krit.club logo

Forces and Motion - Gravity and the difference between mass and weight

Grade 5Cambridge (IGCSE)

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Mass is the amount of matter (or 'stuff') in an object. It is measured in kilograms (kgkg) or grams (gg).

•

Mass stays the same no matter where the object is in the universe.

•

Weight is a force caused by gravity pulling on an object's mass. Because it is a force, it is measured in Newtons (NN).

•

Weight changes depending on the strength of gravity. For example, you would weigh less on the Moon than on Earth because the Moon has weaker gravity.

•

Gravity (gg) on Earth is a constant pull of approximately 10 N/kg10\ N/kg (often rounded from 9.8 N/kg9.8\ N/kg for Grade 5 level).

•

Mass is measured using a balance scale, while Weight is measured using a spring scale or Newton meter.

📐Formulae

Weight=mass×gravityWeight = mass \times gravity

W=m×gW = m \times g

gEarth≈10 N/kgg_{Earth} \approx 10\ N/kg

💡Examples

Problem 1:

A school bag has a mass of 5 kg5\ kg. What is its weight on Earth?

Solution:

W=5 kg×10 N/kg=50 NW = 5\ kg \times 10\ N/kg = 50\ N

Explanation:

To calculate the weight, we multiply the mass (5 kg5\ kg) by the Earth's gravitational field strength (10 N/kg10\ N/kg).

Problem 2:

An astronaut weighs 700 N700\ N on Earth. If they travel to the Moon, where gravity is only 1.6 N/kg1.6\ N/kg, what happens to their mass?

Solution:

The mass remains exactly the same as it was on Earth.

Explanation:

Mass is the amount of matter in the body. While the astronaut's weight will decrease on the Moon because gravity is weaker, their mass does not change.

Problem 3:

If a rock weighs 30 N30\ N on Earth, what is its mass?

Solution:

m=Wg=30 N10 N/kg=3 kgm = \frac{W}{g} = \frac{30\ N}{10\ N/kg} = 3\ kg

Explanation:

By rearranging the formula, we divide the weight (30 N30\ N) by the gravity (10 N/kg10\ N/kg) to find the mass in kilograms.