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Property of LS&A Physics Department Demonstration Lab

Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan48109

Newton’s Laws of Motion

UM Physics Demo Lab 07/2013

Pre-Lab Question

What is required to make an object at rest accelerate?

EXPLORATION

Exploration Materials

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Property of LS&A Physics Department Demonstration Lab

Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan48109

1 Pitsco PC Sportster car

8 ½ inch cut steel washers

1 ½ inch x 2” steel hex bolt with nut

19” diameter round balloon

1 Balloon Pump

12” piece of 3/8” OD-1/4” ID clear vinyl tubing

13” piece of 1/2” OD-3/8” ID black vinyl tubing (photo gate flag)

2 spring scales

1 clear plastic ruler

6 small rubber bands

Shared/Consumable Components:

4 digital gram scales

Additional rubber bands and balloons

10 Pitsco 4-gram CO2 canisters

Guide wire/data acquisition system for Pitsco cars

Mass and Inertia

Car Number______

  1. Describe the state of motion for your car sitting undisturbed on the table.
  1. How can you change the state of motion for your car? (Hint: use your hand to move the car). What can you say about the net force on the car when the car is accelerating?
  1. Recalling our study of equilibrium for objects at rest, what must be true about the net force on the car if it is to start moving?
  1. You are going to push two vehicles on level ground. You are able to push with a steady fifty pounds of force. Which will accelerate (gain speed) more rapidly—a compact car or a minivan?
  1. What intrinsic property of the vehicle makes it accelerate more or less rapidly? Ignore any effects due to friction, air resistance etc.
  1. Consider the motion of the air puck. In principle it will move forever in a straight line. In reality what eventually brings it to a stop?
  1. Next imagine that you are riding along side the air puck in a golf cart which matches the motion of the air puck exactly for both speed and direction. What is the motion of the air puckrelative to you and the golf cart?
  1. What is the net force acting on your car sittingat rest on the table?
  1. In light of your answers to questions 7 and 8, what is the net force acting on the air puck as it moves with aconstant speed anddirection?
  1. Considering your answers to questions 7-9, what states of motion are possible for an object with zero net force acting on it?

Action and Reaction

Hook two spring scales together and pull on them while they rest on the table. Call the right hand scale “R” and the left hand scale “L”.

  1. When you pull on the scales, how do the two scale readings compare?
  1. What is the direction of the force scale R exerts on scale L?
  1. What is the direction of the force scale L exerts on scale R?
  1. How do the magnitude and direction of the force that R exerts on L compare with the magnitude and direction of the force that L exerts on R?
  1. Could you pull on scale R withscale L without scale R pulling back on scale L?
  1. Now stop pulling on the scales. Now what forces do the two scales exert on each other?

Reaction Propulsion

Now construct a propulsion system for your car using the balloon, tubing and rubber bands provided. You can attach your balloon “engine” to the car with more rubber bands. The balloon can be inflated using the hand pump provided.

  1. Draw a sketch of your car with the propulsion system attached.
  1. Explain how your propulsion system will make the care move on the table top, and then demonstrate it to your instructor.

Impulse and Momentum

You will now be assigned a number of washers at random. Remove your balloon propulsion “engine” from your car and load the car with the assigned number of washers, placing one washer under the head of the bolt on the bottom of the car. If you are assigned zero washers, leave your car unloaded and do not attach the bolt.

You will now use a compressed CO2 gas canister to propel your car down the Physics 106 Drag Strip, where it will pass through a “speed trap” made with two photo detectors.These “photo gates” measure the time it takes the car to pass through a distance of 0.35 meters, thereby determining its speed. You will also measure the final mass of your car using one of the scales provided. Record and reduce your data and all the data for the rest of the class in the table below (be sure to convert the mass of your car from grams to kilograms where indicated):

Your Data

Group #: ______# Washers: ______

Final Mass (g): ______Time (s): ______

Class Data

Group # / #Washers / Final Mass (kg) / Time (s) / Final Speed =(0.35m/Time)
(m/s) / (Final Mass)
X (Final Speed) (kg-m/s)
1
2
3
4
5
6
7
8
  1. Do higher-mass or lower-mass cars attain larger final speeds?
  1. What do you notice about the product of the final speed and final mass for all the cars?
  1. Based on your answer to question 20, predict the final speed for a car with a final mass of ½ kilogram. Show your calculation!

Everyday Applications

APPLICATION

  1. Cite an example of reaction propulsion used in an actual vehicle. Explain how the propulsion system uses Newton’s Laws of Motion to propel the vehicle.
  1. Dr. Evil has imprisoned you in a devilish and overly contrived trap. He has marooned you on a sled in the middle of a perfectly frictionless pond with only a pile of snowballs to use to make your escape. Due to the presence of man-eating penguins on shore, the only safe place to exit the pond is due east of your current position. Describe how you can use the snowballs to make your way to safety.

Summary:

  • An object willremain at rest or move with a constant velocity until a net force acts upon it: Newton’s First Law
  • Forces are always exerted in action-reaction pairs-if I push on the car with my hand (action), the carpushes on my hand with an equal and opposite force (reaction): Newton’s Third Law. Action-reaction pairs never act on the same object.
  • The product of an objects mass and velocity vector is its linear momentump=mv
  • The net force is equal to the rate at which an object’s momentum changes with time F=Δp/Δt. If the object has a constant mass, then the net force equals the product of the object’s mass and acceleration F=ma: Newton’s Second Law. These are vector equations.
  • The product of a force and the time it acts is called impulse (denoted as J) and Newton’s Second Law can be stated as “the impulse imparted to an object by a force is equal to the object’s change in momentum” J=FΔt=Δp

Cleanup

Please dismantle your “balloon engine”, remove the bolt, washers and CO2 canister from your car and discard the CO2 canister in the bucket provided.

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Property of LS&A Physics Department Demonstration Lab

Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan48109