Edexcel A Level (IAL) Physics-5.30 - 5.31 Gravitational Fields & Field Strength- Study Notes- New Syllabus
Edexcel A Level (IAL) Physics -5.30 – 5.31 Gravitational Fields & Field Strength- Study Notes- New syllabus
Edexcel A Level (IAL) Physics -5.30 – 5.31 Gravitational Fields & Field Strength- Study Notes -Edexcel A level Physics – per latest Syllabus.
Key Concepts:
Gravitational Field as a Force Field
A gravitational field (also called a force field) is a region of space in which a mass experiences a gravitational force.
Definition of a Gravitational Field
A gravitational field exists at a point if:
- A mass placed at that point experiences a force.
- The force acts without physical contact.
- The force is always attractive.
Key idea:
- Masses interact through fields.
- The field exists even if no mass is present.
Source of a Gravitational Field
- All masses produce gravitational fields.
- Larger masses create stronger gravitational fields.
- The field strength decreases with distance from the mass.
For example:
- The Earth produces a gravitational field around it.
- This field causes objects to fall towards the Earth.
Gravitational Force in a Field
When a mass is placed in a gravitational field:
- The mass experiences a force called its weight.
- The force acts towards the centre of the field source.
Near the Earth’s surface, this force is:
\( W = mg \)
Gravitational Field Lines
Gravitational fields can be represented using field lines.
- Field lines show the direction of the force on a mass.
- They always point towards the mass creating the field.
- Closer lines indicate a stronger field.
Important:
- Field lines never cross.
- Gravitational field lines are always directed inward.
Gravitational Field Compared to Contact Forces
- Gravitational force acts at a distance.
- No physical contact is required.
- The force is transmitted through the field.
Therefore: Gravity is a non-contact force.
Exam Language to Use
- “A region of space where a mass experiences a force.”
- “The field exists even if no test mass is present.”
- “The force acts towards the source mass.”
Example (Easy)
What is meant by a gravitational field?
▶️ Answer / Explanation
A gravitational field is a region of space in which a mass experiences a gravitational force.
Example (Medium)
Does a gravitational field exist in empty space where no mass is present?
▶️ Answer / Explanation
Yes. The gravitational field exists even if no mass is present; it is created by the source mass.
Example (Hard)
Explain why gravity is described as a force acting at a distance.
▶️ Answer / Explanation
- No physical contact is required.
- The force is transmitted through a gravitational field.
- A mass experiences a force when placed in the field.
Gravitational Field Strength
Gravitational field strength describes how strong a gravitational field is at a particular point in space.
Definition of Gravitational Field Strength
Gravitational field strength at a point is defined as the force per unit mass experienced by a small test mass placed at that point.
\( g = \dfrac{F}{m} \)
- \( g \) = gravitational field strength (N kg⁻¹)
- \( F \) = gravitational force acting on the mass (N)
- \( m \) = mass experiencing the force (kg)
Meaning of the Equation
- Gravitational field strength tells you how much force acts on each kilogram of mass.
- A stronger gravitational field produces a larger force on the same mass.
- The direction of \( g \) is the direction of the gravitational force.
Key idea: Gravitational field strength is a property of the field, not of the test mass.
Gravitational Field Strength Near the Earth
- Near the Earth’s surface, \( g \approx 9.8\,\mathrm{N\,kg^{-1}} \).
- This means a \( 1\,\mathrm{kg} \) mass experiences a force of about \( 9.8\,\mathrm{N} \).
- The value of \( g \) decreases with altitude and distance from Earth.
Relation Between Gravitational Field Strength and Weight
Weight is the gravitational force acting on a mass:
\( W = mg \)
- Mass is constant.
- Weight depends on gravitational field strength.
Using the Equation \( g = \dfrac{F}{m} \)
- Use when force and mass are known.
- Ensure all values are in SI units.
- Rearrange carefully if finding \( F \) or \( m \).
Rearranged forms:
\( F = mg \)
\( m = \dfrac{F}{g} \)
Example (Easy)
A \( 2.0\,\mathrm{kg} \) mass experiences a gravitational force of \( 19.6\,\mathrm{N} \). Find the gravitational field strength.
▶️ Answer / Explanation
\( g = \dfrac{F}{m} = \dfrac{19.6}{2.0} = 9.8\,\mathrm{N\,kg^{-1}} \)
Example (Medium)
What force acts on a \( 5.0\,\mathrm{kg} \) mass in a gravitational field of strength \( 3.0\,\mathrm{N\,kg^{-1}} \)?
▶️ Answer / Explanation
\( F = mg = 5.0 \times 3.0 = 15\,\mathrm{N} \)
Example (Hard)
A mass weighs \( 120\,\mathrm{N} \) on a planet where \( g = 4.0\,\mathrm{N\,kg^{-1}} \). Find the mass of the object.
▶️ Answer / Explanation
\( m = \dfrac{F}{g} = \dfrac{120}{4.0} = 30\,\mathrm{kg} \)