1. 4.1 Gravitational Force Near Earth
Recall that all objects in free fall accelerate at a rate of 𝒈 = 9.8 m/s2 [down]. An object undergoes terminal velocity when the force due to air resistance acting on the object, is equal to the force of gravity. Air resistance increases with the cross-sectional area and the speed of an object. Initially, this skydiver will accelerate at 𝒈 when he first jumps from a plane. As he continues to fall, his acceleration will decrease until he reaches a maximum constant speed; terminal velocity.

2. 4.1 Gravitational Force Near Earth
The following FBDs represent a skydiver jumping out of a plane. In your own words, describe the possible motion of the skydiver from (a) to (f).

3. 4.1 Gravitational Field Strength
A force field is a region of space surrounding an object that can exert a force on other objects that are placed within that region. The gravitational force field surrounding Earth attracts all other masses placed within its field. The magnitude of Earth’s gravitational field decreases as an object moves farther away from Earth’s centre. To represent this force field around Earth, we draw vectors of force pointing towards Earth’s centre. Their magnitudes indicate that the field becomes weaker as distance from Earth’s centre increases.

4. 4.1 Gravitational Field Strength
Gravitational field strength is the force per unit mass, acting on an object placed at a particular location within Earth’s gravitational field. At Earth’s surface, its value is 𝒈 = 9.8 N/kg. Notice that this value has the same magnitude as the acceleration due to gravity. Since Earth is not a perfect sphere, its field strength varies at different locations on its surface. It is greater at the poles and less at the equator. On this map, red and yellow areas have greater gravitational field strength.

5. 4.1 Mass vs. Weight
The terms mass and weight are used interchangeably in everyday language, but in physics these terms have different meanings. Mass is the quantity of matter in an object. The only way to change mass is to add or remove matter. The SI units of mass are kilograms (kg). Weight is a measure of the force of gravity, 𝑭 𝒈 , acting on an object. Weight is a vector quantity, and its magnitude is measured in Newtons (N). On the Moon, your weight is one-sixth of your weight here on Earth. The force of gravity on the Moon is much weaker than Earth’s. However your body contains the same amount of matter at either location, so your mass remains the same.
SP #2,3 p.166

6. 4.1 Homework
Practice # 1-4 p.166
Questions # 1, 3, 5, 7, 10 p.167