A-level Physics: Mechanics (Motion and Newton's Laws, Work and power, momentum)

Scalar quantity

One that only has a magnitude

Vector quantity

One that has a magnitude and a direction

Resultant vector

The vector that results from adding 2 or more vectors in a vector sum

Moment

The turning effect of the force about a pivot, The force multiplied with the perpendicular distance from the pivot

Pivot

The point about which an object rotates

Equilibrium

An object is in equilibrium when the sum of the forces = 0 and the sum of the turning moments = 0

Principle of moments

For an object in rotational equilibrium, the sum of the clockwise moments around a pivot are equal to the sum of the counterclockwise moments around that same pivot

Centre of mass

The point in a body around which the resistant torque due to the pull of gravity is zero

Couple

A pair of forces that provide a turning effect but no translational movement. They act in opposite directions, are parallel, but do not act along the same line

What must you do when calculating moments

Find the perpendicular distance from the force to the pivot, which may require trigonometry if the force isn't perpendicular to the distance to the pivot

Static

Not moving/stationary

What must you do in 2-support problems

Take moments about one of the pivots, to determine the upwards force of another pivot, whose moment should sum to zero with the other forces

What to remember when drawing a free body force diagram

The angle of the arrows MUST be accurate to how it would be in real life, the size of the arrows must be proportional to the forces, and the arrows MUST balance out to zero in direction if the body is in equilibrium

What to do when asked to use a scale figure

Draw a closed triangle with all of the forces shown at an appropriate scale

Distance travelled from a velocity-time graph

Area under the graph

How to determine velocity from a non-linear displacement-time graph

Draw a tangent with a pencil

What properties determine the size of the drag on a falling object

speed and surface area

Name of maximum speed reached by falling object

Terminal velocity

Newton's First Law of motion

An object at rest will remain at rest unless acted on by an unbalanced force

Newton's Second Law of motion

An object accelerates in the direction of a resultant force acting on it

Newton's Third Law of motion

For every action, there is an equal and opposite reaction

What is terminal velocity

The constant maximum speed reached by a falling object in a medium where the weight equals the drag force

What shape to draw objects in free body diagrams

The general shape of the object itself—no need to simplify it down to a circle

Key point to remember about Newton's Third Law force pairs

They must be acting on different objects, with the same magnitude of force in opposite directions along the same line, and they must be the EXACT same kind of force—reaction force and gravity are NOT a third law force pair for this exact reason

Equation for work done with distance and force

Work done = force * distance moved in direction of force

Area under force-displacement graph

Work done

Key consideration when calculating work done in stretching a spring

The work done is the area under a force-displacement graph, therefore if the line is linear, you must use the average force, which is half of the final force

Explanation of impulse

If the equation for force with momentum is F = ∆mv/∆t, then you can use F∆t=∆mv, and F∆t is the impulse, which tells you how a force applied over time causes a change in momentum

Why are impulses and F=∆mv/∆t useful

As you increase the time taken for the change in momentum, it decreases the force applied, which can be used for safety like with falling onto a trampoline versus a concrete floor

Area under a force-time graph

Change of momentum

What is always conserved, and what might not always be conserved, in collisions and explosions

Total energy and momentum are always conserved; kinetic energy is not always conserved

Elastic vs inelastic collisions

Elastic collisions are where all kinetic energy is conserved, inelastic collisions are where not all kinetic energy is conserved, and where the rest could've been transferred into other forms

How to deal with collisions in 2D

Separate x and y components into 2 separate independent equations

How to know whether to use energy or SUVAT for mechanics questions (e.g. calculating maximum speed at a given point)

If it's nonlinear acceleration or a nonlinear path, or there's resistive force, use energy!