We now turn out a attention to energy. Like momentum, energy is intricately connected to the concept of force.
However, the concept of energy took a little bit more time to develop. In essence, it's the ability to do work. In the context of classical mechanics, this energy could be related to an object in motion or its position in a field.
In this lesson, we will only examine work that result in a change of gravitational potential energy or kinetic energy or both.
However, the concept of energy took a little bit more time to develop. In essence, it's the ability to do work. In the context of classical mechanics, this energy could be related to an object in motion or its position in a field.
In this lesson, we will only examine work that result in a change of gravitational potential energy or kinetic energy or both.
Work and Energy
Work and energy are interrelated.
The odd thing is, they are often defined in terms of the other.
The odd thing is, they are often defined in terms of the other.
"Energy is the ability to do work"
"Work results in a change of energy"
"Work results in a change of energy"
In essence, if a force is applied on an object and the result is a change in displacement in the same directions as the force, we do work on that object. Therefore there is a change in the energy of that object
This energy can be one of many forms.
Take for example the engine applying a force to a car. It does work and it results in an increase in velocity. Its kinetic energy increases.
When we lift a ball, we change its displacement, increasing its height. It has an increase in gravitational energy.
What if I drag a chair across the room at constant speed. Am I doing work, even though it does not speed up nor increase in height?
Yes!
In this case the work I do results in the generation of heat and sound energy as we have the frictional effects between the chair and ground.
You get the idea?
Watch the video below as I discuss the relationship between work and energy: the work energy theorem. It includes how the formulas are derived.
This energy can be one of many forms.
Take for example the engine applying a force to a car. It does work and it results in an increase in velocity. Its kinetic energy increases.
When we lift a ball, we change its displacement, increasing its height. It has an increase in gravitational energy.
What if I drag a chair across the room at constant speed. Am I doing work, even though it does not speed up nor increase in height?
Yes!
In this case the work I do results in the generation of heat and sound energy as we have the frictional effects between the chair and ground.
You get the idea?
Watch the video below as I discuss the relationship between work and energy: the work energy theorem. It includes how the formulas are derived.
Sample Problem
We are now ready to try a sample problem
Below is a sample problem with a video that explain how to solve it. It is suggested you try the problem beforehand, as this actually aids understanding, even if you are unsure if you are correct.
We are now ready to try a sample problem
Below is a sample problem with a video that explain how to solve it. It is suggested you try the problem beforehand, as this actually aids understanding, even if you are unsure if you are correct.
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I have a number of worksheets that you can download on various aspects of work
Conservation of Energy
We now move to another important law of Physics - the conservation of energy.
In essence, it states that the total energy of a system stays constant. Energy can be transferred or transformed, but at no stage do we create energy nor lose energy
Interactive
We will start by first examining this interactive from pHET, called Skate Park
In essence, it states that the total energy of a system stays constant. Energy can be transferred or transformed, but at no stage do we create energy nor lose energy
Interactive
We will start by first examining this interactive from pHET, called Skate Park
- Starting at the intro TAB, place a skater on the ramp. It will have gravitational energy (U) but no kinetic energy (K).
- Reveal the bar graph
What do you notice about the value of U as the skater goes down? What about K?
What about the total energy?
Since the total energy remains the same, is it possible for the skater to reacher a higher height than from where it started? - Now look at the friction TAB. This simulates reality better.
- Again, look at the bar graphs.
Which bar does NOT change?
Hopefully you now have a conceptual understanding of the conservation of energy
Next watch the video - make sure you take notes.
Next watch the video - make sure you take notes.
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Newton Cradle
The Newton's Cradle is a cool office toy that allows good discussion of both momentum and energy. Using a Newton's Cradle and an air track, with which I apply video analysis, I discuss energy as they relate to elastic and inelastic collisions |
Sample Problem
We are now ready to try a sample problem
Below is a sample problem with a video that explain how to solve it. It is suggested you try the problem beforehand, as this actually aids understanding, even if you are unsure if you are correct.
We are now ready to try a sample problem
Below is a sample problem with a video that explain how to solve it. It is suggested you try the problem beforehand, as this actually aids understanding, even if you are unsure if you are correct.
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- A graphing problem - using graphs from a datalogger, students need to interpret elastic vs inelastic collisions
Work and Power
If two objects have the same amount of work done, then their change of energy is the same, irrespective of the time that took place.
For example, Tom and Jerry are two removalists and each lift a chest of drawers on to their truck, both having the same mass.
Tom takes three times as long to do the job than Jerry. Who does the most work?
The answer is the same as they both have the same increase in gravitational energy.
Bu who is more powerful ie has more power?
In that case its Jerry as he took less time
So the rate of change of energy is called power
For example, Tom and Jerry are two removalists and each lift a chest of drawers on to their truck, both having the same mass.
Tom takes three times as long to do the job than Jerry. Who does the most work?
The answer is the same as they both have the same increase in gravitational energy.
Bu who is more powerful ie has more power?
In that case its Jerry as he took less time
So the rate of change of energy is called power
The unit for Power is J/s or the Watt (W)
Since W = F.s
Since W = F.s
Try the following worksheet
Consolidation
Let's consolidate what we have learnt.
There's a lot of physics involved in car crashes. This video looks at the basics: acceleration, forces, momentum and energy.
There's a lot of physics involved in car crashes. This video looks at the basics: acceleration, forces, momentum and energy.
