# Arithmetic

Write out the following code fully (do not copy and paste!) and run it to inspect what it does.

```
1public class Numbers {
2 public static void main(String[] args) {
3 // Look, numbers!!!
4 System.out.println(2);
5 System.out.println(5034);
6
7 // Look, arithmetic!!!
8 System.out.println("How old am I?");
9 System.out.println(5 + 20);
10
11 // Look, negative numbers!!!
12 System.out.println("What is 3229 minus 234234?");
13 System.out.println(3229 - 234234);
14
15 // We can also have numbers and strings on the same line
16 System.out.println("How tall am I in m? " + 2);
17 System.out.println("How tall am I in cm? " + (2*100));
18 System.out.println("How many eggs do I own? " + 36);
19 System.out.println("How many dozens of eggs do I own? " + (36/12));
20
21 // Remainders and division fun
22 System.out.println("What is the remainder of 6 divided by 5? " + (6 % 5));
23 }
24}
```

### Explanation

As you can see integers in Java can be added, subtracted, multiplied and divided. The symbol for multiplication is

`*`

and division is`/`

. In Java this follows the same rules as maths, i.e. BIDMAS (Brackets, Indices, Division, Multiplication, Addition, Subtraction). So`2 + 3 * 5`

will be treated as`2 + (3 * 5)`

not`(2 + 3) * 5`

. A symbol you might not be familiar with is in line 22:`%`

, the modulo operator. The modulo operator calculates the remainder when a number is divided by another number. So`6 % 5`

is`1`

as the remainder of`6/5`

is`1`

. You can also use parentheses/brackets to make calculations clearer.It is important that you recognise that while

`System.out.println(3229 - 234234)`

makes sense,`System.out.println(3229) - 234234`

does not.`System.out.println(3229) - 234234`

makes no sense as you're saying print`3229`

and then subtract`234234`

from whatever`System.out.println(3229)`

equals. However,`System.out.println(3229)`

isn't a number and doesn't equal a value, so you are trying to subtract`234234`

from nothing.In line 16 you can see that you can add, or rather

**concatenate**, a string and an integer by using the`+`

**operator**. For example,`"aaa" + 5`

becomes`"aaa5"`

.In line 17, in the expression

`"How tall am I in cm?" + (2*100)`

,`(2*100)`

is first simplified to`200`

and then concatenated with`"How tall am I in cm? "`

. Although technically you don't need to put parentheses, as`*`

has a higher precedence in BIDMAS than`+`

.

### Exercises

Write a program to work out what the power in watts of a device is if the voltage is 10V and the current is 6A, and print the result. Hint: Power (W) = Current * Voltage

Write a program to work out the kinetic energy of an object with a mass of 500kg that has a velocity of 12m/s, and print the result. Hint: Kinetic Energy = 0.5 * Mass * (Velocity)

^{2}Write a program to answer the following physics question and print the result. If a car has a mass of 800 kg and moves with a velocity of 25 m/s, what force is needed to stop the car in 50 metres? Hint: You will need the equation Energy = Force * Distance and the previous exercise.

Write a program to work out the missing angle of a triangle which has two known angles of 108 degrees and 24 degrees. Hint: The angles of a triangle must sum to 180 degrees.

Write a program in which you work out the percentage efficiency of a light bulb that is supplied with 50J (Joules) of energy, and uses 2J to produce light, leaving 48J wasted as thermal energy. Then, print the result. Hint: percentage efficiency = (useful power)/(total power).