# Relative speed

Two pieces of furniture * A* and

*moving in the same path. We can say that their velocities are respectively v*

**B**_{A}and v

_{B}. In physics, we define the

**relative speed**of

*with respect to*

**A***as the difference between the speeds of mobile*

**B***and mobile*

**A****. So we have:**

*B**vAB = _{vA} – _{vB }_{_}*

This means that everything happens as if mobile B is stationary and mobile A, relative to it, is moving with a speed v _{AB} .

In many **kinematics** exercises , the relative velocity sign is not used much, that is, its value in modulus matters. In order to calculate the relative velocity value in module, we can make use of two practical rules:

*|v _{rel} |= |v _{A} |- |v _{B} |*

2nd case: if the two pieces of furniture are moving in opposite directions, the absolute value of the relative speed is given by the sum of the modules of the two scalar speeds. So we have:

*|v _{rel} |= |v _{A} |+ |v _{B} |*