# Lec 2 | MIT 18.085 Computational Science and Engineering I, Fall 2008 | Summary and Q&A

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February 25, 2009
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Lec 2 | MIT 18.085 Computational Science and Engineering I, Fall 2008

## TL;DR

This video lecture introduces the concept of solving differential equations by using difference equations, with a focus on solving a boundary value problem using finite differences.

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### Q: What is the main difference between an initial value problem and a boundary value problem in differential equations?

An initial value problem involves finding the solution to a differential equation given an initial condition, while a boundary value problem involves finding the solution to a differential equation given conditions at multiple points or boundaries.

### Q: Why is it necessary to replace a differential equation with a difference equation when solving numerical problems?

Difference equations are easier to solve numerically because they involve discrete quantities and can be represented by matrices. It allows for the transformation of a continuous problem into a more manageable discrete problem.

### Q: What is the advantage of using centered differences compared to other finite difference approximations?

Centered differences provide higher accuracy in approximating derivatives compared to other first difference approximations. It is a second order accurate method, which means the error between the approximation and the exact derivative is of order h^2 rather than h.

### Q: How does boundary condition affect the accuracy of the numerical solution obtained from the difference equation?

The choice of boundary condition can significantly impact the accuracy of the numerical solution. Boundary conditions that approximate the true behavior of the solution at the boundary points can lead to higher accuracy, while poorly chosen boundary conditions can introduce errors and affect the overall accuracy of the approximation.

## Summary & Key Takeaways

• The lecture begins by explaining the difference between an initial value problem and a boundary value problem in differential equations.

• The professor demonstrates how to approximate a differential equation by using difference equations, specifically using finite differences.

• The lecture provides examples of different types of finite difference approximations, such as first differences, centered differences, and second differences.

• The professor solves a boundary value problem using finite differences for a uniform bar subjected to a uniform load. The exact solution is compared to the numerical solution obtained from the difference equation.