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Worked solution
State the reference directions and convention
The positive direction along the line of centres is taken from towards , and every component below is signed accordingly.
Write down the data for the impact
Each velocity is resolved along the line of centres and perpendicular to it.
Note why the impulse acts along the line of centres
Because the spheres are smooth there is no friction at the contact, so the impulse is entirely along the line joining the centres.
Carry the tangential component of through unchanged
There is no impulse perpendicular to the line of centres, so this component of is unaffected by the impact.
Carry the tangential component of through unchanged
For the same reason the tangential component of passes through the impact untouched.
Write down conservation of momentum along the line of centres
Only the normal components change, so momentum is applied in that direction to relate them.
Substitute the normal data into the momentum equation
The signed normal components are inserted exactly as they stand.
Simplify the momentum equation
This is the first of the two simultaneous equations for the normal components after impact.
Write down Newton's law of restitution along the line of centres
Separation speed equals times approach speed, measured along the line of centres.
Substitute the normal data into the restitution equation
The bracket is the approach speed along the line of centres.
Solve the simultaneous equations for
Adding times the restitution equation to the momentum equation eliminates .
Back-substitute to find
The normal component of follows from the restitution equation.
State the velocity of immediately after the impact
The components are along the line of centres and perpendicular to it respectively.
State the velocity of immediately after the impact
keeps its tangential component and takes the normal component found above.
Find the speed of after the impact
The resultant speed of combines its two components by Pythagoras' theorem.
Find the speed of after the impact
The resultant speed of is found in the same way.