PHYS 1405 – Conceptual Physics

Newton’s Third Law and Introduction to Momentum

 

 

Leader: _________________________          Recorder: __________________________

Skeptic: _________________________         Encourager: ________________________

 

Materials

Pasco Track                                                     500 g mass

Motion Detector                                               Force Sensor Mounted on Pasco Cart

LabPro                                                             Laptop

String

 

Introduction

      In this activity, we will explore Newton’s 3rd law of motion.  We will also introduce the idea of momentum.  To make a complete set-up for part 1, you will need to borrow a cart with a force sensor from your neighbor.

 

Part 1 Newton’s 3rd Law

Introduction

      Newton’s 3rd Law relates the forces on two objects when they interact either through a long range force such as gravity or in a short-range interaction such as a collision.  We want to focus on the direction of the forces on either object, and how the magnitudes of the forces compare.

 

Procedure

1.  Set-up

      You will use the Pasco Dynamics tracks with two carts each equipped with a force probe for this activity.  Level the tracks until the carts do not roll on their own.  Connect the LabPro to the computer and start up LoggerPro.  Make sure that the switches on both force sensors are set to the ±50 N setting.  Open the experiment file by following the path

Probes & Sensors => Force Sensors => Dual Range Force Sensor => 2-50N Dual Range.   Plug one of the force probes into CH 1 on the LabPro and the other into CH 2.  Take a small piece of tape and label the top of the force sensor plugged into CH 1with a 1 and label the other cart with a 2.  We will refer to these as cart 1 and cart 2 from now on.  Place the carts on the tracks so that the tips face each other.

 

2.   Adjust the Directions of The Sensors

We will measure the forces on each sensor.  Note that the tips on the force probes face each other so that the forces that each measure will be in opposite directions and so should be of opposite sign.  To account for this, click on the green LabPro button.  Right click on the picture of the force probe under CH 2.  Click on Reverse Direction.  Close the box.

 

3.  Zero the Force Sensors

With both carts horizontally on the track, click on the Experiment menu and then choose Zero.  Zero both force sensors.  After a few seconds, the force sensors will be ready to go.

 

Data Acquisition and Questions

      The idea here is that we will aim the carts at each other under different situations and gently push them together so that the rubber tips on the force sensors collide.  Click on the Collect button and push the carts together.  Note if you push the carts to hard then they will just push each other out of the tracks and you will not get good data.  Once you have a data set, notify your instructor and he will help you trouble shoot any problems.  The red graph shows the reading of the force sensor on cart 1 and the blue cart 2.  Print one copy of the graph as representative data.

 

1.  What exerts a force on cart 1?

 

2.  What exerts a force on cart 2?

 

3.  Thus force probe 1 measures the force on cart ___ due to cart ____ and force probe 2 measures the force on cart ___ due to cart ____.

 

4.  Circle the appropriate response from the italicized pair.  The force on cart 1 due to cart 2 is towards the front/back of the room and the force on cart 2 due to cart 1 is towards the front/back of the room.

 

5.  The forces on cart 1 and cart 2 are in ________ directions.

 

6.  How do the magnitudes of the forces that you measured compare?  Do they seem to be equal?  (Keep in mind that there might be a slight measurement error.)

 

 

7.  If one of the carts is initially at rest and you push the other cart towards it, predict how the graphs of the forces will compare.  Explain your reasoning.

 

 

 

8.  Carry out the procedure in question 7 after you have carefully answered question 7.  How did the graphs of the forces compare?

 

 

9.  If you add a large mass to one of the carts and collide them, predict how the graphs of the forces will compare.  Explain your reasoning.

 

 

10.  Add 500 g to one of the carts and carry out the procedure outlined in question 9 after you have carefully answered question 9. How did the magnitude of the forces compare?

 

Summary Questions for Part 1

1.  If identical carts collided at identical speeds the forces on the carts were _______ in magnitude and ________ in direction.

 

2.  If identical carts collided with one cart initially at rest the forces on the carts were _______ in magnitude and ________ in direction.

