# M1 Help Watch

1. A car starts from rest the bottom of a slope. It accelerates up the slope for 8 seconds at 1.5ms^-2, then disengages the engine and coasts. If its acceleration is now -1ms^-1, find the time between the car's leaving the bottom of the slope and returning to it.

I'm so confused, thanks for the help
2. What I have done is drawn a diagram (which you should always do);

1st - draw a diagonal line going up and label the beginning A and the other end B.
So, A --> B

a = 1.5
t = 8
u = 0
V = ?
S = ?

V=u+at
v = 0 + 1.5(8)
V = 12

V2=U2+2as
144 = 0 +2(1.5)s
S = 48

(See if you can work the rest out for yourself before opening spoiler)

Spoiler:
Show

B --> A

a = -1
t = ?
s = 48
u = 0
V = ?

S = ut + 1/2at2
48 = -t2
48 = t2
t = Sqrt 48

t = 8+sqrt48

I think this is right, however, try and find the answer first before taking my work for it. It also wouldn't hurt to get as second opinion on this. =l
3. 48 = -t2
48 = t2
How did you get from here to here?
4. But according to the answer its wrong ? :/
5. Think of it as two situations, where there is constant acceleration, the car going up the slope and the car sliding due to g. I would form two us ate equations

Posted from TSR Mobile
8. (Original post by 3.1415927)
Do you know if that's a rounded answer or supposed to be exact?

I get 35.4919 seconds approximately.
9. (Original post by davros)
Do you know if that's a rounded answer or supposed to be exact?

I get 35.4919 seconds approximately.
Working?
10. (Original post by Scienceisgood)
Working?
There may be a more optimum way, but I broke the motion into 3 stages:

Stage 1: the car goes up the slope for time t1 = 8 seconds with acc 1.5ms^-2 for a distance s1, say.

Stage 2: the car carries on going up the slope but is being decelerated at 1ms^-2 until it comes to rest. This takes a further t2 seconds, say, and covers an extra distance s2, say, upwards.

Stage 3: the car starts from rest and is now sliding back down the slope with acceleration 1ms^-2 until it has covered the combined distance s1 + s2. This takes a further time t3, say.

Our total time is now 8 + t2 + t3
11. Yeah its probably rounded

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 15, 2013
Today on TSR

### Falling in love with him

But we haven't even met!

### Top study tips for over the Christmas holidays

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.