Power

Investigation Information

Sorry about ze late upload.

Work and Energy Transformation

In our investigation, we need to know about energy transformations happening in the swinging masses ("wrecking ball") and the work done (by friction) when the swinging mass hits the block ("building").

Gravitational Potential Energy

In our investigation, we found that we needed to know how to calculate the Gravitational Potential Energy of the swinging mass. Mr Nicoll gave a quick overview of how to do this, using a diver on a board as a context:

The key thing to note is that all units need to be converted to SI Units (kg for mass, m for height).

From this information, we could actually calculate the speed the diver hits the water at (assuming no energy was "lost" due to friction during the dive). This will be covered in tomorrow's lesson.

Wrecking Ball Inquiry

This week, we are investigating Energy, Energy Transformations and Work. We are investigating the collision between a swinging mass and a block. This should give us an idea about the forces involved when we change a variable about the swinging mass ("wrecking ball").

We have been given some guidelines for what needs to be achieved and when these should be achieved by:

Acceleration

Acceleration is derived from Δvelocity (in m s^-1) and Δtime (in s). This means we need to convert other units (km hr^-1 and min, for example) into SI units.

This is the same as:

a = Δv ÷ Δt

Δv = a × Δt

Δt = Δv ÷ a

On Graphs

On a d-t graph, acceleration looks like this:

Deceleration (negative acceleration) looks like this:

On a v-t graph, acceleration looks like B and D in the following graph. B is a higher rate of acceleration than D. Deceleration is represented by A (negative gradient). C is uniform motion (constant speed).

Calculating Acceleration from a Graph

To do this, you need a v-t graph. Acceleration is the gradient of the line on a v-t graph. This is the skill you are expected to be able to do to access Merit in NCEA. A v-t graph can also be used to calculate the distance covered (area under the line of the graph).

Alternatively, you use the tangent of the curved line for two points in a d-t graph (but this is very inaccurate). You then use this to find the initial and final velocity, and calculate acceleration from this. See Mr N if you really want to know how to do this!

Speed

Speed is derived from distance and time. This is written as:

v = d/t

d is (usually) measured in metres, but can also be measured in other units such as km, cm and mm
t is (usually) measured in seconds, but can also be measured in other units such as minutes and hours
Therefore, v is usually stated in metres per second (m s^-1). However, when other units are used for distance and time, we can have other units such as km hr^-1.

NOTE #1

1 km hr^-1 = 0.277 m s^-1
1 m s^-1 = 3.6 km hr^-1

NOTE #2

This equation only works for uniform motion (constant speed or stopped).

From Graphs

Speed can be calculated from a d-t graph by finding the gradient (rise/run).