# Need help with kinetic energy calculation

• Warren
In summary, the conversation is about someone asking for help with calculating the kinetic energy of a person falling from a diving board. They clarify that English is not their first language and provide their attempted calculation. However, the textbook gives a different answer. The conversation turns to finding the person's velocity as they reach the surface and using conservation of energy to calculate their potential energy and kinetic energy. The person is advised to work backwards to find the velocity.
Warren
Hi

This seems like a helpful community so I thought I could ask you guys for some help.

First of all I need to clarify that English is not my first language so you'll have to excuse me if I make any grammatical or spelling errors.

Ok, here's my question:

I'm am trying to calculate the kinetic energy of a person who is standing on a diving board 8 meters above the surface. He weighs 80kgs.

What is his kinetic energy after falling 2m ?

I think it's 1/2*80*9,8^2 = 3841,6 joule

But the textbook tells me I have the wrong answer

I also have to find out his velocity as he reaches the surface, and I have no clue where to even begin.

Any help would be appreciated, thank you.

You appear to have used the ke equation, without knowing his velocity - where did you get 9'8 from??

This is about conservation of energy.

Calculate how much PE he loses in falling 2m.

This is the ke he has after 2 metres.

For the speed as he hits the water, calculate how much PE he has lost by this point, this gives you the KE. Work backwords to find the velocity.

Hello, thank you for reaching out for help with your kinetic energy calculation. It seems like you are on the right track with your formula, but there may be some errors in your calculation. Let's break it down step by step to see where the mistake might be.

First, we need to find the potential energy of the person standing on the diving board. This can be calculated by multiplying the mass (80kg) by the gravitational acceleration (9.8 m/s^2) and the height (8m). This gives us a potential energy of 6272 joules.

Next, we can use the formula for kinetic energy, which is 1/2 * mass * velocity^2. Since the person starts from rest at the top of the diving board, their initial kinetic energy is zero. When they reach the surface, their potential energy is converted into kinetic energy, so we can set the two equal to each other.

6272 = 1/2 * 80 * v^2

Solving for velocity, we get v = √(6272/40) = 11.16 m/s. This is the velocity of the person as they reach the surface.

To find the kinetic energy at a height of 2m, we can use the same formula but substitute the height of 2m for the initial height. This gives us a kinetic energy of 6272 - (1/2 * 80 * 2^2) = 6112 joules.

I hope this helps clarify things for you. Remember to always check your units and double check your calculations. Best of luck with your calculations!

## 1. What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is calculated by multiplying an object's mass by the square of its velocity and dividing by 2.

## 2. How do I calculate kinetic energy?

The formula for calculating kinetic energy is KE = 1/2 * m * v^2, where KE is kinetic energy, m is the mass of the object, and v is the velocity.

## 3. Why is kinetic energy important?

Kinetic energy is important because it helps us understand the amount of energy an object has due to its motion. This can be useful in many fields, such as engineering, physics, and sports.

## 4. What are the units for kinetic energy?

The units for kinetic energy are Joules (J) in the metric system and foot-pounds (ft-lb) in the imperial system.

## 5. Can kinetic energy be negative?

Yes, kinetic energy can be negative. This typically occurs when an object's velocity is negative, meaning it is moving in the opposite direction as its initial motion. Negative kinetic energy represents a decrease in an object's energy due to its motion slowing down.

• Introductory Physics Homework Help
Replies
33
Views
1K
• Introductory Physics Homework Help
Replies
12
Views
207
• Introductory Physics Homework Help
Replies
1
Views
658
• Introductory Physics Homework Help
Replies
6
Views
1K
• Introductory Physics Homework Help
Replies
16
Views
1K
• Introductory Physics Homework Help
Replies
55
Views
2K
• Introductory Physics Homework Help
Replies
3
Views
906
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
2
Views
2K
• Introductory Physics Homework Help
Replies
19
Views
714