# Homework Help: Equation for momentum with headwind

1. Oct 21, 2012

### susiehk

1. The problem statement, all variables and given/known data

I'm working on an assignment where I need to develop an equation for the momentum of a truck of mass m moving at speed v into a headwind of speed V. Road temperature is T. The goal of this problem is for me to learn to use equations to guide my thinking.

2. Relevant equations

p = mv
Ft = mv

3. The attempt at a solution

Ah, I'm stuck not knowing how to handle the headwind.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 21, 2012

### Simon Bridge

ƩF=dp/dt
... so what happens as the truck barrels forward - are you imagining a constant speed or accelerating?
What does the truck to to the air and how does that affect the truck?

3. Oct 21, 2012

### susiehk

Thanks Simon. Yes, constant speed until it hits the headwind.

4. Oct 21, 2012

### Simon Bridge

OK - so the net force is the push from the wind?
How does the wind push on the truck?

5. Oct 21, 2012

### susiehk

I hadn't thought of the wind as a force since the problem was defining truck speed and wind speed. Hmm...

The wind is pushing head on in the opposite direction the truck is driving (if truck is driving north, wind is pushing south, for example).

I'm trying to use equations to guide my thinking. So, if the truck's momentum is mv (mass of truck x truck's velocity), can the wind have momentum given that it doesn't have mass (kg). In a way this is similar to a collision problem, but the wind has no mass, only velocity.

Thanks again!!

6. Oct 21, 2012

### Simon Bridge

Of course the wind has mass - air is made up of matter.
Now you can think in terms of collisions: every second, the truck gets hit by a volume of air that depends on the speed of the wind and the speed of the truck and something else. Knowing the local temperature, you can find the (approximate) mass-density of the air. You'll have to use the equations to guide your assumptions.

You have to use the equations ... so I think that is as far as I can go.
When you are done thinking about conservation of momentum, you could have a look at conservation of energy.

7. Oct 21, 2012

### susiehk

Thank you again, have a great evening.