Studio Session 6

Impulse and momentum

Everyday, objects or people collide, sometimes by accident and sometimes on purpose.  In a collision, the momentum of each colliding partner changes in a very short time interval.  Each collision partner receives an impulse.  A momentum change or impulse requires a force.  For the momentum to change by an amount ∆p a force F must act for a time ∆t such that ∆p = F∆t.  The shorter the collision time, the larger are the forces acting on the collision partners.

Many safety devices, such as seat belts, airbags, crumple zones, etc, are standard equipment on modern cars.  The purpose of these devices is to increase the time it takes for a passenger's velocity to change by a large amount in the event of a collision.  In sports, pads are designed to increase the collision time and therefore reduce the force acting on a player during a collision which changes the player's momentum by an amount ∆p.  If a pad doubles the collision time, it decreases the force by a factor of 2.

In this session you will investigate the relationship between force, collision time and impulse, and you will also examine momentum conservation in elastic and inelastic collisions.

Equipment needed:

Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions.  This log will form the basis of your lab report.


Experiment 1

A cart rolls down an inclined track and collides with a wood block at the end of the track.  The wood block is padded with another block made of metal, wood, or foam, for four different experimental runs.  Since the car is released from rest at the same position on the track every time, its speed when it makes contact with the block is approximately the same every time.  An acceleration sensor measures the acceleration as a function of time during the collision and a computer displays the output of the acceleration sensor.  The magnitude of the interaction force F is proportional to the acceleration, F = ma.  The output of the acceleration sensor under different collision conditions is shown below.

The cart collides with an aluminum block.  The magnitude of the maximum measured acceleration is ~22 m/s2.  The collision lasts for ~0.09 s

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The cart collides with a wood block.  The magnitude of the maximum  measured acceleration is ~21 m/s2.  The collision lasts for ~0.1 s.

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The cart collides with a high-density foam block.  The magnitude of the maximum measured acceleration is ~17 m/s2.  The collision lasts for ~0.12 s.

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The cart collides with another foam block.  This type of foam is used for packing fragile materials for shipping.  The magnitude of the maximum measured acceleration is ~14 m/s2.  The collision lasts for ~0.15 s.

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The mass of the cart with the attached acceleration sensor is 555 g. 
Fill in the table below.

Bumper block Aluminum Wood High-density foam Low-density foam
Maximum force        
Collision time        
Impulse Favg*∆t        

In order to estimate the impulse during the collision, you need to find the area under the peak in the acceleration versus time graph (units m/s2 * s = m/s) and then multiply by the mass of the cart.


Experiment 2

Discuss the following situation with your partners and record your predictions.
Assume you place a wide textbook on the floor and stand on it.  Then you jump off the book onto the floor two different times.  The first time you land normally, allowing your knees to bend.  The second time you land stiff-legged, not allowing your knees to bend.  Will these jumps feel different to you.  Explain!

Jumper's knees stiff bend
Max. acceleration    
Collision time    
Max.  force    
Impulse Favg*∆t    

Discuss your results with your partners.  Compare and contrast the maximum forces, interaction times, and the total impulses given to the jumper in the two different kinds of landings.  Relate these results to your earlier experiment with the carts.


Experiment 3

You will now investigate elastic and inelastic collisions between two carts on a track.  In elastic collisions the carts bounce off each other and in inelastic collisions they stick together.  The momentum of an object is the product of its mass and its velocity, p = mv.  If external forces acting in the horizontal direction (such as friction) can be ignored in the experiments of this lab, then the sum of the momenta of the two carts prior to a collision should be the same as the sum of the momenta of the carts after the collision.  You will explore the implication of momentum conservation under various collision conditions.


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Procedure:

Video 1:  An elastic collision

Before the collision cart 1 travels towards the left and cart 2 travels towards the right.  During the collision they reverse their directions.  Determine the momentum of each cart and the total momentum of both carts before and after the collision.  Paste the table into your word document.  If friction were completely negligible, the total momentum should not change.  With a small amount of friction present its magnitude probably decreases, but only by a small amount.

Do the experiment!

Video 2:  Another elastic collision

In this experiment both carts travel to the left before and after the collision.  Determine the momentum of each cart and the total momentum of both carts before and after the collision.  Paste the table into your word document.  If friction were completely negligible, the total momentum should not change.  With a small amount of friction present its magnitude probably decreases, but only by a small amount.

Do the experiment!

Video 3:  A inelastic collision

Before the collision cart 1 travels towards the left and cart 2 travels towards the right.  After the collision they stick together and travel with the same speed towards the left.  Determine the momentum of each cart and the total momentum of both carts before and after the collision.  Paste the table into your word document.  If friction were completely negligible, the total momentum should not change.  With a small amount of friction present its magnitude probably decreases, but only by a small amount.

Do the experiment!

Discuss your results!


Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.