A.2 Forces and momentum - Key terms

interaction

any event in which two or more objects exert forces on each other

newton, N

derived SI unit of force, 1 N = 1 kgms⁻²

mass

a measure of an object’s resistance to a change of motion (inertia)

kilogramme, kg

SI unit of mass (fundamental)

weight, F

gravitational force acting on a mass, F = mg

gravitational field strength, g

the gravitational force per unit mass (that would be experienced by a small test mass placed at that point), g = F/m (SI unit: Nkg⁻¹), numerically equal to the acceleration due to gravity

centre of mass

average position of all the mass of an object, the mass of an object is distributed evenly either side of any plane through its centre of mass

force meter

instrument used to measure forces, also sometimes called a newton meter or a spring balance

calibrate

put numbered divisons on a scale

weigh

determine the weight of an object, in everyday use the word ‘weighing’ usually means quoting the result as the equivalent mass: ‘my weight is 60 kg’ actually means I have the weight of a 60 kg mass (about 590 N)

peer review

evaluation of scientific work by experts in the same field of study

tension (force)

force that tries to stretch an object or material

compression (force)

force that tries to squash an object or material

deformation

change of shape

normal

perpendicular to a surface

buoyancy force

vertical upwards force on an object placed in or on a fluid, sometimes called upthrust

density

mass/volume

Archimedes’ principle

when an object is wholly or partially immersed in a fluid, it experiences buoyancy force equal to the weight of the fluid displaced

elastic behaviour

a material regains its original shape after a force causing deformation has been removed

elastic limit

the maximum force and/or extension that a material, or spring, can sustain before it becomes permanently deformed

extension

displacement of the end of an object that is being stretched

spring constant, k

the constant seen in Hooke’s law that represents the stiffness of a spring (or other material)

restoring force

force acting in the opposite direction to displacement, returning an object to its equilibrium position

interpolate

estimate a value within a known data range

extrapolate

predict behaviour that is outside of the range of available data

friction

resistive forces opposing relative motion, occurs between solid surfaces, but also with fluids; static friction prevents movement, whereas dynamic friction occurs when there is already motion

coefficient of friction, μ

constants used to represent the amount of friction between two different surfaces, maybe static or dynamic

constant

a number which is assumed to have the same numerical value under a specified range of circumstances

fundamental constants

numbers which are assumed to have exactly the same numerical values under all circumstances and all times

coefficient

a multiplying constant placed before a variable, indicating a physical property

viscosity

resistance of a fluid to movement

viscous drag

the drag force acting on a moving object due to the viscosity of the fluid through which it is moving

turbulence

flow of a fluid which is erratic and unpredictable

Stoke’s law

equation for the viscous drag acting on a smooth, spherical object undergoing non-turbulent motion

streamlined

having a shape that reduces the drag forces acting on an object that is moving through a fluid

field (gravitational, electric or magnetic)

a region of space in which a mass (or a charge, or a current) experiences a force due to the presence of one or more other masses (charges, or currents - moving charges)

free-body diagram

diagram showing all the forces acting on a single object, and no other forces

point particle, mass or charge

theoretical concept used to simplify the discussion of forces acting on objects (especially in gravitational and electric fields)

resultant force

the vector sum of the forces acting on an object, sometimes called the unbalanced or net force

resultant

the single vector that has the same effect as the combination of two or more separate vectors

components (of a vector)

any single vector can be considered as having the same effect as two parts (components) perpendicular to each other

inclined plane

flat surface at an angle to the horizontal (but not perpendicular), a simple device that can be used to reduce the force needed to raise a load; sometimes called a ramp

Newton’s laws of motion

first law: an object will remain at rest, or continue to move in a straight line at a constant speed, unless a resultant force acts on it;

second law: acceleration is proportional to resultant force;

third law: whenever one body exerts a force on another body, the second body exerts exactly the same force on the first body, but in the opposite direction

balanced forces

if an object is in mechanical equilibrium, we describe the forces acting on it as ‘balanced’

equilibrium

an object is in equilibrium if it is unchanging under the action of two or more influences (e.g. forces), different types of equilibrium include translational, rotational and thermal

translational

changing position

natural philosophy

the name used to describe the (philosophical) study of nature and the universe before modern science

proportional relationship

two variables are (directly) proportional to each other if they always have the same ratio

uncertainty bars

vertical and horizontal lines drawn through data points on a graph to represent the uncertainties in the two values

inertia

resistance to a change of motion, depends on the mass of the object

momentum (linear), p

mass times velocity, a vector quantity

system

the object(s) being considered (and nothing else), an isolated system describes a system into which matter and energy cannot flow in or out

surroundings

everything apart from the system that is being considered; similar to the ‘environment’

scientific notation

every number is expressed in the following form: a * 10ᵇ, where a is a decimal number larger than 1 and less than 10 and b is an exponent (integer)

signifcant figures (digits)

all the digits used in data to carry meaning, whether they are before or after a decimal point

collision

two (or more) objects coming together and exerting forces on each other for a relatively short time

explosion

term used in physics to describe when two or more masses, which were initially at rest, are propelled apart from each other

collisions

in an elastic collision the total kinetic energy before and after the collision is the same, in any inelastic collision the total kinetic energy is reduced after the collision, if the objects stick together it is described as a totally inelastic collision

macroscopic

can be observed without the need for a microscope

microscopic

describes anything that is too small to be seen with the unaided eye

recoil

when a bullet is fired from a gun (or similar), the gun must gain equal momentum in the opposite direction

propel

provide a force for an intended motion

jet engine

an engine that achieves propulsion by emitting a fast-moving stream of gas or liquid in the opposite direction from the intended motion

rocket engine

similar to a jet engine, but there is no air intake, instead, an oxidant is carried on the vehicle, together with the fuel

impulse

the product of force and the time for which the force acts

centripetal force

the name given to any force which results in motion along a circular path

banked track

a sloping surface to enable faster motion around curves

centripetal acceleration

the constantly changing velocity of any object moving along a circular path is equivalent to an acceleration towards the centre of the circle

radians

unit of measurement of angle, there are 2π radians in 360°

time period, T

the duration of an event which occurs regularly, T = 1/f

frequency, f

the number of repeating events per unit time

hertz, Hz

derived SI unit of measurement of frequency, 1 Hz = one event per second

angular velocity, ω

change of angle/change of time, sometimes called angular speed

derive

explain in detail the origin of an equation