# Physics Energy/Momentum

1. Mar 17, 2006

### Huskies213

Can anyone help with these 2?

1.) An object of mass 7.0 kg is dropped from a certain height. Just before it strikes the ground it has a kinetic energy of 1750 J. From what height was the object dropped? Ignore air resistance.

2.) A 2.0g bullet hits and becomes embedded in a 5.0kg wood block which is hanging from a 1.4 m long string. This causes the block to swing through an arc of 4.5 (Degrees). What was the speed of the bullet before it hit the block ?

2. Mar 17, 2006

### Pengwuino

1) At any height, a mass has a gravitational potential energy of $$mgh$$. When it is released, that potential energy is converted into kinetic energy and energy is conserved at all times by $$E_{system} = PE + KE$$. When you reach the bottom, as in this case, all of your potential energy has been converted into kinetic energy. Thus, you know what the potential energy was at the height that the mass was dropped and you can solve the question knowing that.

3. Mar 17, 2006

### Da-Force

1.) An object of mass 7.0 kg is dropped from a certain height. Just before it strikes the ground it has a kinetic energy of 1750 J. From what height was the object dropped? Ignore air resistance.

Initial, it has a PE of MGH. Finally, it has a KE of 1/2 MV^2 which is 1750 Joules. So PE=KE=1750 J. mgh=1750. (7)(9.8)h=1750.

2.) A 2.0g bullet hits and becomes embedded in a 5.0kg wood block which is hanging from a 1.4 m long string. This causes the block to swing through an arc of 4.5 (Degrees). What was the speed of the bullet before it hit the block ?

.002 kg bullet hits a 5 kg block.

Find the change in the height of the block. Use that as it's final PE. Because of conservation of energy, you can use the final PE=initial KE and solve for velocity. (You will have to change into the bullet)

4. Mar 17, 2006

### Staff: Mentor

For this one, you'll do well to break the problem into two parts: Collision and post-collision. Mechanical energy is conserved after the collision; momentum is conserved during the collision.