What is weight, and its unit?

The force of gravity on a body, W=mg, in N.

Force \times perpendicular distance from the pivot.

Convert km h^{-1} to m s^{-1}?

Divide by 3.6 (multiply by \frac{1000}{3600}).

Meaning of 'particle' in a model?

Mass but no size, so rotation and dimensions are ignored.

Meaning of 'light string'?

The string has negligible mass.

Quantities and S.I. units in mechanicsEdexcel A-Level Maths: Flashcards

Card 1 of 140 of 14 known

Question

Three fundamental S.I. quantities in mechanics?

Tap or press Space to reveal

Tap card or press Space to flip

See all 14 cards

Three fundamental S.I. quantities in mechanics?: Length (m), time (s) and mass (kg).
Unit of velocity?: m s $^{-1}$
Unit of acceleration?: m s $^{-2}$
Unit of force and its base units?: Newton, N $=$ kg m s $^{-2}$
What is weight, and its unit?: The force of gravity on a body, $W=mg$ , in N.
Unit of moment?: N m ( $=$ kg m $^2$ s $^{-2}$ )
Moment of a force?: Force $\times$ perpendicular distance from the pivot.
Convert km h $^{-1}$ to m s $^{-1}$ ?: Divide by 3.6 (multiply by $\frac{1000}{3600}$ ).
Convert m s $^{-1}$ to km h $^{-1}$ ?: Multiply by 3.6.
Value of $g$ unless told otherwise?: $9.8$ m s $^{-2}$
Meaning of 'particle' in a model?: Mass but no size, so rotation and dimensions are ignored.
Meaning of 'light string'?: The string has negligible mass.
Meaning of 'smooth'?: There is no friction.
What does 'inextensible' mean?: The string does not stretch.

Exam questions on Quantities and S.I. units in mechanics

A car of mass 1200 kg travels along a straight horizontal road at a constant $72$ km h $^{-1}$ .
Find the weight of the car, taking $g=9.8$ m s $^{-2}$ , and state its unit.2 marks
A cyclist of mass 70 kg travels a distance of 5.4 km in a time of 15 minutes along a straight track at a constant speed.
Find the weight of the cyclist, taking $g=9.8$ m s $^{-2}$ , and express the unit of weight in terms of the S.I. base units kg, m and s.2 marks
A stone of mass 0.4 kg is dropped from the top of a cliff and hits the sea at a speed of $29.7$ m s $^{-1}$ . The stone is modelled as a particle moving vertically under gravity only.
State three modelling assumptions that are being made about the stone and its motion.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).