# Hsc Physics Notes - Space

**Topics:**General relativity, Force, Special relativity

**Pages:**42 (10641 words)

**Published:**August 19, 2013

9.2 Space:

Context 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 (m):

• The amount of matter in a body

• Constant anywhere in the universe

• Scalar quantity

• Measured in Kilograms (Kg)

Weight (W):

• The force acting on an object due to a gravitational field • Attractive force

• Vector quantity

• Direction – following a line between the centre of masses • Measured in Newtons (N)

• According to Newtons 2nd Law – F = ma:

Weight = Mass x Acceleration due to gravity (W = mg)

• Gravitational Force of Attraction between 2 bodies: F = Gm1m2 r2

• Note: Universal constant of gravitation = 6.67 x 10-11 Nm2kg-2

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). When an object is moved away from the origin of a gravitational field it gains gravitational potential energy which is the work done.

W = Fs = ∆KE = -∆Ep

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

EP = - G m1m2

r

Gravitational potential energy is the work done to move an object from a very large distance (ie. infinity) to a point in a gravitational field. Zero gravitational energy is defined as a point in infinity where the gravitational effects of all mass, would be zero. GPE is negative as the system loses energy when trying to remain at a point in a gravitational field. However, as seen from the diagram the further from the earth the less GPE required.

Example:

If a 1kg stone was raised 100m the work done would be 980J, however this is now GPE and will be converted to 980J of KE once dropped according to the law of conservation of energy.

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 from the value 9.8ms-2

Pendulum Motion Experiment:

Aim: To determine acceleration due to gravity using pendulum motion.

Apparatus: Pendulum support, non-elastic thread, metre ruler, stopwatch, pendulum bob

Theory:

On the earth’s surface the only factor affecting the period of a pendulum is its length. The period of a pendulum is independent of its mass and the angle of release.

Relationship is defined by:

[pic]

Safety: Ensure hands are washed after the experiment if lead pendulum bobs are used.

Procedure:

1. Set up pendulum as shown.

2. Measure the length of the pendulum from the centre of mass of the bob to the point of attachment ensuring that it is 0.1m long. 3. Displace the pendulum and time 10 complete oscillations to minimise experimental errors. 4. Record data.

5. Repeat step 3 three more times and calculate an average time for 10 oscillations. 6. Repeat steps 2 – 5 for lengths 0.2, 0.4, 0.6, 1.0, 1.5, 2.0 and 3.0. 7. For each length determine the period by dividing the average by ten. 8. Calculate (period)2 for each length.

Results:

|Length (L) (m) |Time for 10 oscillations (t) (s) |Period (T) (s) |(Period)2 (T2) | | | | |(s2) | | |Trial 1 |Trial 2 |Trial 3 |Trial 4 | | | | | | | | | | |

Analysis of Results:

Graph of T vs. L- Parabolic curve of the form y2 = x

- Vertex on (0,0)...

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