Physics Constant Acceleration Questions [PLEASE HELP] Watch

nwmyname
Badges: 13
Rep:
?
#1
Report Thread starter 2 years ago
#1
Hi, I have the following questions.

1. A car accelerates uniformly from rest along a straight road and experiences negligible frictional forces. The car travels 15m in the second second of its journey. How far does it travel in the fifth second? (OPTIONS: 15m, 35m, 45m, 75m)

2. A car passes through traffic lights as they turn from red to green at a speed of 12ms-1. A motorcyclist, starting from rest, accelerates from the traffic lights in the same direction as the car at 1.5ms-2. After what distance from the light will they meet. (OPTIONS: 16m, 120m, 168m, 192m) NOTE - assume the car has constant accleration.

I can derive the following information from question 2, but I am stuck as to what to do from thereon. u = 0, a = 1.5.

PLEASE HELP.

Update - I have got question 1 to being 45m.
0
reply
Joinedup
Badges: 20
Rep:
?
#2
Report 2 years ago
#2
(Original post by nwmyname)
Hi, I have the following questions.

1. A car accelerates uniformly from rest along a straight road and experiences negligible frictional forces. The car travels 15m in the second second of its journey. How far does it travel in the fifth second? (OPTIONS: 15m, 35m, 45m, 75m)

2. A car passes through traffic lights as they turn from red to green at a speed of 12ms-1. A motorcyclist, starting from rest, accelerates from the traffic lights in the same direction as the car at 1.5ms-2. After what distance from the light will they meet. (OPTIONS: 16m, 120m, 168m, 192m) NOTE - assume the car has constant accleration.

I can derive the following information from question 2, but I am stuck as to what to do from thereon. u = 0, a = 1.5.

PLEASE HELP.

Update - I have got question 1 to being 45m.
s=(u+v)/t

for the second second u=v/2 (because constant acceleration)

from which you can get u and hence a... which leads you to a solution
0
reply
Joinedup
Badges: 20
Rep:
?
#3
Report 2 years ago
#3
sorry - wrong one

for the car & bike you're looking for the point at which they're the same distance from the lights at the same time.

using suvat, you can write equations for s for both vehicles as a function of t and find the time till they pass by treating as simultaneous equations... then it's easy to find out how far either of them has travelled in that amount of time (but slightly easier for the car)
0
reply
nwmyname
Badges: 13
Rep:
?
#4
Report Thread starter 2 years ago
#4
Joinedup I have finally solved it - at least I think so.

The Car
S
U 12
V
A 0
T

The Motorbike
S
U 0
V
A 1.5
T

Using the Car
s = ut + 1/2 at^2
s = ut (as a = 0)
s = 12t

Using the Motorbike
s = ut + 1/2 at^2
s = 1/2 at^2 (as u = 0)
s = 1/2 (1.5) t^2
s = 0.75 t^2

Subsitute the first equation to the second
12t = 0.75t^2
12t - 0.75t^2 = 0
t (12 - 0.75t) = 0
t = 0 (1)
12 - 0.75t = 0
12 = 0.75t
16 = t [so the time is 16 seconds]

The car travels at constant velocity of 12ms-1 so to find the distance, using speed x time, I do 12 * 16 = 192.
Is this correct?
0
reply
Kyx
Badges: 6
Rep:
?
#5
Report 2 years ago
#5
(Original post by nwmyname)
Hi, I have the following questions.

1. A car accelerates uniformly from rest along a straight road and experiences negligible frictional forces. The car travels 15m in the second second of its journey. How far does it travel in the fifth second? (OPTIONS: 15m, 35m, 45m, 75m)

2. A car passes through traffic lights as they turn from red to green at a speed of 12ms-1. A motorcyclist, starting from rest, accelerates from the traffic lights in the same direction as the car at 1.5ms-2. After what distance from the light will they meet. (OPTIONS: 16m, 120m, 168m, 192m) NOTE - assume the car has constant accleration.

I can derive the following information from question 2, but I am stuck as to what to do from thereon. u = 0, a = 1.5.

PLEASE HELP.

Update - I have got question 1 to being 45m.
You use s = ut + 1.at^2 for number 2 (methinks)
0
reply
derpz
Badges: 2
Rep:
?
#6
Report 2 years ago
#6
(Original post by nwmyname)
Hi, I have the following questions.

1. A car accelerates uniformly from rest along a straight road and experiences negligible frictional forces. The car travels 15m in the second second of its journey. How far does it travel in the fifth second? (OPTIONS: 15m, 35m, 45m, 75m)

2. A car passes through traffic lights as they turn from red to green at a speed of 12ms-1. A motorcyclist, starting from rest, accelerates from the traffic lights in the same direction as the car at 1.5ms-2. After what distance from the light will they meet. (OPTIONS: 16m, 120m, 168m, 192m) NOTE - assume the car has constant accleration.

I can derive the following information from question 2, but I am stuck as to what to do from thereon. u = 0, a = 1.5.

PLEASE HELP.

Update - I have got question 1 to being 45m.
I got 45 for the first one too, as for the second one, I think you are correct in your method with the simultaneous equation
0
reply
Joinedup
Badges: 20
Rep:
?
#7
Report 2 years ago
#7
(Original post by nwmyname)
Joinedup I have finally solved it - at least I think so.

The Car
S
U 12
V
A 0
T

The Motorbike
S
U 0
V
A 1.5
T

Using the Car
s = ut + 1/2 at^2
s = ut (as a = 0)
s = 12t

Using the Motorbike
s = ut + 1/2 at^2
s = 1/2 at^2 (as u = 0)
s = 1/2 (1.5) t^2
s = 0.75 t^2

Subsitute the first equation to the second
12t = 0.75t^2
12t - 0.75t^2 = 0
t (12 - 0.75t) = 0
t = 0 (1)
12 - 0.75t = 0
12 = 0.75t
16 = t [so the time is 16 seconds]

The car travels at constant velocity of 12ms-1 so to find the distance, using speed x time, I do 12 * 16 = 192.
Is this correct?
I got 192... though it's pretty straightforward to verify for yourself.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

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.

Personalise

University open days

  • University of East Anglia
    All Departments Open 13:00-17:00. Find out more about our diverse range of subject areas and career progression in the Arts & Humanities, Social Sciences, Medicine & Health Sciences, and the Sciences. Postgraduate
    Wed, 30 Jan '19
  • Aston University
    Postgraduate Open Day Postgraduate
    Wed, 30 Jan '19
  • Solent University
    Careers in maritime Undergraduate
    Sat, 2 Feb '19

Brexit: Given the chance now, would you vote leave or remain?

Remain (1365)
79.73%
Leave (347)
20.27%

Watched Threads

View All