x Turn on thread page Beta
 You are Here: Home >< Physics

# Help understanding a question on the acceleration of a car?? watch

1. I was working through the question below, and when I compared my answer to that in the back, it wasn't the same and i don't understand where I went wrong? I've italicised from where I don't understand.

Question:
The weight of a car is 1.8x10^4 N. It accelerates from rest to a velocity of 28ms^-1 in a time of 9.6s. Calculate the acceleration of the car.

a= (v-u)/t = 28/9.6
a=2.91 ms^-2
m=w/g = (1.8 x 10^4)/9.81
m= 1.83x10^3
F=ma =1.83x10^3 x 2.91
=5.4 x 10^3 N
2. (Original post by sabruh)
I was working through the question below, and when I compared my answer to that in the back, it wasn't the same and i don't understand where I went wrong? I've italicised from where I don't understand.

Question:
The weight of a car is 1.8x10^4 N. It accelerates from rest to a velocity of 28ms^-1 in a time of 9.6s. Calculate the acceleration of the car.

a= (v-u)/t = 28/9.6
a=2.91 ms^-2
m=w/g = (1.8 x 10^4)/9.81
m= 1.83x10^3
F=ma =1.83x10^3 x 2.91
=5.4 x 10^3 N
Ok, first of all we are given force (weight), initial velocity, final velocity, time and must find the acceleration. You gud with that....
Now the formula for acceleration is a=(v-u)/t. So we know u =0m/s, v=28m/s and time is 9.6 seconds. Therfore chucking all them in the calculator gives acceleration= 35/12. with f=ma we only have force. So to get mass do force/g so the mass is 90000/49. Now f=ma so f=ma which is (35/12) * (1.8x10^4) which gives 53500 which in standard form is 5.4 x 10^3N. But I don't understand you get the force when the quesion asks for acceleration, you sure the question is written correctly?
Well anyways has that helped?
3. (Original post by raman_17)
Ok, first of all we are given force (weight), initial velocity, final velocity, time and must find the acceleration. You gud with that....
Now the formula for acceleration is a=(v-u)/t. So we know u =0m/s, v=28m/s and time is 9.6 seconds. Therfore chucking all them in the calculator gives acceleration= 35/12. with f=ma we only have force. So to get mass do force/g so the mass is 90000/49. Now f=ma so f=ma which is (35/12) * (1.8x10^4) which gives 53500 which in standard form is 5.4 x 10^3N. But I don't understand you get the force when the quesion asks for acceleration, you sure the question is written correctly?
Well anyways has that helped?
that's why i'm so confused- I can use the given suvat equation to work out acceleration, so why is anything else relevant? I don't understand why you even get an answer in Newtons, when the SI unit for acceleration is ms^-2.

My problem wasn't with the math and following what they did, but rather why it was done. thanks though!
4. (Original post by sabruh)
that's why i'm so confused- I can use the given suvat equation to work out acceleration, so why is anything else relevant? I don't understand why you even get an answer in Newtons, when the SI unit for acceleration is ms^-2.

My problem wasn't with the math and following what they did, but rather why it was done. thanks though!
Isn't there a second section in the question asking about the force accelerating the car?

if not I can't see what that bit is doing in the M/S either
5. (Original post by Joinedup)
Isn't there a second section in the question asking about the force accelerating the car?

if not I can't see what that bit is doing in the M/S either
As you can see, three is no second part in the question. I'll just assume it was a printing error (5 marks for simple suvat though??)

6. (Original post by sabruh)
that's why i'm so confused- I can use the given suvat equation to work out acceleration, so why is anything else relevant? I don't understand why you even get an answer in Newtons, when the SI unit for acceleration is ms^-2.

My problem wasn't with the math and following what they did, but rather why it was done. thanks though!
yh thats fine, its just I got stuck with the SI units too why acceleration was N...
7. (Original post by raman_17)
yh thats fine, its just I got stuck with the SI units too why acceleration was N...
Acceleration is m/s^2 or N/kg
8. (Original post by Kyx)
Acceleration is m/s^2 or N/kg
oh yh thanks! but, what I was told was that the answer was just Newtons so I was confused that it should be newtons per kilogram...
9. (Original post by raman_17)
oh yh thanks! but, what I was told was that the answer was just Newtons so I was confused that it should be newtons per kilogram...
Maybe they did the force?
10. (Original post by Kyx)
Maybe they did the force?
the question was probably written wrong then
11. (Original post by raman_17)
the question was probably written wrong then

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: January 18, 2016
Today on TSR

### Loughborough better than Cambridge

Loughborough at number 1

### Can i date a girl with no boobs?

Poll

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