Topic A Definitions

Definitons for Topics A1-A5 (only ratings 4,5) Text in bold most likely important for each definition, sometimes missing extra notes

Terminal Speed

For simple case of falling from rest:

1. Initially only force is weight

2. So acceleration is 9.8m/s²

3. Then air resistance acts

4. As speed increases, air resistance increases

5. So resultant force decreases

6. So acceleration decreases

7. Until air resistance = weight

8. And there is zero resultant force on the object

9. So zero acceleration - terminal speed

Acceleration

Rate of change of velocity (can be a vector or a scalar)

Displacement (s)

Distance traveled in a particular direction (change in position) (vector)

Speed (u,v)

Rate of change of distance (scalar)

Velocity ((u,v)

Rate of change of displacement (vector)

Impulse

Change in momentum (OR area under force-time graph)

Law of Conservation of Linear Momentum

The total momentum of an isolated system (no external forces) remains constant

Linear momentum

Mass x Velocity

Elastic collision

A collision in which kinetic energy is conserved

Newton’s First Law of Motion

Objects move with constant velocity (which may be zero) if the resultant force on the object is zero (and the reverse)

Newton’s Second Law of Motion

The resultant force on a body is the rate of change of the body’s momentum

Notes: Not F=ma but F= deltap/delta t, (resultant) force = rate of change of momentum contrast with impulse = change in momentum

Newton’s Third Law of Motion

The force that A exerts on B is equal and opposite to the force that B exerts on A

Translational equilibrium

Resultant force acting on a body is zero

Inelastic collision

A collision in which kinetic energy is not conserved

Upthrust//buoyancy

An upward force exerted on a body immersed in a fluid, equal to the weight of the displaced fluid

Energy density

The energy that can be obtained per unit volume of fuel

Mechanical energy

The sum of kinetic energy, gravitational potential energy and elastic potential energies of a system (really sum of all macroscopic kinetic and potential energies)

Conservation of mechanical energy

If there is no friction AND the system is isolated, then the total mechanical energy of a system is conserved

Power

The rate at which work is done or the rate at which energy is transferred

Principle of Conservation of Energy

The total energy of an isolated system (no external forces) remains constant. (OR Energy can be neither created nor destroyed but only transformed from one form to another or transferred from ne object to another.)

Work done

The product of the force and the distance travelled in the direction of the force (Sometimes ok: Energy transferred mechanically, by forces)

Rotational equilibrium

Bodies in rotational equilibrium have a resultant torque of zero

Moment of inertia

Resistance to angular acceleration OR the sum of (mass x distance from pivot²) for all point masses in a system

Torque

Force x perpendiccular distance from pivot

Newton’s second law for rotations

Torque = moment of inertia x angular acceleration (𝜏 = Iα)

Rolling without slipping

When this occurs:

• s (translational) = r x theta

• v (translational) = r x omega

• a (translational) = r x alpha

Notes: These equations always hold for the tangential (rather than translational) velocity/displacement/acceleration of any point on a rotating body which is at rest translationally. But for bodies with translational motion, they only hold for rolling without slipping

Angular impulse

Change in angular momentum OR area under a torque-time graph

Event (in the context of the theory of relativity)

A point in spacetime / something happening at a particular time and a particular point in space

Inertial frame of reference

a frame of reference that is not accelerating but is at rest or moving with a constant velocity (Or: a frame of reference in which Newton’s law of inertia is valid)

Frame of reference

The point of view of an observer or a coordinate system against which measurements are made consisting of x,y,z axes and a clock

Muon decay experiment - explanation

1. Earth observer’s frame of reference. The time for the half-life has dilated. So more time to cover distance (from top of atmosphere)

2. Muon’s frame of reference: the lenth (depth) of the atmosphere has contracted. So can cover this shorter distance in the (proper) time of its half-life

Proper length

The length of an object recorded in a frame of reference where the object is at rest (greates possible length that could be recorded)

Proper time interval

Time between events as measured in a frame where the events take place at the same point in space (shortest possible time that any observer could correctly record for the event)

Two postulates of Special Theory of Relativity

1) The laws of physics are the same in all inertial reference frames

2) The speed of light in a vacuum is the same for all observers

Inertial

Not being accelerated OR not subject to an unbalanced force OR where Newton’s laws apply

Time dilation

An effect of relativity in which moving clocks run slow

Length contraction

An effect of relativity in which the separation between two points in space contracts if there is relative motion in that direction

Simultaneity

Two events occuring at different points in space which are simultaneous for one observer cannot be silumlatneous for another observer in a different frame of reference which is moving relative to the first