1. Force Forces, Fields,Potential and Energy
Objective:
TSW understand and apply the concepts gravitational force, fields, potential and energy.

2. Gravitational Field: A property of space around a mass that causes forces on other masses. The Gravitational field strength at a point is the force per unit mass on a small mass placed at that point. F F F F
g = the gravitational field
The gravitational force is always in the same direction as the gravitational field
The gravitational field is a vector. (Magnitude and direction)

3. Equipotential lines: Lines along which the potential is the same.
Spacing between equipotential lines increases as the distance from the mass increases.

4. Equipotential lines are like a topographical map
Click to view the Spin and tilt video

5. The units for a gravitational field are N/kg
The gravitational Force = (The mass) x (The Gravitational Field)
The units for a gravitational field are N/kg

6. Let’s zoom in to our physics room:Fg
Constant Field

7. Equipotential line
Constant Field

8. Example 1:
Calculate the gravitational field 10000km from the earth, which has a mass of 6x1024kg and a radius of 6400km
10000km

9. Example 2:
Find the final velocity of a 4kg object starting from rest if it is accelerated 25meters through a constant gravitational field of 22N/kg?
4kg
25m
Fg = mg = (4)(22) = 88N

10. Example 2: Solving with energy
Find the final velocity of a 4kg object starting from rest if it is accelerated 25meters through a constant gravitational field of 22N/kg?
4kg
25m
V = gh
V = 0 J/kg

11. The gravitational potential of a position r is given by:
Gravitational potential at a point in a gravitational field is the work done per unit mass in moving a small mass from infinity to that point r R
The gravitational potential of a position r is given by:
Notice r is not squared
The gravitational potential at infinity is zero

12. Gravitational potential energy at a point is the work done to move a mass from infinity to that point

13. Graphically V (J/kg) r (m)
r (m)
The slope (gradient) of the tangent line gives the field strength g. R 2R 3R
R = The radius of the mass

14. Example:
With what velocity must a rocket be fired in order to escape the earth’s gravitational field? (Neglect air resistance)