The Earth + Gravitational Fields

The Earth + Gravitational Fields

1. define weight:

   • Weight is the force on an object due to a gravitational field. It is a vector quantity and the measurement unit is the Newton.

2. Explain that a change in gravitational potential energy is related to work done

   • this dot point is hard. understand it fully.
   • Prove that: Work (done by an external force) = Change in GPE
     • Note: Gravitational Potential Energy is the energy of a mass due to its position within a gravitational field. E.g. If a ball is placed atop a cliff, it will have stored energy, but it won't if it is on the ground. 
     • Well, we know that net work = change in KE
       • Force x distance = Work done
         • But, the force in regard to GPE is equal to mg (every force has an equal and opposite reaction)
           • let distance = delta y (change in y)
             • Now, sub these in. You will get Fd = mg(delta y) which is the notation for gravitational potential energy. Potential energy thus is the work done to raise an object in a gravitational field.
               • However this is only valid when the object is near the Earth's surface.
                 • On a larger scale, due to the Law of Universal Gravitation, the force of attraction between a planet and an object will drop to zero only when the object is an infinite distance from the planet.
                   • Thus, infinity is our level of zero potential energy.
                     • Because gravity is a force of attraction, work must be done on the object to move it from a point to infinity - to move it against the field so that it gains energy. Thus, as earlier, the gravitational potential energy of an object at a point in a gravitational field is equal to the work done to move the object from an infinite distance away to that point. The mathematical equation is over there

3. Define Gravitational Potential Energy

   • The work done to move an object from a very large distance away to a point in a gravitational field
     • • U = Gravitational potential energy, G = gravitational constant, M and m are the two masses, R = distance between their centres of mass
       • Potential energy is negative as the amount of work done refers to the work that is done to bring the object closer to a reference point instead of away from it

4. Investigations

   • Determine a value for acceleration due to gravity and identify reason for possible variations from the value of 9.8m/s

5. Gather secondary information

   • To predict the value of acceleration due to gravity on other planets

6. Analyse information using

   • F =mg
     • to determine the weight force for a body on Earth and for the same body on other planets

7. Gravity is a force of attraction between any two masses. Gravitational fields are vector fields within which a mass will experience a force.

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