AQA Physics 8463 - Section 4.5: Forces Vocabulary Flashcards

Force

A push or pull caused by an interaction between objects.

Unit of Force

Newton, $N$.

Contact Force

A force that acts when objects are physically touching.

Examples of Contact Forces

Friction, air resistance, tension and normal contact force.

Non-contact Force

A force that acts between objects that are not touching.

Examples of Non-contact Forces

Gravitational, electrostatic and magnetic forces.

Scalar Quantity

A quantity with magnitude only.

Vector Quantity

A quantity with both magnitude and direction.

Force Classification (Scalar or Vector)

A vector quantity.

Resultant Force

The single force that has the same effect as all the forces acting together.

Balanced Forces

Forces acting on an object such that the resultant force is zero.

Effect of Non-zero Resultant Force

The object's velocity can change; it accelerates in the direction of the resultant force.

Free-body Diagram Force Representation

Represented by arrows showing their direction and relative size.

Length of a Force Arrow

Represents the magnitude of the force.

Weight

The force acting on an object due to gravity.

Weight Equation

$$W = m g$$

Unit of Mass

Kilogram, $kg$.

Unit of Gravitational Field Strength

Newton per kilogram, $N/kg$.

Earth's Gravitational Field Strength

About $9.8\,N/kg$ near Earth's surface, often approximated as $10\,N/kg$ when appropriate.

Weight Measuring Instruments

A calibrated spring balance or newton meter.

Mass

The quantity of matter in an object; it is measured in kilograms.

Centre of Mass

The point at which the mass of an object can be considered to be concentrated.

Work Done

Energy transferred when a force moves an object through a distance.

Work Done Equation

$$W = F s$$

Variable s in Work Done Equation

Distance moved along the line of action of the force, in metres.

Unit of Work Done

Joule, $J$.

Relationship Between Joules and Newton-metres

$$1\,J = 1\,N\,m$$

Work Done Against Friction

Energy is transferred to thermal energy stores of the objects and surroundings.

Elastic Deformation

A change of shape that reverses when the force is removed.

Inelastic Deformation

A change of shape that does not fully reverse when the force is removed.

Extension

The increase in length of an object from its original length.

Hooke's Law Equation

$$F = k e$$

Unit of Spring Constant

Newton per metre, $N/m$.

Limit of Proportionality

The point beyond which force is no longer directly proportional to extension.

Straight-line Force-Extension Graph

A graph through the origin showing force is directly proportional to extension.

Elastic Potential Energy Equation

$E_e = \frac{1}{2} k e^2$, when the spring behaves elastically.

Required Practical 6 Overview

Investigates the relationship between force and extension for a spring.

Required Practical 6 Measurements

The applied force and the corresponding extension.

Elastic Limit Caution in Practical 6

Loading a spring beyond its elastic limit may permanently deform it so it no longer returns to its original length.

Moment of a Force

The turning effect of a force about a pivot.

Moment Equation

$$\text{moment} = \text{force} \times \text{perpendicular distance from pivot}$$

Unit of Moment

Newton metre, $N\,m$.

Perpendicular Distance in Moment Equation

Only the perpendicular distance from the pivot to the force's line of action determines the turning effect.

Principle of Moments

For an object in equilibrium, total clockwise moment equals total anticlockwise moment.

Lever

A simple machine that uses a force acting at a distance from a pivot to produce a turning effect.

Increasing Lever Force Effectiveness

Increasing the perpendicular distance from the pivot increases the moment.

Gear

A toothed wheel that transfers rotational motion and force.

Meshed Gears Rotation

Meshed gears rotate in opposite directions relative to each other.

Gear Turning Effect Modification

Different gear sizes can change the rotational speed and moment.

Pressure (Physics-only)

Force acting per unit area.

Pressure Equation (Physics-only)

$$p = \frac{F}{A}$$

Unit of Pressure (Physics-only)

Pascal, $Pa$.

One Pascal Definition (Physics-only)

1 newton per square metre, $1\,N/m^2$.

Increasing Pressure for Same Force (Physics-only)

Decrease the area over which the force acts.

Cause of Liquid Pressure (Physics-only)

The weight of liquid above a point produces pressure.

Liquid Pressure and Depth (Physics-only)

Liquid pressure increases with depth.

Column Liquid Pressure Equation (Physics-only)

$$p = h \rho g$$

Variable h in Liquid Pressure Equation (Physics-only)

Height of the liquid column, in metres.

Variable rho in Liquid Pressure Equation (Physics-only)

Density of the liquid, in $kg/m^3$.

Upthrust (Physics-only)

The upward resultant force exerted by a fluid on an immersed object.

Cause of Upthrust (Physics-only)

Fluid pressure is greater at greater depth, so the force on the bottom of an object is greater than the force on the top.

Object Floating Condition (Physics-only)

When its weight is balanced by the upthrust.

Atmospheric Pressure (Physics-only)

Pressure caused by collisions of air molecules with surfaces.

Atmospheric Pressure and Altitude (Physics-only)

Atmospheric pressure decreases as altitude increases.

Cause of Atmospheric Pressure Decrease with Altitude (Physics-only)

There are fewer air molecules above and the air is less dense.

Distance

How far an object moves, regardless of direction.

Displacement

The distance moved in a particular direction from the starting point.

Distance Classification (Scalar or Vector)

Scalar.

Displacement Classification (Scalar or Vector)

Vector.

Speed

Distance travelled per unit time.

Speed Equation

$$\text{speed} = \frac{\text{distance}}{\text{time}}$$

Unit of Speed

Metres per second, $m/s$.

Velocity

Speed in a given direction.

Difference Between Speed and Velocity

Velocity includes direction; speed does not.

Acceleration

The rate of change of velocity.

Acceleration Equation

$$a = \frac{v - u}{t}$$

Unit of Acceleration

Metres per second squared, $m/s^2$.

Deceleration

Acceleration that reduces speed; it may be described as negative acceleration depending on the chosen direction.