Foundations of Physics

Workshop: The Momentum Collider

The Momentum ColliderCPO Science

Key Questions What is Momentum?

What are some useful properties of momentum?

How can we measure and observe momentum?

What role does momentum play in collisions and how can we use it for calculations?

What is Momentum? Property of moving matter

Like mass, it measures an object’s resistance to a change in speed or direction

The product of an object’s mass and velocity

IMPORTANT – Remember velocity is a vector so DIRECTION is very important

Setting up the Collider Allows us to measure

and observe momentum

The collider is level and plumb

This ensures the projectile and target will collide squarely

Practice releasing the projectile a few times

Two Objects Loop the String of the Target over

the post on the side of the hanger

Take a few practice swings with the projectile to get a feel for the release

Measure the Projectile’s Velocity Loop the String of the Target over the

post on the side of the hanger

Only the projectile will be swung

Swing the projectile through the photogates once, then catch it so it does not swing back through

Calculate the velocity of the projectile; the diameter of the projectile is 2.50 cm

Velocity is a vector!! It is direction sensitive!

Collect Data

Use the CPO Data Collector and photogates to see how long it takes the marble to break the light beam at points A and B

Calculate speeds

Investigate Motion of Projectile

How would you calculate the velocity?

0

What about MASS?Don’t we need MASS to calculate momentum?

We will calculate the mass of the target from our measurements of velocities and the mass of the projectile at the end

How? We will use a “conservative” approach

Conservation of Momentum Like energy, momentum

obeys a conservation law

After the collision both balls may be moving with different speeds and in different directions

the total momentum after the collision must be equal to the total momentum before the collision

mpv0 = mtvt + mpvp

Different Kinds of Collisions In an elastic collision, the objects bounce

off of each other with no loss in the total kinetic energy.

In an inelastic collision, objects may change shape, stick together, or ‘lose’ some kinetic energy to heat, sound, or friction.

Momentum is conserved in both elastic and inelastic collisions, even when kinetic energy is not conserved.

Two Objects…Again This time we will use both objects

to perform a collision (target diam. 3.175 cm)

Double check to make sure they are aligned

Predict with your group – Elastic or Inelastic?

Performing A Collision Allows us to

measure and observe momentum

The collider is level and plumb

This ensures the projectile and target will collide squarely

Observations The projectile collided with the target

The projectile actually bounced backward in the opposite direction!

The target swung in the same direction as the projectile, even though the projectile “bounced off” it

Try it again but this time, record data

Calculate the Three Velocities 1st velocity– the velocity of the

projectile as it approaches the

collision vo

2nd velocity– the velocity of the

projectile as it bounces back vp

3rd velocity – the velocity of the

target after the collision vt

Using Conservation of Momentum

the total momentum after the collision must be equal to the total momentum before the collision. Insert velocity values in cm/sec

mpv0 = mtvt + mpvp

Conservation of Momentum

mp113.9 = mt74.5 + mp-32.3

mp113.9 = mt74.5 - mp32.3 Don’t Forget About Direction!

mp146.2 = mt74.5

mp146.2 = mt74.5Divide both sides by 74.5

mp146.2/74.5 = mt

mp1.96 = mt

If the projectile ball has a mass of 67.2 g, what is the mass of the target ball?

We have used Momentum We calculated the ratio of the

masses involved in the collision

We used the Conservation of Momentum Equation to do it

What would happen if they were the same mass?

What are other ways you can think of to use this equation?