# Isaac Physics - Three Collisions Help!!Watch

Announcements
Thread starter 2 years ago
#1
Can someone help me with this question?

"Three particles A, B, and C lie at rest in that order in a straight line on a smooth horizontal table. The particle A is then projected directly towards B with velocity uu. Particle A collides with B which then collides with C. Each of the particles has mass mm, and the collisions are elastic.

What is the velocity v1 of particle A immediately after the collision with particle B? Give your answer in terms of u and m."

I know this is really easy but I have no idea where I'm going wrong! m always cancels for me so I can't get it in the right format
0
reply
2 years ago
#2
This is an awfully peculiar result, and one you shouldn't forget.

So you have your three particles lined up on a table, each with initial velocity . Particle is projected towards particle with a velocity . You should draw a diagram now! With a before and after velocity. My diagram looks like this:

Now then, let's do the maths. We remember that momentum is conserved in all collisions: therefore, the net momentum before the collision is equal to the net momentum after.

Therefore
.

For a perfectly elastic collision, kinetic energy is also conserved:
.

Solving simultaneously for (the desired velocity) we get
. So the velocity of after the collision is 0.
0
reply
2 years ago
#3
(Original post by Satsuko)
Can someone help me with this question?

"Three particles A, B, and C lie at rest in that order in a straight line on a smooth horizontal table. The particle A is then projected directly towards B with velocity uu. Particle A collides with B which then collides with C. Each of the particles has mass mm, and the collisions are elastic.

What is the velocity v1 of particle A immediately after the collision with particle B? Give your answer in terms of u and m."

I know this is really easy but I have no idea where I'm going wrong! m always cancels for me so I can't get it in the right format
mu = mv1 + mv2 => u = v1 + v2
0.5mu² = 0.5mv1² + 0.5mv2² => u² = v1² + v2²

So v2 = u - v1, v2 = √(u² - v1²)
Equate and solve for v1, only keep feasible solution.
0
reply
9 months ago
#4
Has anyone actually got an answer for this? I keep struggling and am having the same problem with getting the answer into the correct format, always end up with v2 in the equation which i cant get rid of
0
reply
9 months ago
#5
(Original post by reecesmith)
Has anyone actually got an answer for this? I keep struggling and am having the same problem with getting the answer into the correct format, always end up with v2 in the equation which i cant get rid of
Post 2 explained that the answer is 0

Use conservation of kinetic energy
Thus relative speed of approach = relative speed of separation

Use conservation of momentum
0
reply
9 months ago
#6
(Original post by BobbJo)
Post 2 explained that the answer is 0

Use conservation of kinetic energy
Thus relative speed of approach = relative speed of separation

Use conservation of momentum
yh i also came to an answer of 0 which I understand but even using the steps that your saying I just cant get it into the correct form
0
reply
9 months ago
#7
(Original post by reecesmith)
yh i also came to an answer of 0 which I understand but even using the steps that your saying I just cant get it into the correct form
What do you mean? What correct form? The answer is 0
0
reply
9 months ago
#8
(Original post by BobbJo)
What do you mean? What correct form? The answer is 0
its on isaac physics and it wants v1 ( the velocity of ball A after impact) in terms of m(mass of ball) and u( its initial velocity)
0
reply
9 months ago
#9
(Original post by reecesmith)
its on isaac physics and it wants v1 ( the velocity of ball A after impact) in terms of m(mass of ball) and u( its initial velocity)
put 0 and units ms-1 ?
0
reply
9 months ago
#10
(Original post by BobbJo)
put 0 and units ms-1 ?
I don't think it lets you
0
reply
9 months ago
#11
(Original post by reecesmith)
I don't think it lets you
did you try? can you give the link to the question
0
reply
9 months ago
#12
(Original post by BobbJo)
did you try? can you give the link to the question
ill just link the question it'll be easier than explaining https://isaacphysics.org/questions/t...tum_collisions
0
reply
9 months ago
#13
(Original post by BobbJo)
did you try? can you give the link to the question
srry the link is taking ages it keeps saying they need to verify the message just type in isaac physics and under a momentum and collisions section there should be a question called three collisions
0
reply
9 months ago
#14
(Original post by reecesmith)
srry the link is taking ages it keeps saying they need to verify the message just type in isaac physics and under a momentum and collisions section there should be a question called three collisions
I put 0 and it said correct
0
reply
9 months ago
#15
(Original post by BobbJo)I put 0 and it said correct
wth im getting scammed i put that
0
reply
9 months ago
#16
(Original post by reecesmith)
(Original post by BobbJo)I put 0 and it said correct
wth im getting scammed i put that
you bumped a 1 year old thread for this.
you should have tried 0
0
reply
9 months ago
#17
(Original post by BobbJo)
you bumped a 1 year old thread for this.
you should have tried 0
i did that in the first place idk why that didnt work, sorry bout that
1
reply
9 months ago
#18
(Original post by reecesmith)
i did that in the first place idk why that didnt work, sorry bout that
No worries
1
reply
X

Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

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

### University open days

• Cardiff Metropolitan University
Undergraduate Open Day - Llandaff Campus Undergraduate
Sat, 19 Oct '19
• Coventry University
Undergraduate Open Day Undergraduate
Sat, 19 Oct '19
• University of Birmingham
Undergraduate Open Day Undergraduate
Sat, 19 Oct '19

### Poll

Join the discussion

#### Why wouldn't you turn to teachers if you were being bullied?

They might tell my parents (13)
6.67%
They might tell the bully (20)
10.26%
I don't think they'd understand (33)
16.92%
It might lead to more bullying (71)
36.41%
There's nothing they could do (58)
29.74%

View All
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

### See more of what you like onThe Student Room

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