# Homework Help: Net Force

1. Jul 16, 2009

### dance_sg

1) A student driving her 1.60 x 102 kg car at 50.0 km/h presses on the gas pedal and accelerates to 90.0 km/h in 12.0 s. What is the average net force applied by the car’s engine during this time interval?

2) v=vf-vi, f=ma, a=v/t

3) i found v by subtracting 90.0km/h by 50.0km/h. i also converted 12.0s to hours which gave me 0.003 repeated. Then i tried finding acceleration by dividing 40km/h by time. then i plugged that in the force formula and multiplied it by 1600kg, but when i find my answer, it is not one of the answers i can chose from.

2. Jul 16, 2009

### cepheid

Staff Emeritus
Your acceleration would be in units of km/h2. When you multiply this by kg, you get some unit of force, but it is certainly not newtons!

You should convert everything to SI units.

3. Jul 16, 2009

### dance_sg

what are SI units?

4. Jul 16, 2009

### cepheid

Staff Emeritus
Well, you should know this, because it's basically the first thing taught in introductory physics classes in school. In any case, the SI (Systeme International) unit system, also called the metre-kilogram-second unit system, is derived from the metric system and is a system of units in which everybody agreed that the metre, kilogram, and second would be the basic units of length, mass, and time, respectively. So the units of m, kg, and s are SI base units, and all other units are derived from them (i.e. all other units are combinations of these three). Actually there is one other SI base unit, the ampere, which measures current.

EDIT: It looks like I overlooked the kelvin, candela, and mole, but they are not relevant to problems involving motion.

5. Jul 16, 2009

### dance_sg

ok, makes sense.
but the chioces given to me are all in newtons..

6. Jul 16, 2009

### cepheid

Staff Emeritus
The newton is an SI unit. The point I was making is that if you don't use SI units for length mass, and time in your calculations, then the force you calculate won't be in newtons. You should be keeping track of the units when you do your calculations so that you are aware of this.

A dimensional analysis:

[force] = [mass]*[acceleration] = [mass]*[length]*[time]-2

Therefore:

1 N = 1 kg m s -2

If you don't convert your distances from km to m and your times from h to s, then your force will be in units of kg km h -2, which is NOT a newton and is therefore NOT SI.