AQA Unit 4 Physics Definitions

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• Created by: Joe
• Created on: 13-12-12 19:30

Section 1 - Further Mechanics

Newton’s 2nd Law - The rate of change of momentum is directly proportional to the resultant force which acts upon it.

The Newton - One newton is the force that will give a mass of one kilogram an acceleration of one metre per second.

Linear Momentum - The mass of an object multiplied by its velocity.

Conservation of Momentum - In any direction, in the absence of external forces the total momentum of a collision remains constant.

Impulse - Change in momentum of a body.

Elastic Collision - No momentum of kinetic energy is lost.

Inelastic Collision - Momentum is conserved but kinetic energy is lost.

Radian - One radian is the angle subtended at the centre of a circle by an arc of length equal to the circles radius.

Period - The period of an object in circular motion is the time taken for it to complete one revolution.

Angular Speed - The unit angle an object rotates through per unit time.

Centripetal Force - A force which keeps the object moving in a circle.

Simple Harmonic Motion - An oscillation in which the acceleration is inversely proportional to the displacement from the mid-point, and is directed towards the mid-point.

Damping - When energy is lost to the surroundings and the amplitude of an oscillation is reduced.

Resonance - When the driving force equals the natural frequency and causes the…

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This is great, but there are a few points I'd like to nitpick about in this document - it really is crucial to get things absolutely right to get top marks, after all :)

First, in conservation of momentum, I agree entirely with what you've written. However, in recent AQA mark schemes, saying "in a system" isn't enough to get all the marks, they want you to say "in a collision". Whilst this is silly (and I'd argue implied anyway), it's what needs to be said to get full marks.

In Elastic and inelastic collisions, it doesn't need to be said that momentum is conserved - generally this is assumed anyway :)

Angular speed - this really should be the unit angle that an object rotates through per unit time. Saying unit angle isn't so crucial, but saying unit time is important because in AQA papers they often ask you questions involving angular speeds given in RPM (revolutions per minute), which definitely isn't the angle traveled per second.

Centripetal Acceleration - You've defined centripetal Force. It's important not to get these mixed up - force is not the same as acceleration!

Simple Harmonic Motion - It's important not to say that acceleration is directly proportional to displacement, as it isn't. It's inversely proportional, which is what the second half of your definition states. This is because the acceleration acts in the opposite (inverse) direction to the displacement from the midpoint.

Potential Difference (electric and gravitational) - there is a crucial bit missing from each of these definitions - they need to have "From one point in the field to another" put on the end, otherwise you could be meaning from here to Azerbaijan for all the examiner knows. It's very important to be explicit in your exam answers - leave nothing to chance, make sure the examiner has no doubt that you know the answer.

Capacitance - Charge stored per unit potential difference - again, a small thing, but I've seen a mark scheme where it disallowed the use of "volt" or "voltage" instead of p.d.

Flux Linking - No actual physics-related comment here, you just made a typo on "thee magnitude" :)

Have a great new year, and good luck in your exams! :)

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That correction for SHM is wrong, acceleration is proportional to displacement, acting in the  opposite direction to the displacement.Just because it's acting in the other direction doesn't mean it's inversely proportional.

Velocity in SHM is inversely proportional to the displacement, which is why the fastest point is when the displacement is 0 in the centre.

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Sorry this is an old document, but the SHM definition is wrong!