20
Mass of a body
Textbook: pp. 76–77
GoalKnow that mass measures inertia, and know mass units.
New words
mass · massakilogram (kg) · kilogramm (kg)balance (scales) · taroziinertness · inertlik
Explanation
A body’s inertness is measured by its mass: the greater the mass, the harder it is to change the body’s velocity. Under the same force a body of smaller mass gets a larger acceleration than one of larger mass. Mass is a scalar; its SI unit is the kilogram (kg); 1 t = 1000 kg, 1 kg = 1000 g. Mass is measured with a balance, which compares the body with standard masses. Mass is additive: when bodies are combined, the total mass is the sum of their masses. Mass does not depend on where the body is, whereas weight can change (we will see this later).
Worked examples
A cart of 8 kg carrying a 4 kg load has total mass 8 + 4 = 12 kg.
2.5 t = 2.5 · 1000 = 2500 kg.
Class activity
“Which is more inert?”: gently push an empty and a full light box with equal effort and discuss which is harder to set moving. Safety: use no heavy objects.
Practice
1
What does mass measure and what is its unit?
The inertness of a body; the unit is the kilogram (kg).
2
How many kg is 3 t?
3000
3
Bodies of 5 kg and 7 kg are combined. Total mass (kg)?
12
4
Why does an empty cart speed up faster than a loaded one under the same push?
The empty cart has less mass and so less inertness, hence a larger acceleration.