# Homework Help: Velocity question

1. Jan 19, 2005

### jai6638

You ride your bike for 1.5 hour at an average velocity of 10km/hr, then for 30 min at 15km/hr. What is your average velocity?

so since average velocity = delta D/delta T

i got 7500 m / 7200 secs = 1.04 m/s.. is that correct?

2. Jan 19, 2005

### chroot

Staff Emeritus
1.5 hours at 10 km/hr would be 15 km, right?

0.5 hours at 15 km/hr would be 7.5 km, right?

Total distance = 15 km + 7.5 km = 22.5 km, total time = 2 hours.

Total distance / total time = 22.5 km / 2 hours = ...

There's no reason at all to convert anything to meters or seconds. Leave it all in km/hr.

- Warren

3. Jan 19, 2005

### jai6638

oh damn... i subtracted 7.5 by 15 instead of adding them.. silly me...

Have an exam tomorrow so need help with this question too:

Highway safety engineers build soft barriers so that cars hitting them will slow down at a safe rate. A person wearing a seat belt can withstand an acceleration of -3.0 x 10^2 m/s^2. How thick should barriers be to safely stop a carthat hits a barrier at 110km/h?

dunno which equation to use.... i know i have to fiind D but i dunno the Vinitial!

4. Jan 19, 2005

### chroot

Staff Emeritus
The initial velocity is 110 km/hr, a given.

How will the car's velocity change when it's undergoing an acceleration of -300 m/s^2?

- Warren

5. Jan 19, 2005

### jai6638

im lost... how do i find that out?

6. Jan 19, 2005

### chroot

Staff Emeritus
What does it mean when we say an acceleration is, for example, "300 meters per second per second?"

It means that for each second an object is subjected to that acceleration, it will experience a change in velocity of 300 meters per second.

If your car starts out going 110 km/hr, which is just about 30.5 m/s, and is subjected to an acceleration of -300 m/s^2, then it will take just about a tenth of a second for the car to come to a stop. (In an entire second, the car would lose 300 m/s of velocity; every tenth of a second, the car will lose one tenth of 300 m/s of velocity.)

How far does the car go in that tenth of a second? Use the standard equation:

$$s(t) = v_0 t - \frac{1}{2} a t^2$$

plugging in 30.5 for $v_0$, -300 for a and 0.1 for t.

- Warren

7. Jan 19, 2005

### Parth Dave

You are told the initial velocity is 110 km/hr. You are also told the acceleration is -300 m/s^2. If the car is to come to a complete stop, then what is its final velocity? Now can you find the displacement?

8. Jan 19, 2005

### jai6638

"in an entire second SQUARED not just a second".... its not just a second right?? its m/S^2... so would the value of 1/10 still remain the same takin into consideration the S being squared?

9. Jan 19, 2005

### chroot

Staff Emeritus
There's no such thing as a second squared. Read this sentence over again:
In one second, the car loses 300 m/s of velocity. That's an acceleration of -300 meters per second, per second.

- Warren