Physics Laboratory 7

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 take 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 his momentum by an amount ∆p.  If a pad doubles the collision time, it decreases the force by a factor of 2.

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

Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions.  Address everything that appears in blue font.   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 interaction force F is proportional to the acceleration, F = ma.  The table below shows the output of the acceleration sensor under different collision conditions.

(a)  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.

 

                                                            Aluminum block -->

(b)  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.

 

                                                                 Wood block -->  

(c)  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.

 

                                          A block of high-density foam -->

(d)  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.

 

                                               A block of packing foam -->

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. 

bulletCompare the impulse using the four different bumper blocks.  Is this what you expected?   What significance might this have in a real car collision?  Explain.
bulletCompare maximum forces and the interaction times for the four different collisions.  Is this what you expected?  What significance might this have in a real car collision?  Explain!

Activity 1:

Place a wide textbook on the floor and stand on it.  Then 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.  Do these jumps feel different to you.  Explain!

Experiment 2:

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.

Procedure:

bulletVideo 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!

bulletVideo 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!

bulletVideo 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!

bulletDoes the total momentum of the carts change in the elastic collision experiments?
bulletDoes the total momentum of the carts change in the inelastic collision experiment?
bulletDid your experiments reproduce the expected results?  If not, speculate on the reasons for any discrepancies.

Name:
E-mail address:

Laboratory 7 Report

bulletConvert your log into a lab report. Make sure you have addressed all items in blue, and you are describing your  procedures, results and conclusions in complete sentences

Save your Word document (your name_lab7.docx), go to Blackboard, Assignments, Lab 7, and attach your document.