MC-14a  Collisions Between Rolling Carts

NOTE TO INSTRUCTORS:
  This lab uses PASCO carts and avoids many of the problems of the air track. The older version of this lab that uses gliders on the air track can be found in MC-14b. The three procedures outlined below will certainly take more than the three hours allotted. Please do procedure I, then as many of the others as time allows.

OBJECTIVE:
  In this experiment you will observe elastic and inelastic collisions between two PASCO carts; the carts are provided with Velcro bumpers on one end, and magnetic bumper on the other.

THEORY:
  Collisions are a common experience in our lives-the collisions of billiard balls, the collisions of two football players, the collisions of cars. Various `natural philosophers' from Galileo onwards discussed the laws that govern such collisions. As early in the XVII^th century it was understood that collisions between hard bodies, like billiard balls, behaved differently from collisions like car wrecks, where the two masses stick together after the collision. The concept of Momentum was first introduced in mechanics by Descartes as $momentum = mass \times velocity$; he did not get it quite right however, because he thought of momentum as a scalar quantity. It was Leibnitz that defined momentum as a vector quantity:

$$\vec p = m \times \vec v.$$

Newton's II^nd Law $\vec F = m \vec a$ can be rewritten

$$\vec F = m {{\Delta \vec v} \over {\Delta t}};$$

now $m \Delta \vec v$ is the change in the quantity $\vec p = m \vec v$, so one can finally rewrite Newton's II^nd law as

$$\vec F = {{\Delta \vec p} \over {\Delta t}}$$

or

$$\vec F \Delta t= \Delta \vec p.$$

The quantity $\vec F \Delta t$ is called the impulse and is equal to the change in momentum.

FUNDAMENTAL CONCEPTS:
  A physical quantity is said to be conserved in a process if its value does not change even though other quantities are changing. An example of a conserved quantity is energy. When a body falls freely its total energy (potential energy plus kinetic energy)remains unchanged while the position and velocity of the body change with time.

Linear momentum is conserved if the net force acting on an object is zero. This follows from the equation which relates the change in momentum to the impulse given to the object. Clearly if the force F is zero, the impulse is zero and the change in momentum is zero, hence the momentum remains constant.

  This same principle becomes more useful when it is applied to an isolated system of objects. An isolated system is one where the only forces acting on an object in the system are due to the interaction with another object within the same system: there are no external forces.

A simple such system may consist of two particles, let us call them A and B, interacting with each other. The force $\vec F_A$ on particle A is equal and opposite to the force $\vec F_B$ acting on particle B: $\vec F_A = - \vec F_B$. Because the times during which the forces act are the same, it follows that the changes in momentum of the two particles are also equal and opposite, so that the total change in momentum is zero. The conservation of momentum is therefore a consequence of Newton's III^rd Law. In the collision of two bodies, the sum of the initial momenta equals the sum of the final momenta:

$$\vec p_1 +\vec p_2 = \vec {p_1}' +\vec {p_2}'$$

where the unprimed quantities refer to the velocities before the collision, and the primed quantities refer to the velocities after the collision.

For one dimensional processes the physical quantity that is conserved is linear momentum.

A collision is called totally inelastic if the two bodies stick together after colliding. The conservation of momentum for a one dimensional totally inelastic collision is then:

$$m_1v_1 + m_2v_2 = (m_1 + m_2)\cdot v'.$$

Energy is not conserved in inelastic collisions.

A collision of two bodies is called totally elastic if energy is conserved in the process. In this case the result of the collision can be calculated by

(v₁-v₂) = -(v'₁-v'₂)'.

It is interesting to note that (v₁-v₂) and (v'₁-v'₂) are the relative velocities of of body # 1 relative to body # 2 before and after the collision.

APPARATUS:

 

1.

Personal Computer with monitor, keyboard, and mouse

2.

PASCO dynamic track with magnetic bumpers

3.

PASCO signal interface

4.

Two PASCO carts with a ``picket fence''

5.

A set of 500 g masses

6.

One meter long ruler

7.

Two photogates: these devices consist of a source which produces a narrow beam of infrared radiation an infrared detector at the other side of the gate that senses the radiation. When the beam between the source and detector is blocked, a red Light Emitting Diode (LED) on the top of the gate lights up; concurrently an electrical signal is sent to the Signal Interface which converts the time during which the beam was blocked into the velocity of the fence. NOTE: for the photogates to work properly, the picket fence must be on the opposite side of the cart closest to the LED.

SUGGESTIONS:

 

1.

Measure the mass of the empty carts using the pan balance; MAKE SURE YOU PUT THE CART ON ITS SIDE ON THE PAN. Record these masses in your lab notebook. Note that the cart with the magnet bumper is somewhat heavier than the cart without magnet, you may wish to tape some masses on the lighter cart to equalize the weights.

2.

Initiate the PASCO interface software in the usual way. The monitor should now look as shown in Figure 1.

 

Figure 1:

 

On the computer monitor you see a table of velocities for each of the two photogates. As seen by a person looking at the computer monitor the photogate on the left is # 1 and the one on the right is # 2.

3.

Level the track so the carts do not move to the right or the left.

4.

Place the photogates 15 cm apart.

PROCEDURE I - INELASTIC COLLISION - EQUAL MASSES

 

1.

Prepare a table like the one below:

Run # Velocity # 1 (m/s) Velocity #1 (m/s) 1 2 3

 

2.

Install a picket fence one cart (on the side opposite the LED). Make sure that it is the longest black stripe on the fence that intercepts the LED beam. Put the cart at the left end of the track. Place the other cart just to the right of photogate #2 as shown below. Check that the carts do not repel each other; the Velcro hooks and loops must be able to stick together easily.

