# Find total force resisting motion.

1. Sep 3, 2010

### Bananahat

A boy and his bicycle have a combined mass of 45kg. When traveling at 20km/h on a level road, he ceases to pedal and finds his speed reduced to 16km/h in 10 seconds. What is the total force resisting motion?

Thanks for reading! It should be easy for me but I just can't figure it out. If someone could please provide me with the calculations or formula that would be great!

I know the answer is 5 N, But have no idea how to get it.

2. Sep 3, 2010

### CompuChip

How about you start by posting some relevant formulas.
For example, there is something about a change in velocity with the time given, and something with a force... surely you can find some formulas in which they occur?

3. Sep 3, 2010

### Bananahat

In fact I have found the answer, I had the right formula all along. I forgot to input brackets into my calculator, so I was getting the wrong answer. -_-

But here's my working out if anyone has a similar problem.
First I changed Km/h to m/s 20km/h = 5.55 16km/h = 4.44

I then used the equation: Final velocity = Initial velocity + Acceleration x time
Transformed that into :
Final velocity - Initial velocity / time = Acceleration

which is (4.44 - 5.55) / 10 = -.111 (negative acceleration)

Acceleration is also equal to Force\Mass. And that Force is then
.111 x 45 = 4.995 Newton! keep in mind the boy stopped peddling, thus the only force left was a resistant force. Hope that made sense.