### 7.1.6 Gravitational Field LO (a)

Think about it: How can two objects exert attractive forces on each other when they are not in contact with each other?
Every object sets up a gravitational field around itself due to its mass. When two objects enter each other’s gravitational fields, they will be attracted towards each other.
Hence, a gravitational field is a region of space in which any object that lies in it experiences a gravitational force towards the object that creates the field, due to its mass.
(For your information, magnetic fields and electric fields are also examples of force fields.)

### 7.1.6.1 Inquiry:

A gravitational field is invisible and thus is represented by imaginary field lines on paper. How would the Earth’s gravitational field (over both near and large distances from Earth) look like?

1) Using the simulation later, drag the red test mass to the outer most series of blue dots and play and simulation. Every time the field lines are drawn and contact Earth's surface, the simulation will pause. When that happens, drag the red mass to a new position to draw the field line and click play to continue to run the simulation. repeat these steps until all outer most series of blue dots have been drawn with the field lines. The green vectors represent the field vectors while the red lines represent the field lines near the Earth’s surface. Compare with the picture below;

• The gravitational field near Earth’s surface is uniform
• The field lines should be drawn parallel to each other and of equal spacing.
2) Select from the drop down menu the option for Near_Earth_Surface. Using the same steps earlier, try to draw the RED field lines for this case. Compare with the picture below.
• The gravitational field around Earth is non-uniform.
• The field lines should be drawn radially pointing towards the centre of Earth.
3) Select from the drop down menu the option for Outer_Space. Using the same steps earlier, try to draw the RED field lines for this case. Compare with the picture below.
• The gravitational field around Earth is non-uniform.
• The field lines should be drawn radially pointing towards the centre of Earth.
4) Finally, select from the drop down menu the option for very_outer_Space. Using the same steps earlier, try to draw the RED field lines for this case. Compare with the picture below.
• The gravitational field around Earth is non-uniform.
• The field lines (use the red test mass to draw field lines) should be drawn radially pointing towards the centre of Earth.

### What can you infer from the representations of these gravitational field lines? LO (e)

1. The closer the field lines, the stronger the gravitational field.
2. Near Earth’s surface, the field strength is approximately constant (around 9.81 m s-2) and hence the gravitational field lines are almost equidistant from each other.
3. Over large distances from Earth, the gravitational field strength decreases as it gets further from Earth and hence the gravitational field lines space out further from each other.

### 7.1.6.2 Model:

1. Run Sim
2. http://iwant2study.org/ospsg/index.php/55