Physics Notes HSC

Topics: General relativity, Gravitation, Orbit Pages: 88 (32437 words) Published: April 13, 2014
HSC Physics
1. The Earth has a gravitational field that exerts a force on objects both on it and around it Define weight as the force on an object due to a gravitational field Mass is a measure of the amount of matter in an object, it is measured in kilograms (kg). The weight of an object is a measure of the force with which it is attracted by a gravitational force, it is measured in Newtons (N). It is directly related to the strength of the gravitational field at a point where the object is located. W=mg Newton’s Law of Universal Gravitation

where G is the universal gravitational constant (6.67x10-11m3/kg) The Gravitational Field

Explain that a change in gravitational potential energy is related to work done Work done is the measure of how much energy was used to displace an object a specified distance. W=fs where s is displacement. When an object is moved away form a gravitational field, it gains energy. This is because by raising it up from the field’s origin, work is done. Perform an investigation and gather information to determine a value for acceleration due to gravity using pendulum motion or computer assisted technology and identify reason for possible variations for the value of 9.8m/s2 1. Construct a pendulum at least one metre long, attached at its top to a support (such as a clamp connected to a retort stand) and with a small mass tied to its lower end to act as the pendulum bob.  2. Measure the length (l) of your pendulum, from its point of attachment to the centre of mass of its bob.  3. Pull the pendulum aside and release it so that it starts swinging. Using a stopwatch (or other device for measuring time), begin timing at an extreme of the pendulum’s motion and time ten full swings (one swing = back and forth) of the pendulum. Divide this time by ten to get a value for the average period (T) of the motion. Using this averaging technique tends to minimise random errors. 

The period of a pendulum depends upon the length (l) and the value of acceleration due to gravity (g), as described in the following equation:

Rearranging this equation gives an expression that can be used to calculate g.

4. Substitute your values for l and T into this equation to determine a value for g. The reasons for possible variations for the value 9.8m/s include: Firstly that Earth has different radius lengths at different points so, according to the formula for gravitational force between two objects, the larger the distance the lesser the pull. Secondly, as the Earth spins it bulges as the equator, flattening at the poles. This causes the poles to be closer to the centre of the Earth than the equator, so the Earth’s gravitational field is stronger at the poles than at the Equator. Thirdly, the field o the Earth varies with the density of nearby geography. Places where the lithosphere is thick, or where there are dense mineral deposits experience greater gravitational force compared to places over less dense rock or water. Analyse information using the expression: F=mg to determine the weight force for a body on Earth and for the same body on other planets Gather secondary sources to predict the value of acceleration due to gravity on other planets

Define gravitational potential energy as the work done to move an object from a very large distance away to a point in a gravitational field As we lift an object from the ground to a height above the ground we do work on it. This work is stored in the object as gravitational potential energy. For an object of mass m at a height h above the Earth’s surface the gravitational potential energy E is given by: . However this equation is valid only when the object is near the Earth’s surface.

Gravitational potential energy is defined as the work done to move an object from a point a very large distance to a specified point in the gravitational field. The work done is the energy input provided by the gravitational field to the object as it falls to that...
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