Download Name(s) Hr. ____ Investigating Newton`s Second Law by Pulling a

Name(s) ________________________________________________ Hr. ____
Investigating Newton's Second Law by Pulling a Wagon
Background: Isaac Newton comes up with three laws (or guidelines) of motion. These
laws integrate (or bring together) the concept of the force (push or pull) with our
concepts of motion. The first law states that to change the state of motion of an object,
we need to place an unbalanced force on that object. To get an object to move, we need
to apply a force, etc. Newton's second law states that if we have a set of forces on an
object that is unbalanced, the object will accelerate [Fnet = ma (not your dad, but your...)].
Newton's third law is action/reaction.
In this activity, we will try to test Newton's second law by pulling a wagon. I've created a
movie and would like you to do the same. My movie is located on the web here:
However, to make things work out right we have to make a careful video. Here are
important things to keep in mind when making this video:
 film from left to right
 attach a rope or string tightly to the handle of the wagon
 to this handle attach the hook of the spring scale (luggage scale). You will pull on the handle of
the luggage scale.
 start pulling the wagon (and person) and keep the pull constant. This will be difficult but try your
best. By keeping the pull constant, you should feel like the wagon is getting faster and faster.
Watch your dial and TRY TO KEEP the reading constant. MAKE SURE THAT YOU REMEMBER
WHAT YOUR DIAL said as you pulled.
 move your camera back so you can film about 5 meters and keep the camera very still as you film
 scale the video using a meterstick (place it on the wagon with the holder) or scale from the
wagon itself.
 VERY important: keep the handle and string as horizontal as possible!
Investigating Newton's Second Law by Pulling a Wagon, p. 1
Force Analysis (Force diagram):
We have four basic forces acting on the wagon and rider. I pull on the string which is
causing tension in the rope. This tension is the forward force. However, friction is acting
against my tension. The wagon and rider have weight (downward) and the floor supports
the wagon and rider (normal force):
FRICTION
TENSION
Normal Force
Weight
If you pull hard and consistently, the TENSION that pulls the rider and wagon forward
should be a bit larger than the FRICTION. We have an UNBALANCED set of forces
horizontally. Since our horizontal forces are unbalanced, we should see the wagon and
rider accelerate or get faster. Vertically, the wagon is not moving. Its acceleration is zero
and thus the WEIGHT and NORMAL force are equal in value but opposite in direction.
These two forces are balanced.
KEY IDEAS integrating forces and motion:
 Unbalanced forces mean the object will accelerate. This is what is happening
horizontally to the wagon.
 When forces are balanced, we have no acceleration (v = 0 or v = constant). This is
what is happening vertically to the wagon.
Investigating Newton's Second Law by Pulling a Wagon, p. 2
PROCEDURE:
1. BEFORE you create your video, we
need to determine the friction that
will be present when you pull the
wagon and occupant.
a) Make sure you have the
wagon wheels straight.
b) Attach your rope or string to
the handle and then the hook of
the luggage spring scale to the
string. Make sure you can see
the dial of the luggage spring
scale.
c) Make sure the black needle of
the luggage spring scale is reset back to zero.
d) Slowly pull the wagon using the handle of the
luggage spring scale attached to the string. PULL
horizontally (parallel to floor) until the wagon starts to move.
e) The black needle will move to a reading that indicates the pull force needed to
start motion. This will represent the friction that you need to overcome to activate
motion.
f) Write down the number that the
black needle is showing. Use the
outside numbers on the dial. This will
be in pounds.
Friction = ______________________ lbs
g) convert this to Newtons by multiplying by 4.45 Newtons (4.45 Newtons per lb) by your
previous number
Friction = ______________________ Newtons
Investigating Newton's Second Law by Pulling a Wagon, p. 3
2. Create your video but read the following steps first.
a) make sure your wheels are straight before you begin.
b) You need to pull HARDER than the number you pulled on earlier (step 1, letter f).
c) Keep this pull constant to the best of your ability. The puller needs to remember
what the dial said when you pulled (numbers on the outside of the dial).
d) Keep the pull horizontal (very important!)
e) make sure you film left to right
f) make sure the camera is far enough back to get 5 meters or so of motion.
g) make sure the wagon is NOT moving when you start filming.
h) the wagon and rider should get faster and faster if you are pulling hard.
i) hold the camera really still
3. Before you analyze your video, let’s write down your numbers and make some
calculations.
