# Elastic Collision: Find Velocity of 2 Bodies

• TheAce3317
In summary: Good luck with your problem :)In summary, the conversation discusses two physics problems involving elastic collisions. The first problem involves finding the velocity of a steel ball and a steel block after the collision, while the second problem involves finding the speed of the center of mass of two bodies after an elastic collision. In both problems, conservation of energy and momentum are key concepts in finding the solutions.
TheAce3317
1. A steel ball of mass 0.400 kg is fastened to a cord 60.0 cm long and fixed at the far end, and is released when the cord is horizontal. At the bottom of its path, the ball strikes a 2.00 kg steel block initially at rest on a frictionless surface. The collision is elastic.

Assuming the positive direction is to the right, find, just after the collision,
Velocity of the ball
Velocity of the block

I don't know how to start this one, never the less do it.

2. A body of mass 2.4 kg makes an elastic collision with another body at rest and continues to move in the original direction but with one-fourth of its original speed. The struck block has a mass of 1.44kg.

What is the speed of the two-body center of mass if the initial speed of the 2.4 kg body was 4.5 m/s?

I plugged in the numbers into the equation (m1v1+m2v2)/(m1+m2) and got 3.234375 which I know is wrong. Don't even know how I got v2.

Last edited:
First, a bit of a tip. When looking at a physics problem, don't limit it to a problem of just one concept (i.e. conservation of momentum); as you'll see, these problems you posted are about more than just one concept, so you can't just plug and chug equations willy-nilly (think about each situation and the relevant equations for those situations. Only then should you plug :) )

For the first question- think about conservation of energy. What's the energy of the steel ball at the beginning? Is it gravitational potential energy, kinetic energy, what? And what's the energy at the bottom, when it crashes with the block? If you consider these questions you'll be able to find the initial velocity of the ball during the collision. If the collision is elastic, kinetic energy is conserved. So, use conservation of momentum and conservation of kinetic energy to wrap everything up :).

Once you get that one let's discuss question 2.
Have fun

At the beginning, I believe it has GPE, and when it hits it has KE, but i don't know how to find the initial velocity. I have been looking over my notes from class for quite some time, but I am just not seeing the right equations. Could you give me a bit of a hint?

Yep, you're right. Initially, it's just standing still at some height so it will have energy equal to mgh. Afterwards (if we set the point of zero GPE to be the maximum downwards extension of the rope, or the point where the collision occurs) it will only have KE for the collision, equal to 1/2mv^2. Remember that energy is conserved, and you should be able to find it.

I have to stop working for now, so I don't want to make you wait for nothing. You have helped with what you told me though, so thanks for that.

No problem! If you need any more help don't hesitate to ask.

## 1. What is an elastic collision?

An elastic collision is a type of collision in which kinetic energy is conserved. This means that the total amount of energy before and after the collision remains the same. In an elastic collision, the objects involved bounce off each other without any loss of energy due to deformation or friction.

## 2. How do you calculate the velocity of two bodies in an elastic collision?

The formula for calculating the velocity of two bodies in an elastic collision is:
V1f = ((m1 - m2) / (m1 + m2)) * V1i + ((2 * m2) / (m1 + m2)) * V2i
V2f = ((2 * m1) / (m1 + m2)) * V1i + ((m2 - m1) / (m1 + m2)) * V2i
where V1f and V2f are the final velocities of the two bodies, m1 and m2 are the masses of the two bodies, and V1i and V2i are the initial velocities of the two bodies.

## 3. Can you have an elastic collision between two objects with different masses?

Yes, an elastic collision can occur between two objects with different masses. In fact, the formula for calculating the velocities of the two objects in an elastic collision takes into account the difference in masses between the two objects.

## 4. What are some real-life examples of elastic collisions?

Some real-life examples of elastic collisions include a game of pool, a game of billiards, and a game of air hockey. In all of these games, the objects involved (pool balls, billiard balls, and air hockey pucks) collide with each other and bounce off without losing any energy.

## 5. How is an elastic collision different from an inelastic collision?

In an inelastic collision, kinetic energy is not conserved. This means that the total amount of energy before the collision is not equal to the total amount of energy after the collision. In an inelastic collision, some of the energy is lost due to deformation or friction. In contrast, in an elastic collision, kinetic energy is conserved and there is no loss of energy.

• Introductory Physics Homework Help
Replies
20
Views
1K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
5
Views
1K
• Introductory Physics Homework Help
Replies
4
Views
1K
• Introductory Physics Homework Help
Replies
4
Views
4K
• Introductory Physics Homework Help
Replies
22
Views
3K
• Introductory Physics Homework Help
Replies
10
Views
1K
• Introductory Physics Homework Help
Replies
9
Views
1K
• Introductory Physics Homework Help
Replies
4
Views
2K
• Introductory Physics Homework Help
Replies
6
Views
1K