# Physics Problems: Conservation of Momentum

• Feb 20th 2009, 08:54 PM
xxlvh
Physics Problems: Conservation of Momentum
I'm completely stumped on a few problems that are due after this weekend. If anyone could offer me hints on how to start them, it would be great. Thanks!

1. A 50 g bullet lodges into a 2.0 kg block of clay hung by a string. The bullet and clay rise together to a height of 12 cm. What was the velocity of the 50 g mass just before entering?

2. A 0.150 kg bullet is fired at 715 m/s into a 2.0 kg wooden block at rest. The velocity of the block afterward is 40 m/s. The bullet passes through the block and emerges with what velocity?

3. A 2.0 kg ball moving to the right at 1.0 m/s strikes a 4.0 kg ball moving left at 3.0 m/s. What are the velocities after impact, assuming complete elasticity?

4. A 2.0 kg block A and a 1.0 kg block B are pushed together against a spring and tied with a cord. When the cord breaks, the 1.0 kg block moves to the right at 8.0 m/s. What is the velocity of the 2.0 kg block?

Edit: It would appear the OP now only requires help with Questions 1. and 3.
• Feb 20th 2009, 09:44 PM
mr fantastic
Quote:

Originally Posted by xxlvh
I'm completely stumped on a few problems that are due after this weekend. If anyone could offer me hints on how to start them, it would be great. Thanks!

1. A 50 g bullet lodges into a 2.0 kg block of clay hung by a string. The bullet and clay rise together to a height of 12 cm. What was the velocity of the 50 g mass just before entering?

2. A 0.150 kg bullet is fired at 715 m/s into a 2.0 kg wooden block at rest. The velocity of the block afterward is 40 m/s. The bullet passes through the block and emerges with what velocity?

3. A 2.0 kg ball moving to the right at 1.0 m/s strikes a 4.0 kg ball moving left at 3.0 m/s. What are the velocities after impact, assuming complete elasticity?

4. A 2.0 kg block A and a 1.0 kg block B are pushed together against a spring and tied with a cord. When the cord breaks, the 1.0 kg block moves to the right at 8.0 m/s. What is the velocity of the 2.0 kg block?

Some random thoughts to be applied (after due thought) where appropriate:

Think about conservation of mechanical energy after impact and think about conservation of momentum during impact.

Elastic collision means kinetic energy of the system is conserved.
• Feb 21st 2009, 07:10 AM
xxlvh
So for the elastic collision the total sum of the momentum and kinetic energy must be equal after the equation. I don't know how to set up the equation to solve though, since none of the final velocities were given..
• Feb 21st 2009, 01:32 PM
mr fantastic
Quote:

Originally Posted by xxlvh
[snip]3. A 2.0 kg ball moving to the right at 1.0 m/s strikes a 4.0 kg ball moving left at 3.0 m/s. What are the velocities after impact, assuming complete elasticity? [snip]

Quote:

Originally Posted by xxlvh
So for the elastic collision the total sum of the momentum and kinetic energy must be equal after the equation. I don't know how to set up the equation to solve though, since none of the final velocities were given..

I don't see the trouble here. The final velocities are the things you're trying to find. So you give them a symbol like u and v and substitute into the appropriate equations.

$2(1) + 4(-3) = 2u + 4v$ .... (A)

$\frac{1}{2} (2) (1)^2 + \frac{1}{2} (4) (3)^2 = \frac{1}{2} (2) u^2 + \frac{1}{2} (4) v^2$ .... (B)

I'll let you figure which equation corresponds to which conserved quantity, and you should obviously simplify the equations. Then solve equations (A) and (B) simultaneously.
• Feb 21st 2009, 08:14 PM
xxlvh
Thank you, I'll work on finishing that question up then... any suggestions for problem #1?
• Feb 21st 2009, 08:28 PM
oswaldo
1. A 50 g bullet lodges into a 2.0 kg block of clay hung by a string. The bullet and clay rise together to a height of 12 cm. What was the velocity of the 50 g mass just before entering?

Energy should be conserved - I hope. So:
Kinetic E of bullet = Potential E of {bullet+block}
1/2 * m_b * V^2 = (m_b+m_Block)*g*height

Find V above.

Be careful about the conversion of units... g, kg, cm, m, etc...
and unit of g=m/s^2