# Physics - Gravitational Fields

> Source: https://ollybritton.com/notes/a-level/physics/topics/gravitational-fields/ · Updated: 2021-11-18 · Tags: school

## See Also
- [Physics - Kepler's Laws](https://ollybritton.com/notes/a-level/physics/topics/keplers-laws/)

## Flashcards
##### What is the equation for gravitational field strength, $g$??
$$
g = \frac{F}{m}
$$

##### What are the two possible units for gravitational field strength??
$$
\text{N}\text{kg}^{-1}
$$
$$
\text{m}\text{s}^{-1}
$$

##### What is the wordy explanation of gravitational field strength??
The gravitational force exerted per unit mass on a small object placed at a point within the field.

##### What simplifying assumption do we make for planets and stars when calculating gravitational fields??
They are point masses.

##### ![PHOTO GRAVITATIONAL FIELD](gravitational-field.png) What does this picture show??
That the gravitational field close to a planet's surface can be modelled as uniform.

##### What are the lines on a gravitational field diagram called??
Field lines.

##### What two things does Newton's law of gravitation state??
The force between two point masses is:

* Directly proportional to the product of the masses.
* Inversely proportional to the square of their separation.

##### What is the equation for Newton's law of gravitation??
$$
F = -\frac{GMm}{r^2}
$$

##### $$F = -\frac{GMm}{r^2}$$ What is $G$ here??
The gravitational constant.

##### $$F = -\frac{GMm}{r^2}$$ Why is there a minus sign here??
To show that the force is attractive.

##### What is the approximate value of $G$, the gravitational constant??
$$
6.67 \times 10^{-11}
$$

##### When determining the gravitational force between multiple objects, what do you need to do??
Do the vector addition of all possible pairings.

### 2021-12-01
##### What is the formula for $g$ in terms of $r$ and $M$??
$$
g = \frac{GM}{r^2}
$$
##### What is the radius of the Earth??
$$
6.37 \times 10^6 m
$$

##### What is the mass of the Earth??
$$
5.97 \times 10^{24} \text{kg}
$$

##### What is the most common failure mode answering gravitational field questions??
Not converting distances to meters.

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