# Homework Help: Forces - Solved, but wrong?

1. Oct 2, 2007

### Lorelyn

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

A 21000 kg helicopter is lifting a 5000 kg truck with an upward acceleration of 2.00 m/s2. Calculate the force the air exerts on the helicopter blades.

2. Relevant equations

F=ma

3. The attempt at a solution

I did the total force is equal to the sum of the two masses(26000kg) times the acceleration (2) which gave me 52000N. Which is apparently wrong but I have no idea why...

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

A car that weighs 14200.0 N is initially moving at a speed of 44.0 km/hr ((or 12.222)m/s) when the brakes are applied and the car is brought to a stop in 4.1 s. Find the magnitude of the force that stops the car, assuming it is constant.

2. Relevant equations

F=ma, Vf = Vi+ at

3. The attempt at a solution

So I solve using kinematics to get the acceleration (-12.22=(a*4.1)) so a = 2.98m/s^2. Which I then multiply by 14200/9.8 (the mass) to get the force equaling 4319.45N. Which is, again, wrong. And as with the above question I don't know why. I've done everything right as far as i can tell...
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 2, 2007

### Staff: Mentor

helicopter

You found the net force, but the question asks for the force of the air. What other force acts on this system?

3. Oct 2, 2007

### Staff: Mentor

car

This one looks fine to me.