Please see the general programming webpage for details about the programming environment for this course, guidelines for programming style, and details on electronic submission of assignments.

Collaboration Policy

For this assignment, you are allowed to work with one other student if you wish (in fact, we suggest that you do so). If any student wishes to have a partner but has not been able to locate one, please let the instructor know so that we can match up partners.

Please make sure you adhere to the policies on academic integrity in this regard.

Contents:

• Overview
• Your Tasks
• Physics 101
• User Interface
• Files you will need
• Files to Submit
  • Source Code
  • Readme File
• Suggested Steps of Progress
• Examples
• Grading Standards
• Extra Credit

Overview

The goal of this assignment is to create a new class, Ball, which simulates the movement of a ball under the effects of gravity. For this assignment, we will provide all of the other portions of the code, including a driver for gathering input from the user as well as all of the necessary graphics for visualizing the simulation. However you must define and implement the Ball class properly for the simulation to be complete.

Your Tasks

Your main task is to define and implement a class Ball as described here.

How you choose to represent the private aspects of your class is up to you. In some manner, you will have to define data members which can represent the state of a ball, namely its current position and current velocity.

As far as the public interface of your class, you must include each of the following method prototypes, defined precisely as described here. The precision is necessary because your code must be integrated into the remainder of the project.

• Ball()
• float GetPositionX() const
• float GetPositionY() const
• float GetVelocityX() const
• float GetVelocityY() const
• void SetPositionX(float val)
• void SetPositionY(float val)
• void SetVelocityX(float val)
• void SetVelocityY(float val)
• void Update(World& W)

Physics 101

• position
  The position of the ball is relative to the world's coordinate system. For simplicity, you might choose to represent the position vector as two separate values, separately representing the X- and Y-components of the position (though you may choose to use the Position class which we have seen within EzWindows).

• velocity
  A velocity is not simply a representation speed, but also of direction of motion. Therefore, a velocity is also represented by a pair of coordinates.n the real world, a velocity might be measured in units such as meters per second (m/s); in our world, we will measure velocity in terms of cm per clock tick. Therefore, if a ball had a constant velocity of (1.3, -3.5), then for each clock tick the position would be increased by 1.3 along the X-axis, and decreased by 3.5 along the Y-axis (such a ball would be moving "north northeast" on the computer screen)

• gravity
  By the law of inertia, in the absence of any outside forces the velocity would remain unchanged. Yet gravity is such an outside force. At each clock tick, the gravity does not directly effect the position of a ball, but it will directly effect the velocity of the ball.

  In general, such a force should also be represented as a vector, and thus have separate X- and Y-coordinates. For simplicity, we assume that gravity in our world is aligned with the Y-axis, and that the magnitude of the gravity is a property of the World. A positive value for the gravity means that balls are being pulled downward (i.e., along the positive Y-axis); in contrast, a negative gravity value would be interpreted as balls being pushed upward.

  From within the Update(World& W) routine, you may query the given world for the current gravity setting via W's GetGravity() method.

User Interface

We will be providing you with a simple user interface titled sim. This text-driven menu first directs you through a series of questions to define the characteristics of the World. After setting up the world, the driver prompts you to introduce a ball with a given set of initial characteristics. It then simulates the motion of that ball so long as the ball remains in the visible portion of the world. Once the ball leaves the world it is destroyed and the driver will again prompt you to create a new ball.

You should note that our driver intentionally suggest default responses for each question posed, and you may signify that you are willing to accept the default by simply pressing return. This should make it much easier for you to repeat certain tests by a series of accepted defaults.

Files You Will Need

We are providing the following files for your use. You may either download from a browser or copy them directly to your current directory on turing with the comand:

cp -Rp ~goldwasser/csp125/programs/simulation .

There are several files:

• ball.h, ball.cpp
  These are the two files which you will be developing. The initial files we are providing are relatively vacuous, though they do contain some necessary syntax to handle the combination of files.

• world.h, world.cpp
  These are files we have created to define and implement the World class. You do not need to read or understand these files to complete the assignment, though you are welcome to peruse if you wish as it provides an example of a class definition and implementation.

• sim.cpp
  This file defines the front-end driver we are using. There is really little reason for you to read this file as it is mostly 'grunt work'.

• Makefile
  You should simply type make at the command prompt and then your program will be compiled. Either syntax errors will be displayed, or else the compilation will be successful and an executable named sim will be created which you can run.

Files to Submit

• ball.h, ball.cpp
  These two files are the only source code which you will write. They contain the Ball class definition and implementation, respectively.

• Readme File
  

  Again, we also ask for you to estimate the amount of time you spent on the assignment, and to let us know of any difficulties you had or other issues you wish to discuss.

  If you worked as a pair, please make sure that both names are given and that you discuss how you each contributed to the submitted work.

Suggested Steps of Progress

You may wish to tackle the assignment in stages, making sure to succeed in each stage before continuing to the next.

1. The original set of files we have provided will not even compile properly. To get to this point, you will have to define your Ball class, so that its public interface precisely matches the specifications described above.

2. For the ball to appear properly on the screen, you will already need to use instance variables and to successfully implement the methods for setting and getting the Ball's position.

3. Start to implement the Update routine, ignoring gravity for the time being. You should use the velocity to control the movement of the ball's position.

4. Finally, take the gravity into account and put everything together.

Examples

For the sake of comparison, we have made our solution available for you to execute. You will find it at:

~goldwasser/csp125/demos/simulation/sim

Here is an image based upon the default configuration settings used in the driver:

Grading Standards

The assignment is worth 10 points. If you worked as a pair, you will each be given the same grade.

Extra Credit (1 point)

We will award one additional point to students who successfully meet the following challenge. In the required assignment, the gravity of the world was viewed as a constant and was aligned with the Y-axis. For extra credit, we wish to have you adjust your ball implementation so that it reacts properly to the gravity of a star (in addition to the gravity of the World in the standard case).

Physics 102:
Incorporating the gravity of a star is more complicated than the gravity of a world, for two reasons. First, the gravity of the world was assumed to be a vector aligned with the Y-axis, thus of the form (0,g) for some value g. The gravity between the star and a ball should be a vector pointing from the ball towards the star. Secondly, the actual strength of the force depends on the distance between the center of the ball and the center of the star, as well as the mass of the star. Specifically, the magnitude of the force should be equal to the star's mass divided by the square of the distance between the star and ball's centers.

Coding Issues:
There are some additional issues to cope with, though this is extra credit after all. For simplicity, at most one star can exist in the world at any given time. You may query information about the world through the following methods of the World class:

• bool HasStar() -- returns true if there is a star

• float GetStarX() -- returns the x-coordinate of the star's position

• float GetStarY() -- returns the y-coordinate of the star's position

• float GetStarMass() -- returns the mass of the star.

For the sake of comparison, here is an image based upon the default extra credit configuration settings used in the driver: