the same side of the pivot, they act in the same direction

Output force is closer to the pivot, the force will be larger

What do gears do?

Example

Transmit turning effects

Example:

Gear A (engine) and Gear B (wheels)
Turning effect of engine - rotates gear a - rotates gear b - wheels rotate
Gears turn in opposite directions
Gear B radius is 2x larger than Gear A so turning effect is 2x bigger
Turning effect has been doubles
Gear A has to rotate 2x as much as Gear B
Work done remains the same

What is pressure?
Formula

Force per unit of Area
P = F/A

Pressure (Pa) = Force (N) / Area (m)

Fluid pressure

Particles bang into surroundings and generate pressure
Collisions of particles with wall creates pressure (applying force to inside area of wall)

What is perpendicular force?

Component of the force which is at right angles to the surface its colliding with

Example:

particle at perfect right angle will exert entire force in comparison to particle at an angle

What is pressure acting on an object in water due to? 4 things

Surrounding Water molecules constantly colliding with object
Weight of the water above the object - the downward force of it
Density of the Liquid - denser, the larger the mass per volume therefore the larger the weight
Gravitational field strength - determines weight for a given mass

Pressure in a liquid equation

P = hpg
Pressure in a liquid (Pa) = Depth (m) x density of liquid (kg/m3) x gravitational field strength (N/kg)

What is upthrust?

Water exerts force on an object in water
The bottom of an object is deeper therefore will experience a larger upwards force that downwards force.
Therefore resultant force upwards

What causes an object to not float?

An object’s weight may be larger than upward force therefore the object will sink
If an object is more dense, it will sink

Where is pressure highest in the earth’s atmosphere?

The density of the gas particles is highest near the Earth’s surface therefore pressure is highest