Finding the momentum of the each balls

[Niles]

Homework Statement

There are two questions.

1) A baseball, m=0.045 kg, moves in +y-direction with v = 5 m/s. a tennisball is moving in the -x direction with speed of 2m/s. Wat is the magnitude and direction of total momentum in the system?

2) A 0.01 kg rifle bullet is fired with v = 500 m/s into a ballistic pendulum of mass 5 kg suspended from a cord 0.6 m long. Find the height of the pendulum, the initial kinetic energy of the bullet and the kinetic energy of the bullet and pendulum.

The Attempt at a Solution

1) I have no idea, beside finding the momentum of the each balls.

2) I tried using p_bullet + p_pendulum = (m_bullet + m_pendulum)u, but I get u = 0,998 m/s, which i believe is incorrect.

 

[neutrino]

1. Look up the definition of the total momentum of a system (of particles).

2. Why do you believe it is incorrect?

 

[Niles]

1) great, will do!

2) It seems very small. Must I use that KE_bullet = KE_bullet_and_pendulum?

 

[Niles]

1) great, will do!

2) It seems very small. Must I use that KE_bullet = KE_bullet_and_pendulum?

If I do what I wrote in about KE, I get that v_(pendulum and bullet) = 22,34 m/s. But when using p_1 = p_2, I get v = 0,9980 m/s. How is this?

 

[neutrino]

2) It seems very small. Must I use that KE_bullet = KE_bullet_and_pendulum?

Think potential energy. EDIT: For the KE of bullet+pendulum: You have figured out the velocity of the two together. Now given the velocity and mass, find KE.

 

[Niles]

Think potential energy.


EDIT: For the KE of bullet+pendulum: You have figured out the velocity of the two together. Now given the velocity and mass, find KE.

So KE_bullet = PE_(bullet and pendulum), and from there i find the height.

I'm sorry, but I don't get the last thing you wrote.

 

[neutrino]

So KE_bullet = PE_(bullet and pendulum), and from there i find the height.

Forget about the KE of the bullet...some of it is lost when it hits the block. But mechanical energy is conserved after the bullet embeds itself in the block.

I'm sorry, but I don't get the last thing you wrote.

Just said in a round-about way that KE = 0.5mv² You know the mass of the bullet+pendulum, and you have figured out its (initial) velocity. (Although, the question doesn't mention it, I'm assuming that it is initial KE that is to be found)

 

[Niles]

Great, thanks!