SAMPLE PROBLEM 1A A force of 35 N gives a body of mass m kg an acceleration of 5 m s2. Calculate the mass of the body.
SAMPLE ANSWER 1A
I have taken the approach of omitting the units during the calculations: Using the formula F ma we get:
35 m 5 Q m 35 5 7
The mass of the body is 7 kg.
The approach of including the units during the calculation is also acceptable: Using the formula F ma we get:
The mass of the body is 7 kg. 35 N m kg 5 m s2 Qm kg 35 N
5 m s2 7 kg
The approach taken by each student is entirely up to each individual in consultation with their teacher. What is of vital importance is that the unit is always given with the answer.
SI Units
The International System of Units (abbreviated to SI from the French, Le Système International d'Unités) is the modern form of the metric system.Almost all the standard units we use in the Leaving Cert. course are SI units.The exceptions are for measuring:
angle in degrees () sound intensity level in decibel (dB) energy in kilowatt-hour (kW h) or electron volt (eV)
Base Units
There are seven base units used in the SI system. All others are derived from these. In Leaving Cert. Physics we only use the first five: metre, kilogram, second, kelvin and ampere. Leaving Cert. Chemistry makes use of the sixth unit, the mole.The seventh unit, the candela, does not concern us in any Leaving Cert. subjects.The only unit where the definition is required for Leaving Cert. Physics is the ampere, which is dealt with in Chapter 20. I include the definitions here only as a curiosity for those students who are interested in the history of the subject as well as its science.
