# Homework Help: Science for Technicians

1. May 31, 2007

### fable121

1. The problem statement, all variables and given/known data
A 6000 Kg lorry is travelling at 80 km/h on a level road. It is brough to rest with a uniform retardation in 2 minutes. Calculate a) the retarding force and b) the distance travelled during retardation.

2. Relevant equations
now I know I've got to change the 80km/h into m/s, which is 80 x 0.2778 = 22.224 m/s. for distance travelled its something like S=ut+1/2*-at2. So if someone could explain how to go about doing that question I would be very greatful.

2. May 31, 2007

### stunner5000pt

did you calculate the retarding force ?

the second part can be solved using

$$d = v_{avg} t$$
where Vavg is the average velocity

3. May 31, 2007

### hage567

You need to find the deceleration, which you can do with the information in the question. Do you know the formula for that? You can then find the net force by using Newton's second law.

Your equation for distance will work fine, if you get all the variables right (including the signs).

4. May 31, 2007

### cepheid

Staff Emeritus
general method:

since you know the change in momentum of the truck (er, lorry), you know the impulse that was supplied. Since you also know the time interval over which this impulse was supplied, you can calculate the retarding force (assuming it is uniform, which is stated in the problem).

Furthemore, since you know the change in kinetic energy of the truck, you know the work done on it, and can therefore calculate the distance over which this work was done (since you know the retarding force). This strikes me as simpler/more elegant than resorting to kinematics to find the distance travelled, but maybe others would disagree.

5. May 31, 2007

### fable121

Ok I think I can do it now, thanks for the help all :)

6. May 31, 2007

### hage567

Why use the average velocity??

7. May 31, 2007

### cepheid

Staff Emeritus

no problem. feel free to post your work here if you would like it checked over, or if you run into trouble.

EDIT: I added a quote to make it clear who I was replying to.

8. May 31, 2007

### cepheid

Staff Emeritus
Good question. I don't know why he suggested that. I think it's valid though, right? Mean value theorem???

$$T = 2 \ \textrm{min}$$

$$\Delta s = \int_0^T v(t) \, dt = \left( \frac{1}{T} \int_0^T v(t) \, dt \right) T \equiv v_{\textrm{avg}} T$$

Last edited: May 31, 2007
9. May 31, 2007

### stunner5000pt

method has worked for me in the past ...

where would it fail, though?

10. May 31, 2007

### cepheid

Staff Emeritus
hmmm? Oh, I never said it would fail. I just had the opinion that a completely dynamical solution to the problem was the best approach, without resorting to kinematics. After taking the time to think about it (post #8), though, I think it's a perfectly valid method.

I'm not sure what hage's specific objection was, that's why I tried to verify that the method made sense in my last post.

Last edited: May 31, 2007