Question

can the speed of a particle be negative? can the velocity of a particle be negative? give arguments.

Answer 1

speed can not be negative because it is a scalor quantity which can be expressed only by their value and unit
while velocity is a vector quantity and can be negative... negative velocity means that body is moving in opposite direction
hope this helps..and please mark me brainless

Answer 2

To understand this question , you need to understand scalar quantity and vector quantity. 

Scalar quantity is a quantity with only magnitude. 
Vector quantity is a quantity with magnitude and direction.
(Magnitude means how much, like how big a cube is , 3cm^3, or like how long is my house to my school, 1km. 1km and 3cm^3 are magnitude)

Speed = scalar quantity
Velocity= vector quantity

As I said scalar quantity only involve in magnitude but no direction, so speed can only be in positive value. There's no such way to express speed as negative. While velocity can have negative value, which indicate an opposite direction to the reference point. 

eg. A car move 300m to the west in 20seconds , and after that it move 500m to the east in 40seconds.
Firstly, car move left 300 m , and move right 500 m. 
We let the starting point as reference point. Many people has a misconception that they think right side is always positive and left side is always negative, but actually the first direction the object move to is positive, so here the answer is 
velocity = (300+500)/(20+40)
=13.33m/s 
and the direction is opposite to the first move direction, so answer is -13.33.