Lab/ Help me reason why the error?

  • #1
I wrote a lab report for my physics class and i can't finish my conclusion. In conclusion, I have to write why the results i got was so off and so far i have only two weak reasonings. If any of you can see better or more reasonings, please leave me a comment and elaborate a little bit if you can. Thank you.
Here are my reasonings so far
a) the friction of the track in each experiment might have been differnt
b) in my lab, gravity(9.8m/s2) might not have been a theoretical value. So that's why the results were so off.


I. Purpose

This experiment consisted of two parts to compare the net force of a cart, driven by a fan. In the first part, we calculated the net force of the cart on a horizontally flat track by measuring the mass and acceleration of the cart. In the second part, we raised the end point of the track from the ground in order to make an upward slope for the cart. By finding the length of the track and the height of the raised endpoint from the ground, we calculated the net force of the cart. According to Newton¡¯s Second Law, a body accelerates if a net external force acts on the body. The direction of acceleration is the same as the direction of net force. The net force vector is equal to the mass of the body times the acceleration of the body. For the first experiment we can use the equation below to calculate the net force of the cart:
SF = m * a
(F = force in a vector form, m = mass of an object, a = acceleration in a vector form)
For the second part, we use a different equation since the track is no longer flat on the ground but rather has an angle:
SF = m * g (sin A)
(g = 9.8m/s2, A = the angle between the flat track and the ground)

II. Summary of Procedure

Before the start of experiment, we prepare a track, a cart, a motion sensor, a fan, a balance, a ruler and two batteries.
In the first part of the experiment, a track is placed on the horizontally flat surface of the table, and its length is measured using a ruler. A fan is attached on the top of the cart, and two batteries are put on. Using a balance, we measure the mass of the object, which consists of cart, fan and batteries. Then, a motion sensor is set up on the edge of starting point so that we can measure the velocity of the car once the cart starts moving. Then, the cart is put on at the starting position of the track, and we turn on the fan. While the cart accelerates, driven forward by the energy of the fan and moves toward the opposite end of the track, the motion sensor detects cart¡¯s acceleration. We repeat this procedure three times to derive an accurate result.
In the second part of the experiment, one end of the track is slightly raised so that the track has a slope. A car is put on the lowest part of the track, and its fan is turned on. The slope of the track is slowly raised so that the cart completely stops on the track with its final velocity equals zero. When the cart stops, the height from the upper end of the track to the table will be measured with the ruler. The same steps are repeated three times to get an accurate result.

III. Data/ Results
First Part:
Length of the track = 129cm
Mass of the o cart + fan + batteries = 0.724kg
Height of the raised track = 1) 4cm, 2) 4.1cm, 3) 4.4cm
Second Part:
Mass of the o cart + fan + batteries = 0.724kg
Acceleration of the object = 1) 0.1562m/s2, 2) 0.1769m/s2, 3) 0.1825m/s2

1) Calculation
First Part:
a) A = angle between the flat ground and the track = sin-1(4.0/129) = 1.78 degree
b) A = angle between the flat ground and the track = sin-1(4.1/129) = 1.82 degree
c) A = angle between the flat ground and the track = sin-1(4.4/129) = 1.95 degree
a) S Fx = m * g * (sin A) = (0.724kg) * (9.8m/s2) * (sin 1.78degree) = 0.220 N
b) S Fx = m * g * (sin A) = (0.724kg) * (9.8m/s2) * (sin 1.78degree) = 0.230 N
c) S Fx = m * g * (sin A) = (0.724kg) * (9.8m/s2) * (sin 1.78degree) = 0.242 N
Second Part:
a) First Trial: S Fx = m * a = (0.724kg)(0.1562m/s2) = 0.113 N
b) Second Trial: S Fx = m * a = (0.724kg)(0.1769m/s2) = 0.128 N
c) Third Trial: S Fx = m * a = (0.724kg)(0.1825m/s2) = 0.132 N

Answers and Replies

  • #2
No, I don't think gravity is likely to be noticably different in your lab than anywhere else! (The force of gravity DOES change slightly from place to place, but not enough for you to measure.)

One significant source of error might by your failure to understand what you are doing.

You state that the "first" and "second" parts of your experiment consist of accelerating the carts on a horizontal track and then on an inclined track. Yet you give data only for the inclined track. What you call "first part" and "second part" appear to be calculations of gravitational force and net force on the cart.

What exactly do you see as an error? What numbers are off?
Are you saying that the first numbers, 0.220 N, 0.230 N, and 0.242 N, should be the same as the second set, 0.113 N, 0.128 N, and 0.132 N?
Surely not. The net force, the second set of numbers, should be the force exerted by the fan minus the gravitational force. Put another way, the force exerted by the fan should be the sum of the gravitational force and the net force.
In other words, the values you get from adding the corresponding numbers above should be approximately the force you got from the "first part" of the experiment, on the horizontal. Unfortunately, you didn't report those.

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