Gravitational Field Strength | CAIE 9702 Paper 4 Notes

Paper 4

Physics9702

Gravitational Field Strength

tells you how strong the field is at a point: the pull on each kilogram.

Near the Earth's surface, g is about 9.81 N kg⁻¹. Field strength can be found at any point in a field.

Field strength is force per unit mass

Definition

1 mark

Model answer: The gravitational force per unit mass at that point (9702/41/O/N/25 Q3(a)) (9702/42/O/N/24 Q2(a)).

CAIE may call this the gravitational field at a pointor the gravitational field strength at a point. Both wordings need the same answer: force per unit mass.

g = \dfrac{F}{m}

Symbols

Unit: N kg⁻¹. This is the same as m s⁻², so g is also the .
g is a vector: it points the same way as the force, toward the mass making the field.

Worked example

Use field strength to find a force

The field strength on Mars is 3.7 N kg⁻¹. Find the gravitational force on a 2.0 kg object.

Start with the definition and make F the subject.
$g = \dfrac{F}{m} \quad\Rightarrow\quad F = mg$
Substitute the values. The force points towards the centre of Mars.
$F = 2.0 \times 3.7 = 7.4\ \text{N}$

Answer

F = 7.4\ \text{N}