IGCSE/GCSE/O & A Level/IB/University Student Forum

Qualification => Subject Doubts => GCE AS & A2 Level => Sciences => Topic started by: halosh92 on March 08, 2010, 05:21:47 am

Title: mechanics doubt AS
Post by: halosh92 on March 08, 2010, 05:21:47 am

A car and a motorbike are at rest adjacent to one another at a set of traffic lights on a long,
straight stretch of road. They set off simultaneously at time t = 0. The motorcyclist accelerates
uniformly at 6 m s-2 until he reaches a speed of 30 m s-1 which he then maintains. The car
driver accelerates uniformly for 9 seconds until she reaches 36 m s-1 and then remains at this
speed.
(a) Find the acceleration of the car. (2 marks)
(b) Draw on the same diagram speed-time graphs to illustrate the movements of both
vehicles.
(4 marks)
(c) Find the value of t when the car again draws level with the motorcyclist. (7 marks)

Title: Re: mechanics doubt AS
Post by: halosh92 on March 08, 2010, 01:20:43 pm

2) A non-uniform plank AB of mass 20 kg and length 6 m is supported at both ends so that it is
horizontal. When a woman of mass 60 kg stands on the plank at a distance of 2 m from B, the
magnitude of the reaction at A is 35g N.
(a) Suggest a suitable model for
(i) the plank,
(ii) the woman. (2 marks)
(b) Calculate the magnitude of the reaction at B, giving your answer in terms of g.
(2 marks)
(c) Explain briefly, in the context of the problem, the term ‘non-uniform’. (2 marks)
(d) Find the distance of the centre of mass of the plank from A. (4 marks)

Title: Re: mechanics doubt AS
Post by: Kukkr on March 08, 2010, 01:25:20 pm

I'll try to solve the questions but remember I didn't have M1 in AS... I only studied first few chapters of M1 in A2

Title: Re: mechanics doubt AS
Post by: Kukkr on March 08, 2010, 01:30:46 pm

Quote from: halosh92 on March 08, 2010, 05:21:47 am

(a) Find the acceleration of the car. (2 marks)

Answer:-

v=u+at
36=0+a(9)
36=9a
a=4m/s/s

Title: Re: mechanics doubt AS
Post by: Kukkr on March 08, 2010, 01:46:48 pm

Quote from: halosh92 on March 08, 2010, 05:21:47 am

A car and a motorbike are at rest adjacent to one another at a set of traffic lights on a long,
straight stretch of road. They set off simultaneously at time t = 0. The motorcyclist accelerates
uniformly at 6 m s-2 until he reaches a speed of 30 m s-1 which he then maintains. The car
driver accelerates uniformly for 9 seconds until she reaches 36 m s-1 and then remains at this
speed.
(a) Find the acceleration of the car. (2 marks)
(b) Draw on the same diagram speed-time graphs to illustrate the movements of both
vehicles.
(4 marks)
(c) Find the value of t when the car again draws level with the motorcyclist. (7 marks)

do you really have a problem with part B? I can solve it but its going to be a lot of work for a simple thing x_x

Answer(c):-
Use the graph!

Title: Re: mechanics doubt AS
Post by: Saladin on March 08, 2010, 02:09:43 pm

here u go,

answers to ur first post.

also kindly post markscheme.

Title: Re: mechanics doubt AS
Post by: Saladin on March 08, 2010, 02:35:00 pm

yeh

ansa to 2

Title: Re: mechanics doubt AS
Post by: Kukkr on March 08, 2010, 02:58:55 pm

good work dude... you stole my work lol :P

Title: Re: mechanics doubt AS
Post by: Saladin on March 08, 2010, 03:11:21 pm

actually i did not,

i did these qs b4

and so did it again, and scanned the stuff for the guy.

Title: Re: mechanics doubt AS
Post by: Kukkr on March 08, 2010, 03:48:53 pm

I see... kool

The user is a girl by the way :P

Title: Re: mechanics doubt AS
Post by: halosh92 on March 08, 2010, 04:10:42 pm

thx so much guys!!!  ;)
and yep im a girl ;D

Title: Re: mechanics doubt AS
Post by: Phosu on March 08, 2010, 04:44:11 pm

A car starts from rest at a point S on a straight racetrack. The car moves with constant acceleration for 20 s, reaching a speed of 25 m s–1. The car then travels at a constant speed of 25 m s–1 for 120 s. Finally it moves with constant deceleration, coming to rest at a point F.
(a) In the space below, sketch a speed-time graph to illustrate the motion of the car.(2)
The distance between S and F is 4 km.
(b) Calculate the total time the car takes to travel from S to F.(3)

A motorcycle starts at S, 10 s after the car has left S. The motorcycle moves with constant acceleration from rest and passes the car at a point P which is 1.5 km from S. When the motorcycle passes the car, the motorcycle is still accelerating and the car is moving at a constant speed. Calculate
(c) the time the motorcycle takes to travel from S to P,(5)
(d) the speed of the motorcycle at P.

could someone please explain how to solve c) part, i do have the ans, but i dont kno how they reached to it.

Title: Re: mechanics doubt AS
Post by: Kukkr on March 08, 2010, 08:01:35 pm

lets just find out how much time did the car take to get to point P which is 1.5 km away from S

0.5 x 20 x 25 = 250m (this is how much the car traveled when it was accelerating, t = 20)
remaining is 1.5-0.25=1.25km or 1250m
25 x t = 1250 (we know from the question that the car was still moving at constant speed while the motor cycle passes it)
t = 50s

so 20 + 50 = 70s is the time the car took to reach point P...
but since the motorcycle left 10 seconds later... 70-10=60s

is 60 the correct answer?
I think there might be a better way to answer this question, but this is what I have come up with right now...