# Mach Number and Velocity

1. Jun 23, 2014

### LaReina

1. The problem statement, all variables and given/known data
An object is flying through the air at M=0.5. The free stream temperature is equal to 180 K. At what speed should the object fly when the temperature is 100 K in order to maintain the same Mach number? (therefore ensuring compressibility effects are the same). What was the speed of the first object.

2. Relevant equations
$M=\frac{V}{a}$

$a=\sqrt{γRT}$

3. The attempt at a solution
I've worked out the speed for the first object which is as follows
$a=\sqrt{1.4\times287\times180}=268.931m/s$
$V=0.5\times268.931=134.465m/s$

However when I work out the speed for the second temperature using the exact procedure, I get 100.225 as an answer. The answer that has been given is 88.52m/s.

2. Jun 23, 2014

### Staff: Mentor

Chet

3. Jun 25, 2014

### LaReina

$a=\sqrt{1.4\times287\times100}=200.448$
$V=200.448\times0.5=100.224$

4. Jun 25, 2014

### Staff: Mentor

This calculation looks OK to me.

Chet

5. Jun 25, 2014

### dauto

May be the question has a typo and it meant to ask what happens if the temperature drops 100K (which means it drops to 80K). That brings the answer closer to the answer provided.