In the EV3-G software, you could use negative numbers for power and rotations. In this episode, we would look at what is the meaning of this numbers and make a few notes of where the teacher must be more careful.

**#400**- 17 Mar 2017
- 2:12

- LEGO MINDSTORMS
- Teacher's Note, Methodology, EV3, Classes with students, Programming
- K.Mitov, A.Pavlov

### Minus * Minus = Plus

Many years ago I asked my teacher - "Why is minus times minus equals plus". He gave me an explanation that I do not remember. To make the students remember your explanation you could use the robots. Set the power to minus and the robot will move backward. Set also the rotations to minus and will move minus the direction it is currently moving and this means forward. So minus * minus is equal to a plus - moving forward.

### English

A quick note for the teacher for when the students start using negative numbers for power and for rotations.

The positive rotation of the wheel is in this direction. And when you set rotations + the wheel rotates in this direction. And if you set rotation - it will rotate in this direction. But the positive power is again forward. And the negative power backward. What will happen if you set a negative power to rotate backward and in the same time a negative number of rotations for this power? This means that we'll rotate forward. So, we have one negative and the negative of this negative makes the whole wheel move forward. Let's go to the software, configure it and then run the program. In the software we have our steering block and for this steering block we can set the number of.. Sorry, this is the tank block. I'll just drag and drop the steering block

and in the steering block I'll set the number of rotations to -1.

This makes the robot move backward and then I'll set the power to -50.

And if you ever get a question from a student why minus multiplied by minus gives you a positive number you can just demonstrate this with the robots and with the power and the rotations of this motor. So, if you move minus rotations with a minus power this will move the whole wheel forward and from there the whole robot forward. Download and run.

## Курсове и занятия включващи този Урок

Този Урок е използван в следните курсове и занятия.

### EV3 Phi. Introduction to robotics with LEGO Mindstorms

The things that you will be able to do with your EV3 robot by the end of this course are:

Freely move your robot towards desired target;

Avoid obstacles on the path of your robot using sensors;

Follow lines of any shape;

Detecting and picking up objects of any kind;

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- 220:20
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### Programming Arc turns. Adjusting robot speed

Let's make a competition - there is a pole that the robot must turn around and return home.

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