Exploring Forces · Lesson 5 of 7
Weight, Spring Balance, and Mass
“Learn what weight measures, how a spring balance is read, and why mass stays constant while weight can change.”
• Define weight as the gravitational force acting on an object. • Explain how a spring balance measures weight. • Determine the range and smallest division of a spring balance. • Distinguish clearly between mass and weight. • Explain why the same mass can have different weights on different planets.
Weight Is a Force
Earth does not pull every object with exactly the same force. If different objects are hung from the same spring, they can stretch the spring by different amounts. This difference shows that their weights are different.
The force with which Earth pulls an object toward itself is called the weight of the object.
Because weight is a force, its SI unit is the newton (N), not the kilogram. The stronger Earth's gravitational pull on an object, the greater the object's weight.
How a Spring Balance Measures Weight
A spring balance contains a spring fixed at one end and a hook at the other. Hanging an object from the hook stretches the spring. A scale beside the spring is calibrated so that the amount of stretch can be read as force in newtons.
Before using any spring balance, two features must be checked: its range and its smallest division. The range tells you the largest force the instrument can measure safely. The smallest division tells you the smallest change in force that can be read directly.
The range is the interval from the minimum to the maximum value that the instrument is designed to measure.
Problem
A spring balance is marked from 0 N to 10 N. Between two consecutive 1 N marks there are 5 equal divisions. What does one small division represent?
- 1.The difference between the two larger marks is 1 N.
- 2.That 1 N interval is divided into 5 equal parts.
- 3.One division = 1 N ÷ 5 = 0.2 N.
- 4.So the smallest division of the spring balance is 0.2 N.
Do not hang an object whose weight exceeds the maximum value marked on the spring balance. Doing so may damage the spring and make later readings inaccurate.
Mass and Weight Are Different
Mass tells us how much matter an object contains. Weight tells us how strongly gravity pulls on that object. The two quantities are related in everyday experience, but they are not the same physical quantity and they do not use the same units.
Mass is the amount of matter in an object. It is measured in grams (g) or kilograms (kg) and remains the same from place to place.
| Quantity | What it describes | Common unit in this chapter | Does it change from planet to planet? |
|---|---|---|---|
| Mass | Amount of matter | g or kg | No |
| Weight | Gravitational force on the object | N | Yes |
The same 1 kg object has the same mass on Earth, the Moon, Mars, Venus, and Jupiter, but its weight differs because gravitational pull differs from place to place.
| Place | Mass | Weight |
|---|---|---|
| Earth | 1 kg | 10 N |
| Moon | 1 kg | 1.6 N |
| Mars | 1 kg | 3.8 N |
| Venus | 1 kg | 9 N |
| Jupiter | 1 kg | 25.4 N |
In everyday speech people may say, 'This bag weighs 10 kg.' Scientifically, 10 kg is a mass. Weight is a force and should be expressed in newtons.
Problem
An object has a mass of 1 kg. Its weight is 10 N on Earth and 1.6 N on the Moon. What changes and what stays the same?
- 1.The amount of matter in the object does not change, so its mass remains 1 kg.
- 2.The Moon pulls less strongly than Earth, so the weight is smaller on the Moon.
- 3.Therefore mass stays the same while weight changes.
Quiz
What does weight measure?
What is the SI unit of weight?
A 0–10 N spring balance has five equal divisions between 2 N and 3 N. What is one small division?
Which quantity remains the same on Earth and the Moon?
Which statement is scientifically correct?
Practice Problems
- Define weight and state its SI unit.
- Explain how a spring balance uses stretching of a spring to measure weight.
- A scale has a 2 N gap between labelled marks divided into 10 equal parts. What is the smallest division?
- Explain the difference between range and smallest division of an instrument.
- Compare mass and weight using their meanings, units, and behaviour on different planets.
- Why does a 1 kg object weigh less on the Moon than on Earth while its mass remains 1 kg?
Key Takeaways
• Weight is the gravitational force acting on an object and is measured in newtons. • A spring balance measures weight from the amount a calibrated spring stretches. • The range and smallest division must be checked before taking measurements. • Mass is measured in grams or kilograms and remains unchanged from place to place. • Weight can change because gravitational pull is different on different planets.