Finding two objects velocity after an elastic collision

[DeltaForce]

Homework Statement

A 4-kg block moving at 7m/s makes a head-on collision with a stationary block of mass 3kg. Find the velocities of the two blocks after the collision.
m1=4kg, vi1=7m/s
m2=3kg, vi2 = 0m/s
Trying to find vf1 and vf2

2. Homework Equations
Using the conservation of momentum formula.
m1(vi1)+m2(vi2) = m1(vf1)+m2(vf2)

3. The Attempt at a Solution
Plugging in the numbers I get: 4(7)+3(0) = 4(vf1)+3(vf2)
28 = 4(vf1)+3(vf2)
It is at this point where I'm stuck. How do I find vf1 and vf2? I feel like I'm missing something.

 

[ehild]

Homework Statement

A 4-kg block moving at 7m/s makes a head-on collision with a stationary block of mass 3kg. Find the velocities of the two blocks after the collision.
m1=4kg, vi1=7m/s
m2=3kg, vi2 = 0m/s
Trying to find vf1 and vf2

2. Homework Equations
Using the conservation of momentum formula.
m1(vi1)+m2(vi2) = m1(vf1)+m2(vf2)

3. The Attempt at a Solution
Plugging in the numbers I get: 4(7)+3(0) = 4(vf1)+3(vf2)
28 = 4(vf1)+3(vf2)
It is at this point where I'm stuck. How do I find vf1 and vf2? I feel like I'm missing something.

What about energy in case of elastic collision?

 

[DeltaForce]

What about energy in case of elastic collision?

Kinetic energy is conserved?
Sorry, I'm a bit lost. Could you elaborate a bit more? Thanks

 

[haruspex]

Kinetic energy is conserved?

Yes.

 

[Pallavi]

Kinetic energy is conserved?
Sorry, I'm a bit lost. Could you elaborate a bit more? Thanks

Just an example:
When a body moving with a uniform velocity v collides with another body at rest, the second body after collision moves with the same velocity as the first one.
This is not possible under normal circumstance as some amount of energy is lost during collision in the form of heat or sound.

 

[haruspex]

When a body moving with a uniform velocity v collides with another body at rest, the second body after collision moves with the same velocity as the first one.

Only if they have the same mass.
But more generally, if KE is conserved in a straight line collision then v_2f-v_1f=v_1i-v_2i, regardless of the masses.

 

[neilparker62]

On the face of it the information given is insufficient - we don't know whether this is an elastic collision, completely inelastic or somewhere in between ? I would guess elastic as assumed by others above but the OP is correct saying he/she feels there is something missing!

 

[haruspex]

we don't know whether this is an elastic collision

The thread title says it is.

 

[DeltaForce]

Only if they have the same mass.
But more generally, if KE is conserved in a straight line collision then v_2f-v_1f=v_1i-v_2i, regardless of the masses.

Oh ok, I see now. I need to substitute that that equation into the first one.
So 7 = v2f - v1f
v1f = v2f-7
28 = 3(v2f) + 4(v2f-7)
v2f = 8m/s
Thus v1f = 1 m/s
Thank you!

 

[DeltaForce]

Just an example:
When a body moving with a uniform velocity v collides with another body at rest, the second body after collision moves with the same velocity as the first one.
This is not possible under normal circumstance as some amount of energy is lost during collision in the form of heat or sound.

Thanks for responding!

 

[Pallavi]

Thanks for responding!

Your're most welcome