Forces

Forcea force is a pushing or pulling action which can

change the shape of an object, make a stationary object move or make a moving object

change its speed/directionMeasured in Newtons (N)

Moment of a force• the product of the force (F) and

the moment arm (d). The moment arm or lever arm is the perpendicular distance between the line of action of the force and the center of moments.

• Moment = Force x Distance or M = (F)(d)

• The Center of Moments may be the actual point about which the force causes rotation (a pivot or fulcrum)

The Law of Moments

When a body is in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the

same point

Law of Moments

Conditions for Equilibrium

I. The sum of the forces in one directions equals the sum of the forces in the opposite direction

II. The law of moments must apply

Levers

• Any device which can turn about a pivot/fulcrum

• Moved with a force called effort

Clockwise moment= anticlockwise moment

Force and Distance Multipliers

• Levers act as force multipliers, making it easier to move large loads

• Levers have a mechanical advantage (MA) and a velocity ratio (VR)

• MA = and VR =

Examples

Centres of Mass• a point representing the mean position of the

matter in a body or system• A body behaves as if its whole mass were

concentrated at this point• The centre of mass of a regularly shaped body

of the same density throughout is at its centre. In other cases it can be found by experiment

Toppling

• The position of the centre of mass of a body affects whether or not it topples over easily

• A body topples when the verticle line through its centre of mass falls outside its base

Centre of Mass

Base

Stability of a body can be increased by:

i. Lowering its centre of mass (eg: adding more weight to the bottom of the body)

ii. Increasing the area of its base

Stability• Stable Equilibrium: if a body is slightly

displaced and released, and it then returns to its previous position

• Unstable Equilibrium: when a body moves further away from its previous position when slightly displaced and released

• Neutral Equilibrium: if a body remains in its new position when displaced

Adding Forces• Scalars are values that have magnitude, but no

direction• Force is a vector: it has both magnitude and

direction. Force can be represented by a straight line (The size of the line indicates magnitude of force) with an arrow to indicate direction

Resultant Forces

• It is often useful to know the value of the single, resultant force.

• This is the sum of all of the forces acting on the object at once.

Parallelogram Law

If two forces acting at a point are represented in size and direction by the sides of a parallelogram drawn from the point, their resultant is represented in size and direction by the diagonal of the parallelogram drawn from the point

Friction• The force that opposes one surface moving over

another• Static friction: the friction that exists between a

stationary object and the surface on which it's resting

• Dynamic friction: the force that must be overcome to maintain steady motion of one body relative to another because they remain in contact

Static Friction > Dynamic Friction

Pendulums

Simple Pendulum

• A point mass suspended by a massless string from some point about which is allowed to swing back and forth in a place

• The motion of a pendulum swinging back and forth is Periodic: it takes time T to complete one oscillation

Pendulum Values

• Period(T): measured in time (s)• Frequency of oscillations (f) measured in Hertz

(Hz) OR

• Amplitude of oscillation: the maximum distance that the mass is displaced from its equilibrium position

Pendulums and Forces

• When pendulums are displaced from equilibrium, there is a restoring force (F) that moves it back.

• The Period can be calculated using an equation for simple harmonic motion:

Radioactivity

Isotopes

• Atoms of the same element with different numbers of neutrons. (ie. The same atomic number but a different atomic mass)

• Radioactive Isotopes: unstable isotopes which emit radiation

• Radioactivity results from unstable nuclei• Has an ionizing effect: Can knock electrons out of

atoms, turning them in to positively or negatively charged ions.

• This ionizing effect is used to detect radiation with a Geiger-Müller Tube

Alpha, beta and gamma rays

• A radioactive substance releases one or more of three types of radiation: alpha (α), beta (β) and gamma (γ)

Alpha (α) Rays

• Easily stopped by a thick sheet of paper, strong electric and magnetic fields

• Short range in air: cause intense ionisation due to frequent collisions with gas molecules

• Sources: He ions with a double positive charge, Americium (am-241

Beta (β) Rays

• Streams of high energy electrons• Stopped by a few millimetres of aluminium.

Deflected by medium strength electric and magnetic fields

• Range of several metres in air. Lower ionizing power than alpha rays

• Pure source: Strontium (Sr-90)

Gamma (γ) rays

• Electromagnatic radiation travelling at the speed of light

• Only stopped by very thick lead sheets, not defelected by electric and magnetic fields

• Very low ionization of gas• Source: Cobalt (Co-60)

Particle Tracks

• Detected in a Diffusion Cloud Chamber

Alpha particle tracks

Beta particle tracks

Radioactive Decay• Because they are unstable, radioactive atoms

‘decay’ in to atoms of different elements with more stable nuclei

• This decay is not affected by the purity of the chemical

• Half life: the average time for half of the atoms in a given sample to decay. Unaffected by temperature, different for every radioactive element

Decay Curve

• The activity of a sample is the average number of decaying atoms per second.

• Recording the activity over time using a GM tube allows us to plot a decay curve. We can then plot the half life

Uses of Radioactivity

• Thickness Gauge• Radiotherapy• Sterilisation• Archaeology

Resources

• http://www.phys.utk.edu/labs/simplependulum.pdf