Question 3 of this paper:

https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FPhysics%2FA-level%2FTopic-Qs%2FAQA%2F04-Mechanics-and-Materials%2FSet-M%2FMomentum%2520QP.pdf

quick question:

how do we that both the particles do not move at the same velocity together? if they are both moving together at 2.8ms^-1 then kinetic energy is still conserved, yet the answer is A? why is this? is there something I am misunderstanding about elastic collisions?

https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FPhysics%2FA-level%2FTopic-Qs%2FAQA%2F04-Mechanics-and-Materials%2FSet-M%2FMomentum%2520QP.pdf

quick question:

how do we that both the particles do not move at the same velocity together? if they are both moving together at 2.8ms^-1 then kinetic energy is still conserved, yet the answer is A? why is this? is there something I am misunderstanding about elastic collisions?

Original post by Glooc

Question 3 of this paper:

https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FPhysics%2FA-level%2FTopic-Qs%2FAQA%2F04-Mechanics-and-Materials%2FSet-M%2FMomentum%2520QP.pdf

quick question:

how do we that both the particles do not move at the same velocity together? if they are both moving together at 2.8ms^-1 then kinetic energy is still conserved, yet the answer is A? why is this? is there something I am misunderstanding about elastic collisions?

https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FPhysics%2FA-level%2FTopic-Qs%2FAQA%2F04-Mechanics-and-Materials%2FSet-M%2FMomentum%2520QP.pdf

quick question:

how do we that both the particles do not move at the same velocity together? if they are both moving together at 2.8ms^-1 then kinetic energy is still conserved, yet the answer is A? why is this? is there something I am misunderstanding about elastic collisions?

Momentum would not be conserved for c)

Original post by Glooc

Question 3 of this paper:

https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FPhysics%2FA-level%2FTopic-Qs%2FAQA%2F04-Mechanics-and-Materials%2FSet-M%2FMomentum%2520QP.pdf

quick question:

how do we that both the particles do not move at the same velocity together? if they are both moving together at 2.8ms^-1 then kinetic energy is still conserved, yet the answer is A? why is this? is there something I am misunderstanding about elastic collisions?

https://www.physicsandmathstutor.com/pdf-pages/?pdf=https%3A%2F%2Fpmt.physicsandmathstutor.com%2Fdownload%2FPhysics%2FA-level%2FTopic-Qs%2FAQA%2F04-Mechanics-and-Materials%2FSet-M%2FMomentum%2520QP.pdf

quick question:

how do we that both the particles do not move at the same velocity together? if they are both moving together at 2.8ms^-1 then kinetic energy is still conserved, yet the answer is A? why is this? is there something I am misunderstanding about elastic collisions?

In an elastic collision, both momentum and kinetic energy are conserved.

Initial momentum = 8kg ms^-1.

By looking at option C, final momentum = 2.8x2x2=11.2kg ms^-1.

So momentum is not conserved.

Let Va and Vb be velocities after collision.

In order to solve the problem we can see that:

To conserve momentum, after collision: (Va + Vb)2 = 8

So Va + Vb = 4

KE before = 1/2 x 2.0 x 16 = 16J

Thus 1/2 x 2 (Va^2 + Vb^2) = 16

So Va^2 + Vb^2 = 16

We can solve these equations to get Va = 4 and Vb = 0 (or vice versa).

As momentum is a vector, B is impossible as it would be negative so not conserved. Thus A is the answer.

If a collision is perfectly elastic, you should know that there is a rule about velocity of approach and velocity of separation.

In this case, the velocity with which the 2 objects separate must be the same as when they approach. 4m/s in this case.

The only way for the 2 objects to separate at 4 m/s (and conserve momentum) is for T1 to stop and T2 to move away from it at 4m/s.

Your suggestion that they both move at 2.8 m/s is not an elastic collision. (The same goes for the other option of 1.4m/s)

If they both moved at that speed in the same direction they would have merged and that is a perfectly inelastic collision. (No separation after impact.)

If one moved one way and the other moved the other way both at 2.8 m/s then the velocity of separation would be 2.8+2.8 = 5.6 m/s which is impossible, as it cannot be greater that 4m/s.

In this case, the velocity with which the 2 objects separate must be the same as when they approach. 4m/s in this case.

The only way for the 2 objects to separate at 4 m/s (and conserve momentum) is for T1 to stop and T2 to move away from it at 4m/s.

Your suggestion that they both move at 2.8 m/s is not an elastic collision. (The same goes for the other option of 1.4m/s)

If they both moved at that speed in the same direction they would have merged and that is a perfectly inelastic collision. (No separation after impact.)

If one moved one way and the other moved the other way both at 2.8 m/s then the velocity of separation would be 2.8+2.8 = 5.6 m/s which is impossible, as it cannot be greater that 4m/s.

(edited 1 year ago)

For elastic collision, a short cut can be to consider the 3 special cases: this is one of them, where the 2 masses are equal. The particles undergoing collision exchange velocities.

So particle 1 will inherit the velocity of particle 2 and vice versa.

So particle 1 will inherit the velocity of particle 2 and vice versa.

- Physics question momentum
- Further Mechancis Impulse question
- Closest distance of approach physics electric field help!
- physics momentum question
- Isaac Physics 'Road Collision' Problem
- struggling with physics
- Understanding physics #1
- i am confused with physics
- Elastic collisions (A level physics)
- When to use Energy vs Momentum
- momentum question
- Isaac physics D7.5 Physics Quantum kinetic energy
- A free electron is described by the wave function, .ψ (x)=Ae^i2pi/lamba x Using the
- Impact Forces, A level physics
- A level physics MCQ
- FM1 - Direction of impulse when there is a collision between two particles
- firing a cannon
- Effect of magnetic field on pair prooduction
- 1 mark question
- Momentum and force equation

Last reply 2 minutes ago

Anyone still waiting for outcome of their undergraduate application?Last reply 6 minutes ago

If a student loan is wiped, does that affect entitlement to future student loans?Last reply 7 minutes ago

Official University of Edinburgh Applicant Thread for 2024Last reply 7 minutes ago

what predicted poems for power and conflict 2024?Last reply 10 minutes ago

Official University of St Andrews Applicant Thread for 2024Last reply 11 minutes ago

LAW: How to answer a problem question in Contract LawLast reply 14 minutes ago

UCL (University College London) Postgraduate Applicants 2024-25Last reply 16 minutes ago

HSBC Degree Apprenticeship 2024Last reply 17 minutes ago

Edexcel A-level English Literature Paper 3 (9ET0 3) - 14th June 2024 [Exam Chat]