# Physics Summary

Topics: Special relativity, General relativity, Speed of light Pages: 25 (6540 words) Published: May 8, 2013
1.1 GRAVITY AND GRAVITATIONAL FIELDS

1.1.1 Define weight as the force on an object due to a gravitational field.

Weight is the force experienced by an object due to the presence of a gravitational field. This force is directly related to the strength of the gravitational field acting on an object and the mass of that object.

m = mass, g = acceleration due to gravity
m = mass, g = acceleration due to gravity
Fg = mg
Fg = mg

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

Work done is the measure of how much energy is required to displace an object a specified distance. F = Force, s = displacement
F = Force, s = displacement
W = Fs
W = Fs

When an object is moved away from a gravitational field, it gains energy. This is because by raising it up from the field’s origin, work is done.

E.g. An object 1kg in mass is raised 100m, the work done would be 980J. However, conservation of energy states that energy cannot be destroyed. Therefore the object now has 980J in gravitational potential energy. Whereas at the origin is would have 0J potential energy.

Fg = Gm1m2d2
Fg = Gm1m2d2

Ep =

1.1.3 Perform an investigation and gather information to determine a value for acceleration due to gravity using the pendulum motion or computer assisted technology and identify reasons for possible variations in the value 9.8m.s-1

AIM: To measure the value of acceleration due to gravity on the surface of the earth. APARATUS: A weight attached to a thick, non-elastic string that was tied to a clamp on a retort stand. METHOD:

1) Set the pendulum in motion, making sure it doesn’t swing in a deflection greater than 30° (as greater than 30° will cause the string to lose tension) 2) Using a stopwatch, time the pendulum’s swing over 10 complete cycles (periods). Starting at the middle of the swing, and timing until it returned to the centre of its swing. 3) Using the formula:

4) And the time for a single period swing, (Where T = Period, l = Length of pendulum and g = acceleration due to gravity, we could calculate a value for the acceleration due to gravity.

Experimental Errors effecting the value of g:
* Ensuring the pendulum swings in a single vertical plane, which results in a more accurate and unified period value for each swing. * Using a time over 10 whole swings (periods) instead of a single swing reduces the error involved with the reaction time of the person with the timer. * Parallax error when measuring the length of the pendulum may effect the value calculated for g.

Factors effecting the strength of g on the Earth’s surface: (separate from experimental errors) * As the earth spins, it bulges at the equator, flattening at the poles. This causes the poles to be closer to the centre of the earth than on the equator. According the equation for force due to a gravitational field, F is proportional to 1/r (distance between the centre of the obects). This means the value for g will be greatest at the poles. * The field of the earth will vary with the density of nearby geography. Places where the lithosphere is thick, or where there are dense mineral deposits or nearby mountains experience greater gravitational force compared to places over less dense rock or water. * Gravitational force depends on altitude. Places with greater elevation such at mountain ranges experience less gravitational force, compared to areas closer to sea level.

1.1.4 Gather secondary information to predict the value of acceleration due to gravity on other planets.

Planet| Gravitational Acceleration (m.s2)|
Mercury| 4.07|
Venus| 8.90|
Earth| 9.80|
Mars| 3.84|
Jupiter| 24.83|
Saturn| 10.50|
Uranus| 8.45|
Neptune| 11.20|

1.1.5 Define gravitational energy as the word done to move an object from a very large distance away to a point in a gravitational field.

* A very...