 

Figure 2:

3.

CLK on REC. Gently push the the projectile cart to the right toward the first photogate and the target cart. Catch the carts before they hit the end of the track. CLK on STOP. Record the velocity of the projectile cart before and after the collision in Table 1.

4.

Take two more runs.

5.

ANALYSIS OF THE DATA. Prepare tables as shown below in table 2 and 3.

Car 1 Cars 1+2

$$100 \times \Delta p/p_{ini}$$ Run # p_ini (kg m/s) p_final (kg m/s) 1 2 3 Avg

Car 1 Cars 1+2

$$100 \times \Delta E/E_ini$$ Run # E_ini (J) E_final (J) 1 2 3 Avg

Calculate the initial and final momenta and enter them in table 2. Calculate the initial and final kinetic energies and enter them in table 3 Calculate and record in table 2 the percent difference in the momenta:
$100 \times (p_{ini} - p_{fin})/p_{ini}$. Calculate the average percent difference in the momentum. Calculate and record in table 3 the percent difference in energy:
$100 \times (E_{ini} - E_{fin})/E_{ini}$. Calculate the average percent difference in the energy.

1.

QUESTIONS What is a reason for the observed difference in momentum? What fraction of the energy was dissipated? What fraction of the energy should have been dissipated in the absence of friction?

PROCEDURE II - ELASTIC COLLISION - EQUAL MASSES
In this experiment you will be measuring the velocities of two bodies with equal masses, before and after a totally elastic collision.

1.

Place the photogates about 35cm apart with one cart just to the left of photogate #2. Make sure the carts will repel each other. (One cart has one end without any magnets-that end will not repel the other cart.) Place the other cart on the left end of the track. as shown in Fig 3 below. Cart # 2 should be close to gate # 2, (not as shown in the figure). Make sure that there is enough space, so that cart #2 starts moving after cart #1 has passed completely through gate #1.

 

Figure 3:

2.

Prepare a table like table 4.

Run # Velocity # 1 (m/s) Velocity #1 (m/s) 1 2 3

3.

CLK on REC. Gently push the projectile cart (the one on the left) to the right toward the first photogate and the target cart. The projectile cart should stop between the gates and the target will move through photogate # 2. Catch it before it has time to hit the end of the track; CLK on STOP. Record the velocities in table 4. Take two more runs.

4.

ANALYSIS OF THE DATA Prepare tables like tables 5 and 6.

Car 1 Cars 1+2

$$100 \times \Delta p/p_{1}$$ Run # p₁ (kg m/s) p₂ (kg m/s) 1 2 3 Avg

Car 1 Cars 1+2

$$100 \times \Delta E/E_{1}$$ Run # E₁ (J) E₂ (J) 1 2 3 Avg

Calculate the initial momentum of Car # 1 and final momentum of Car # 2 and enter them in table 5. Calculate and record the percent difference of the momenta: $100 \times (p_{1} - p_{2})/p_{1}$. Calculate the average percent difference in the momentum. Calculate the initial and final kinetic energies and enter them in table 6. Calculate and record the percent difference of the energies: $100 \times (E_{1} - E_{2})/E_{1}$.

5.

QUESTIONS What is a reason for the observed difference between p₁ and p₂? What fraction of the energy was dissipated? What fraction of the energy should have been dissipated in the absence of friction?

PROCEDURE III - ELASTIC COLLISION - UNEQUAL MASSES

1.

The photogates should be about 35cm apart. Find and record the masses of two 500g masses using the triple beam balance; place them on the target cart, place this cart just to the left of photogate # 2 as shown in fig 3 (not as shown in the figure) . The carts should be placed so they will repel each other. The lighter cart is `the projectile' and should be placed on the left end of the track.

2.

Prepare a table like table 7.

Run # V_1ini (m/s) V_1fin (m/s) V_2fin (m/s) 1 2 3

3.

CLK on REC. Gently push the projectile cart (the lighter one on the left) to the right toward the first photogate and the target cart. Try to predict what will happen. Catch the carts before they hit the end of the track. CLK on STOP. Record the velocities in Table 7

4.

Take two more runs.

5.

ANALYSIS OF THE DATA Prepare tables like tables 8 and 9.

Car 1 ini Car 1 fin Car 2

$$100 \times \Delta p/p_{1ini}$$ Run # p_1ini (kg m/s) p_1fin (kg m/s) p_2fin(kg m/s) 1 2 3 Avg

Car 1 ini Car 1 fin Car 2

$$100 \times \Delta E/E_{1ini}$$ Run # E_1ini (J) E_1fin (J) E_fin (J) 1 2 3 Avg

Calculate the initial and final momentum of each cart and enter them in table 8. Calculate the total momentum of the system p_fin after the collision and record it in table 8. Remember that the final momentum is the sum of the momenta of carts # 1 and # 2 after the collision. Calculate the initial and final kinetic energies and enter them in table 9. Calculate the total energy of the system E_fin after the collision and record it in table 9. Remember that the final energy is the sum of the energies of carts # 1 and # 2 after the collision.

Calculate and record the percent difference $100 \times (p_{1ini} - p_{fin})/p_{1ini}$. Calculate and record the percent difference $100 \times (E_{1ini} - E_{fin})/E_{1ini}$.

6.

QUESTIONS What is a reason for the observed difference? Was momentum approximately conserved? What fraction of the energy was dissipated? What fraction of the energy should have been dissipated in the absence of friction ?