# Playing hockey

#### Swatch

A hockey puck B rests on smooth ice surface and is struck by a second puck A, which was originally traveling at 40.0 m/s and which is deflected 30.0° from its original direction. Puck B acquires a velocity at a 45.0° angle to the original direction of A. The pucks have the same mass. Compute the speed of each puck after the collision.

Va2 =speed puck A after collision
Vb2 = speed puck B after collision

So far my work is:

Since the momentum is conserved for both components I did this.

Va2 = (40-Vb2*(cos(45) )/cos(30)

Vb2 = (-Va2*sin(30) )/sin(45)

Okey. I have 2 unknowns I need to find and 2 equations.
When I compute these 2 I don't get the right answer.

The right answer is Va2=29.3 and Vb2=20.7

Could someone please give me a hint to what I'm doing wrong.

Related Introductory Physics Homework News on Phys.org

#### OlderDan

Homework Helper
Swatch said:
A hockey puck B rests on smooth ice surface and is struck by a second puck A, which was originally traveling at 40.0 m/s and which is deflected 30.0° from its original direction. Puck B acquires a velocity at a 45.0° angle to the original direction of A. The pucks have the same mass. Compute the speed of each puck after the collision.

Va2 =speed puck A after collision
Vb2 = speed puck B after collision

So far my work is:

Since the momentum is conserved for both components I did this.

Va2 = (40-Vb2*(cos(45) )/cos(30)

Vb2 = (-Va2*sin(30) )/sin(45)

Okey. I have 2 unknowns I need to find and 2 equations.
When I compute these 2 I don't get the right answer.

The right answer is Va2=29.3 and Vb2=20.7

Could someone please give me a hint to what I'm doing wrong.

#### Päällikkö

Homework Helper
I got
Va2 = (Va1 - Vb2cos45)/cos30
Vb2 = Va2sin30/sin45

So, I have no - (minus) in the latter equation, as the pucks, naturally, deflect in different directions.

With these equations I get the correct answers.

EDIT: I'm a slow typer.

Thanks guys.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving