# Thread: A truck covers 80 m in 17 s while smoothly slowing

1. ## A truck covers 80 m in 17 s while smoothly slowing

A truck covers 80 m in 17 s while smoothly slowing down to a final speed of 5.6 m/s
a ) find its original speed
b) Final its acceleration
I want check my answer :
a )
Dx = 80 m
t = 17 s
v = 5.6 m/s
v0= ??
by appling the rule
Dx = 1/2(v+v0)t
80 = 1/2(5.6 + v0) 17
v0 = .5 X 80 - 5.6 /17
=2.023 m/s
-------------
b)
v = v0 + at
5.6 = 2.023 + 17a
a = 5.6 -2.023/17 = 0.214
plese help me and wha thw unit of a will be ?

2. The average speed is 80 m / 17 s = 4.7 m/s. Since the final speed 5.6 m/s > 4.7 m/s, the truck could not have been slowing down; it must have been accelerating. To find the initial speed v0, use the fact that 4.7 is the average of v0 and 5.6, i.e., 4.7 = (v0 + 5.6) / 2.

3. plese I want your help

4. So, what is your difficulty?

5. the diffculty I can't get v0

6. I am using your equation Dx = 1/2(v+v0)t. Dividing both sides by t, we have that the average speed Dx / t is the arithmetic mean of the initial speed v0 and the final speed v = 5.6 m/s. This is how I got 4.7 = (v0 + 5.6) / 2. From here, 2 * 4.7 = 9.4 = v0 + 5.6, so v0 = 9.4 - 5.6 = 3.8 (approximately, since 80 / 17 = 4.7 only approximately).

now I try to sovle this queation by another way anf please help me If it correct or not

Dx = 1/2(v + v0)t
80=1/2(5.6+v0) 17
80 = 1/2(8.6 + 17v0)
80 = 4.25 + 17v0
v0 = 75.75/17 =4.45

8. Originally Posted by r-soy
You were told by emakarov that initial velocity v0 is calculated from: 4.7 = (v0 + 5.6) / 2.
SO:
v0 + 5.6 = 9.4
v0 = 3.8
If that's not the answer, then your problem is not correctly stated.

9. Dx = 1/2(v + v0)t
80=1/2(5.6+v0) 17
80 = 1/2(8.6 + 17v0)
80 = 4.25 + 17v0
v0 = 75.75/17 =4.45
I don't understand how you got from the second line to the third and from the third to the fourth. For example, the third line should be

80 = 1/2(95.2 + 17v0)

By the way, the lack of attention that results in typos may well contribute to your mistakes in the solution.

10. Originally Posted by r-soy
Dx = 1/2(v + v0)t
80=1/2(5.6+v0) 17
80 = 1/2(8.6 + 17v0)
80 = 1/2(5.6 + v0) 17
Easier if you now went this way:
160 = 17(5.6 + v0)
160 = 95.2 + 17v0
17v0 = 64.8
v0 = 3.81176....

80=1/2(5.6+v0) 17 really means: 80 = 17(5.6 + v0) / 2

11. yes I want this way
Easier to me
160 = 17(5.6 + v0)
160 = 95.2 + 17v0
17v0 = 64.8
v0 = 3.81176

ny question why you multiply 80 X 2 ??

12. If x/2 = 5, then x = 5 * 2