Hey there! Sign in to join this conversationNew here? Join for free
Turn on thread page Beta
    • Thread Starter
    Offline

    7
    ReputationRep:
    Sorry it looks really long.. I'm only stuck on the last bit about the minimum speed of A but it's a multiple step question. Please help I'm so confused.

    Three uniform spheres 𝐴, 𝐵 and 𝐶 have masses 0.3 kg, 0.4 kg and 𝑚 kg respectively. The spheres lie in a
    smooth horizontal groove with 𝐵 between 𝐴 and 𝐶. Sphere 𝐵 is at rest and spheres 𝐴 and 𝐶 are each
    moving with speed 3.2 ms-1
    towards 𝐵 (see diagram). Air resistance may be ignored.

    (a) 𝐴 collides with 𝐵. After this collision 𝐴 continues to move in the same direction as before, but with speed 0.8 ms-1. Find the speed with which 𝐵 starts to move.
    ANSWER 1.8M/S

    (b) 𝐵 and 𝐶 then collide, after which they both move towards 𝐴, with speeds of 3.1 ms-1 and 0.4 ms-1 respectively. Find the value of 𝑚.
    ANSWER 0.7KG

    (c) The next collision is between 𝐴 and 𝐵. How you can tell that, after this collision, 𝐴 and 𝐵 cannot both be moving towards 𝐶 ?

    (d) When the spheres have finished colliding,

    (i) Which direction is 𝐴 moving in? ANSWER LEFT (i got this right but only by intuition, if anyone could clear up exactly why please i'd appreciate it.)

    (ii) What is the minimum speed of 𝐴?
    Offline

    3
    ReputationRep:
    (c), total linear momentum of A and B before collision is 0.3*0.8-0.4*3.1, so the resultant momentum is away from C, therefore they cannot both be going towards C.

    d(i) so after the collision discussed in (c), A is moving left and B is moving right. As A is the furthest left, none of the other two can do anything to make it go right again. They would have to collide with it from the left, but that is impossible as they are on the other side.

    Have you done anything on the coefficient of restitution?
    Offline

    18
    ReputationRep:
    (Original post by stolenuniverse)
    x
    (im assuming you drew the diagram with A to the left of B and C to the right)

    d i) After A and Bs second collision you can deduce the direction A is heading based on the fact A and B cant both be heading towards C.

    ii) I think you work out the speed of A after its second collision with B, then see whether that value, 3.2 or 0.8 is smaller, since after the 2nd collision A is heading left then any subsequent collisions involving A will just be B hitting the "back" of A which will only speed it up
    • Thread Starter
    Offline

    7
    ReputationRep:
    (Original post by Darth_Narwhale)
    (c), total linear momentum of A and B before collision is 0.3*0.8-0.4*3.1, so the resultant momentum is away from C, therefore they cannot both be going towards C.

    d(i) so after the collision discussed in (c), A is moving left and B is moving right. As A is the furthest left, none of the other two can do anything to make it go right again. They would have to collide with it from the left, but that is impossible as they are on the other side.

    Have you done anything on the coefficient of restitution?
    How can you know B is going left? Couldn't A and B both go right?

    And no i haven't i've never heard of that.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 29, 2017
Poll
“Yanny” or “Laurel”
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

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

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