# Space notes (Physics 2014 year 12)

**Topics:**Special relativity, General relativity, Speed of light

**Pages:**17 (4988 words)

**Published:**February 15, 2014

1. Earth has a gravitational field that exerts a force on objects both on it and around it.

K1 Define weight as the force on an object due to a gravitational field. Mass is the amount of matter within an object, measured in kg Weight is the force of gravity acting on an object’s mass measured in newtons (N). Weight is a force thus a vector quantity.

K2 Explain that a change in gravitational potential energy is related to work done. To move an object within a gravitational field, work must be done.

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:

E p mgh However this equation is valid only when the object is near the Earth’s surface.

K3 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 The closer the object is to the center of the gravitational field the less work required to move it there. As the object gets closer the kinetic energy becomes greater, as energy can’t be lost the gravitational potential energy converts into kinetic energy The further away an object is pulled away from a gravitational field the weaker the pull will be on the object decreasing the amount of energy it has there.

S1 Perform an investigation and father 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

Experiment procedure:

1. A mass was tied to a piece of string one metre long and was attached to a clamp connected to a retort stand. 2. The length (l) measured in metres (m) of the pendulum was measured from its point of attachment to the centre of its mass. 3. The pendulum was pulled aside and released and a stopwatch began timing. After ten full swings, the stopwatch was stopped and results recorded. 4. The period was divided by 10 and then squared. A graph of T2 (horizontal) and l (vertical) [remember by the equation of g = constant x l/T2] was made and a random value (very important not to use experimental results) was used to gain a more accurate value. 5. The period was divided by 10 and the following equation was used:

By rearranging the equation an expression to calculate g was found:

To increase accuracy:

Gentle swings were used to minimise jerking, twisting or stretching of the string. 10 full swings were used to reduce human error with timing.

Reducing accuracy:

Human error from timing.

Friction between the clamp and string.

Air resistance.

To increase reliability of results:

Graph the results with T2 (horizontal) and l (vertical) to eliminate any outliers and gain a more accurate value for g. Repeat the experiment several times.

Compare to other classmates results.

Validity:

By comparing to reliable sources, the results can be considered valid or invalid. Should be close to 9.8ms-1. S2 Gather secondary information to predict the value of acceleration due to gravity on other planets S3 analyze information using the expression F=mg to determine the weight force for a body on earth and for the same body on other planets

2. Many Factors have to be taken into account to achieve a successful rocket launch, maintain a stable orbit and return to earth.

K1 Describe the trajectory of an object undergoing projectile motion within the earths gravitational field in terms of horizontal and vertical components. Any moving object that moves only under the force of gravity is a projectile. The horizontal motion of a projectile is independent to the...

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