CIE IGCSE Physics (0625) Mass and weight Study Notes - New Syllabus
CIE IGCSE Physics (0625) Mass and weight Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of Mass and weight
Key Concepts:
- Mass and weight
Definition of Mass
Definition of Mass:
- Mass is a measure of the amount of matter (substance) in an object.
- It tells us how much material is present in a body.
- It is a scalar quantity — it has magnitude but no direction.
Key Characteristics of Mass:
- Mass does not change with location (same on Earth, Moon, or space).
- It is measured in kilograms (kg) in SI units.
- Mass is independent of gravity and other external conditions.
At rest relative to the observer:
- This phrase means that mass is considered in a frame of reference where the object is not moving relative to the person measuring it.
- It ensures that no extra energy or forces (like kinetic energy or momentum) are being considered.
Example:
A 2 kg stone is on Earth. What is its mass on the Moon?
▶️ Answer/Explanation
Mass is a measure of how much matter is in the stone. This does not change with location.
Mass on Moon = \( \boxed{2~\text{kg}} \)
Definition of Weight
Definition of Weight:
- Weight is the force of gravity acting on a mass in a gravitational field.
- It depends on two things:
- The object’s mass (m)
- The gravitational field strength (g)
- Weight is a vector quantity — it has both magnitude and direction (downward).
- It is measured in newtons (N).
Equation relating weight, mass and gravitational field strength:
\( W = m \cdot g \)
Where:
- \( W \) = weight (in newtons, N)
- \( m \) = mass (in kilograms, kg)
- \( g \) = gravitational field strength (in N/kg)
Gravitational Field Strength:
- Defined as the force per unit mass acting on a body in a gravitational field.
\( g = \dfrac{W}{m} \)
This equation shows how much force is acting on 1 kg of mass in a gravitational field.
Important: This is numerically equal to the acceleration due to free fall near Earth’s surface.
So, \( g = 9.8~\text{N/kg} = 9.8~\text{m/s}^2 \)
| Property | Mass | Weight |
|---|---|---|
| Definition | Amount of matter in an object | Gravitational force on an object |
| Quantity type | Scalar | Vector (acts downward) |
| SI Unit | Kilogram (kg) | Newton (N) |
| Affected by gravity? | No | Yes |
| Changes with location? | No | Yes |
Example:
Find the weight of a 60 kg object on Earth. (Use \( g = 9.8~\text{N/kg} \))
▶️ Answer/Explanation
Using: \( W = m \cdot g \)
\( W = 60 \times 9.8 = \boxed{588~\text{N}} \)
Example:
An object weighs 30 N and has a mass of 5 kg. What is the local gravitational field strength?
▶️ Answer/Explanation
Using: \( g = \dfrac{W}{m} \)
\( g = \dfrac{30}{5} = \boxed{6~\text{N/kg}} \)
This could be a different planet or a high-altitude location on Earth.
Comparing Mass and Weight Using a Balance
Comparing Mass and Weight Using a Balance:
- A balance is a device used to compare the masses (and hence the weights) of different objects.
- In particular, a beam balance or pan balance compares mass directly by using gravitational force acting equally on both sides.
Key Point:
- Since weight is proportional to mass ( \( W = m \cdot g \) ) and \( g \) is constant in one location, comparing weights is the same as comparing masses using a balance.
Important Characteristics:
- Balances measure mass, not weight, because both sides of the balance are affected equally by gravity.
- A balance works the same on the Moon as on Earth because it measures the ratio of forces.
- Spring scales, on the other hand, measure weight — and will give different readings if gravity changes.
Example:
A student uses a beam balance to compare a metal cube with known 1 kg standard weights. The balance is level when one 1 kg weight is placed on the other pan. What is the mass of the cube?
▶️ Answer/Explanation
The object balances exactly with the 1 kg weight, meaning both experience the same downward force from gravity.
So, the mass of the metal cube is: \( \boxed{1~\text{kg}} \)