A car that weighs 14900.0 N is initially moving at a speed of 57.0 km/hr when the brakes are applied and the car is brought to a stop in 4.8 s. Find the magnitude of the force that stops the car, assuming it is constant.

I found a using Vf=vi+at...a = 11.88 m/s^2. Then I found the mass of the car using Fwt = mg...mass = 1520.4 kg. Then I found EFx using EFx = ma...EFx = 18062.35.
But this is wrong. Where did I mess up?

I get a different answer for acceleration. Check your conversion from 57.0 km/hr to ?? m/s.

To solve this problem, you correctly used the equation Vf = Vi + at to find the acceleration. However, you made a mistake when calculating the mass of the car using the formula Fwt = mg.

To find the correct magnitude of the force that stops the car, you need to use the equation F = ma, where F is the force, m is the mass, and a is the acceleration.

First, convert the initial speed from km/hr to m/s:
Vi = 57.0 km/hr = (57.0 * 1000) / (60 * 60) = 15.83 m/s

Next, use the equation Vf = Vi + at to find the acceleration:
0 = 15.83 + a * 4.8
a * 4.8 = -15.83
a = -15.83 / 4.8 = -3.29 m/s^2

Now, to find the mass, you can use the equation Fwt = mg, where Fwt is the weight of the car and g is the acceleration due to gravity:
14900 N = mg
m = 14900 N / 9.8 m/s^2 = 1518.37 kg

Finally, substitute the calculated values of the mass and acceleration into the equation F = ma:
F = (1518.37 kg) * (-3.29 m/s^2) = -4987.11 N

The magnitude of the force that stops the car is 4987.11 N. Note that the negative sign indicates that the force is in the opposite direction to the initial motion of the car.