# Two energy problems I do not understand

• jefgreen
In summary: Remember that the "work" represented by the breaking force is equal to the change in kinetic energy. This is true because there is no friction.
jefgreen
1.) In the 1950s, an experimental train that had a mass of 2.50x10^4kg was powered across a level track by a jet engine that produced a thrust of 5.00x10^5 N for a distance of 509m.

A-Find the work done on the train.
B-Find the change in Kinetic Energy.
C-Find the final kinetic energy of the train if it started from rest.
D-Find the final speed of the train if there were no friction.
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A...W=Fd...W=(5.00e^5N)(509m)...W=2.5e^8.

B,C, and D I cannot understand. For part b, I believe you use KE=1/2mv^2, but I have no velocity..., if I had a quantity for velocity, I would be able to find ΔKE by doing K1=...,K2=... and finally...W=K2-K1=...J.

For c and d, I'm totally lost.
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2.) A 14,700N car is traveling at 25m/s. The brakes are applied suddenly, and the car slides to a stop. The average braking force between the tires and the road is 7100N. How far will the car slide once the brakes are applied?
----------
Well, I know this:
F=14,700 N
Vi=25 m/s
Vf=0 m/s
avg. braking force=7100 N
and I'm assuming that I need to find d=...?

What formulae would I use to find the answer? I know these:

KE=1/2mv^2
W=ΔKE
W=fd. I'd be able to solve for d, but how would I go about finding the value for W?

Last edited:

jefgreen said:
1.) In the 1950s, an experimental train that had a mass of 2.50x10^4kg was powered across a level track by a jet engine that produced a thrust of 5.00x10^5 N for a distance of 509m.

A-Find the work done on the train.
B-Find the change in Kinetic Energy.
C-Find the final kinetic energy of the train if it started from rest.
D-Find the final speed of the train if there were no friction.
-------
A...W=Fd...W=(5.00e^5N)(509m)...W=2.5e^8.

B,C, and D I cannot understand. For part b, I believe you use KE=1/2mv^2, but I have no velocity..., if I had a quantity for velocity, I would be able to find ΔKE by doing K1=...,K2=... and finally...W=K2-K1=...J.

True, the velocity was not given. But you know the amount of work put into the system (part A). Work is a measure of energy. If you ignore friction, where does this work go? (Hint: kinetic energy is also a measure of energy ) I'm assuming you need to ignore friction for part B.

For c

Part C is dependent upon your answer for B. What is the kinetic energy of a train at rest?

and d, I'm totally lost.

Once you have the answer to part C, you already have the information and the equation you need to find the velocity.

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2.) A 14,700N car is traveling at 25m/s. The brakes are applied suddenly, and the car slides to a stop. The average braking force between the tires and the road is 7100N. How far will the car slide once the brakes are applied?
----------
Well, I know this:
F=14,700 N
Vi=25 m/s
Vf=0 m/s
avg. braking force=7100 N
and I'm assuming that I need to find d=...?

What formulae would I use to find the answer? I know these:

KE=1/2mv^2
W=ΔKE
W=fd. I'd be able to solve for d, but how would I go about finding the value for W?

Now you're thinking. Both work (caused by the breaking force) and KE = 1/2mv^2 are measures of energy. In this problem, the initial kinetic energy is converted to "work" done on the breaks. Since you know the force on the breaks, it shouldn't be too tough to calculate the distance.

## 1. What are the two energy problems you do not understand?

The two energy problems I do not understand are the concept of dark energy and the issue of energy sustainability.

## 2. What is dark energy?

Dark energy is a theoretical form of energy that is believed to make up about 70% of the universe. It is thought to be responsible for the acceleration of the expansion of the universe, but its exact nature and properties are still not fully understood.

## 3. Why is dark energy important to study?

Studying dark energy is important because it has a significant impact on the evolution and fate of the universe. Understanding its properties can also help us better understand the fundamental laws of physics and the nature of space and time.

## 4. What is the issue with energy sustainability?

The issue with energy sustainability is that our current energy consumption and production methods are not sustainable in the long term. Fossil fuels, which are our main source of energy, are finite resources and their extraction and use have negative impacts on the environment. We need to find alternative, renewable sources of energy to ensure a sustainable future.

## 5. How can we address the issue of energy sustainability?

There are several ways we can address the issue of energy sustainability, including investing in and promoting the use of renewable energy sources such as solar, wind, and hydro power. We can also improve energy efficiency by using technology and practices that reduce energy consumption. Additionally, implementing policies and regulations that promote sustainable energy practices can also help address this issue.

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