4. Write down your pull force that you pulled on the video. This will be equal to the
tension created in the string. It will be in lbs.
a) Tension (pull force) = ______________________ lbs
Now convert to Newtons: (x 4.45 N/lb to get Newtons):
b) Tension (pull force) = ______________________ Newtons
5. Get the weight of the rider in lbs (step on our bathroom scale) and add 20 lbs for the
wagon:
a) Weight of rider AND wagon = ________________________ lbs
x 4.45 Newtons/lb to get Newtons.
b) Weight of rider AND wagon = ________________________ Newtons
Now get the mass of the rider by dividing the previous answer by 9.8 (Weight/g = m):
c) Mass of the rider AND wagon = ________________________ kg
Investigating Newton's Second Law by Pulling a Wagon, p. 4
6. Newton's Second Law states that [Fnet = ma (not your dad, but your...)].
FRICTION
TENSION
Our Fnet would be our (Tension – Friction). Friction is negative as it acts to oppose the
tension force.
So our equation would be:
Fnet = ma OR
Tension – Friction = m(a) where m is the mass (in kilograms) and acceleration is a. We are
going to solve this for the acceleration. Our Tension and Friction must be given in
Newtons. The acceleration will be in m/s/s.
The tension is what we you found in step 4b
The friction is what you found in step 1g
The mass is what you found in step 5c
________________Newtons - ________________ Newtons = ___________ kg ∙(a)
FIRST, subtract the left side and then divide by your mass to get your acceleration.
Acceleration = _________________ m/s/s (Newtons divided by kg = m/s/s)
Now let’s find the acceleration using Logger Pro. We will compare the acceleration we
get on Logger Pro with the acceleration determined on the previous line with Newton’s
2nd Law.
7. Open up logger pro and insert your movie.
8. When the movie opens, right click,
and activate “First VA point…” I also
advanced the movie ’10’ frames.
Investigating Newton's Second Law by Pulling a Wagon, p. 5
9. Scale the movie. I didn’t have a meterstick that day so I used the length of the sitting
part of the wagon (0.95 meters):
10. Now fast forward to the part of the movie where the wagon is just about ready to
move. Start putting in your points. I got about 15 points (1 point per 10 frames). The
spacing of the points should grow, meaning that the wagon is getting faster!
11. You will see x and y data on your graph. Choose the x data. Notice we get a parabola.
This means that the wagon is accelerating!
Investigating Newton's Second Law by Pulling a Wagon, p. 6
12. Now choose the X-velocity data. This should look linear, at least part of it should be
nicely linear:
Remember to get the acceleration, we need to get the slope. Highlight the linear part
and do a linear curve fit. Write down the slope below
Acceleration = _____________________________________ m/s/s
………………………………………………………………………………………………..
Go to the next page…please
Investigating Newton's Second Law by Pulling a Wagon, p. 7
13. So, we first got our acceleration using Newton’s second Law (force analysis):
FRICTION
TENSION
Fnet = ma
Tension – Friction = m(a)
Your acceleration found this way is on PAGE 5, step 6. Rewrite it here:
Acceleration = _________________ m/s/s (Newtons divided by kg = m/s/s)
The second way to get the acceleration is through logger pro (video analysis). We did
that on the previous page. Rewrite it here:
Acceleration = _____________________________________ m/s/s
Both of these numbers should be close as you are finding the same concept (acceleration)
but rely on different methods.
Were your two accelerations close? ______________________
……………………………………………………………………..
Summary, PLEASE READ: Newton’s second law deals with forces. When we pull the
wagon, we have two horizontal forces: your pull force (creating a tension in the string)
and friction. Friction works against your pull force. We can subtract these two forces to
get the net force. However, Newton’s second law is expressed: Fnet = ma
So, if we know our net force, we can then divide by the mass of what we were pulling
(wagon and rider) to determine the acceleration.
KEY IDEAS integrating forces and motion:
 Unbalanced forces (Fnet ≠ 0) mean the object will accelerate, either getting faster or
getting slower.
 When forces are balanced (Fnet = 0), we have no acceleration (v = 0 or v = constant).
Investigating Newton's Second Law by Pulling a Wagon, p. 8