 

3.  If a more massive cart collided with a less massive cart the forces on the carts were _______ in magnitude and ________ in direction.

 

4.  In general if two objects interact, the force exerted on the first object by the second object is _______ in magnitude and _______ in direction to the force on the second object exerted by the first object.  This result is known as Newton’s 3rd law.

 

 

Part 2  Introduction to Momentum

Introduction

      In this activity we will introduce the notion of momentum and identify what is needed to change momentum.  Momentum is a measure of inertia.  It is defined as

momentum = mass x velocity.  We customarily denote momentum with the letter p so in symbols we write p = mv.  Momentum is a vector quantity.  Its direction is the same as the velocity. In the SI system, the units of momentum are kg m/s.  This unit has no special name.

 

1.  If a car has a mass of 1200 kg and is traveling west at 20 m/s, what is its momentum?  Include units.

 

 

2.  A .6 kg cart is sitting on a track at rest.  What is its momentum?

 

 

Procedure

1.  Set-up

      If not already connected, connect the LabPro to the laptop and if not already open, start LoggerPro and open the experiment file called “L03In3” in the Mechanics folder in the Tools for Scientific Thinking folder.  We will not worry about calibrating the force probe.  Make sure that the hook is on the force probe. Connect the force probe to CH 1 on the front of the LabPro and the motion detector to DIG/SONIC 2.  Make sure that the track is level so that the carts do not roll on their own.

 

2.  Use the electronic balance to measure the mass of the cart with the force probe on it.

 

mass of cart = ______________ kg

 

3.  Place the motion detector on the table at one end of the track facing the cart.

 

4.  Zero the force sensor

      Click on the Experiment menu and choose Zero…  Zero the Force probe.  After a few seconds, the force sensor will be ready to go.

 

 

Data Acquisition and Questions

1.  Give the cart a gentle shove away from the motion detector and click on the collect button.  Record the average velocity of the cart.

 

 

2.  Determine the momentum of the cart.  Include units.

 

 

If we call the cart moving away form the motion detector positive, then the cart moving towards the motion detector will be moving in the negative direction.

 

3.  Be sure to not let the cart hit the motion detector in this question.  Give the cart a gentle shove towards the motion detector and click on the collect button.  Record the average velocity of the cart.

 

 

4.  Determine the momentum of the cart.

 

 

5.  Add a 500 g mass to the cart.  What is the total mass of the cart now?  

 

 

6.  Give the cart a gentle shove away from the motion detector and click on the collect button.  Record the average velocity of the cart.

 

 

7.  Determine the momentum of the cart.

 

 

8.  Be sure to not let the cart hit the motion detector in this question.  Give the cart a gentle shove towards the motion detector and click on the collect button.  Record the average velocity of the cart.

 

 

9.  Determine the momentum of the cart.

 

 

Attach a piece of string to hook on the force sensor.

 

Click on the collect button and once the motion detector starts clicking, give a tug on the string to start the cart moving.  Print the graph for future reference.

 

10.  What was the momentum of the cart before the tug?

 

 

11.  Determine the momentum of the cart after the tug.

 

12.  Did the momentum of the cart change before and after the tug?

 

13.  When you tugged on the string a ________ was exerted on the cart.

 

Start the cart at the opposite end to the motion detector.  Give the cart a gentle shove towards the motion detector and then click on the collect button. Once the motion detector starts clicking steadily, pull on the string to stop the cart.  Print the graph for future reference.

 

14.  What was the momentum of the cart before the pull?

 

 

15.  Determine the momentum of the cart after the pull.

 

16.  Did the momentum of the cart change before and after the tug?

 

17.  When you tugged on the string a ________ was exerted on the cart.

 

18.  If you exert no force on the cart, does the momentum change?  Explain your answer in terms of Newton’s first law.

 

 

19.  When you do exert a force on the cart does the momentum change?

 

This is actually how Newton originally wrote his second law.

 

20.  When a net _________ is exerted on object, the _________ of the object changes.