Low-Friction Cart
Pulley and String
Triple-Beam Balance
Digital Balance
Stopwatch
Meter Stick
Mass Hanger
(1) 10 g Mass
(2) 20 g Masses
(1) 50 g Mass
Paper Clips (small masses)
Plumb Bob
Wastebasket
Figure 5.2: Note: String from cart to pulley must be
horizontal and aligned with the pulley.
27
, 28 Experiment 5: Newton’s Second Law
FREE-BODY DIAGRAM SOLUTION METHOD: INSTRUCTIONS
Step 1: Sketch the problem/situation and specify the coordinate system for each object in your system.
Step 2: Draw all forces (arrows that represent these vectors) acting on each object in the system you are investigating.
All forces should extend away from the object in the direction of the force. Remember that the length of the arrow
is an indication of the magnitude of the force.
~ = m~a = 0.0 N).
~ constant, ΣF
Figure 5.3: Free-Body Diagram of Modified Atwood’s Machine (Example for v
Step 3: Write Newton’s 2nd Law (ΣF ~ = m~a ) in component form (ΣFx = max and ΣFy = may ) for each object in
the system. For this example, mA and mB :
mA : ΣFAx = mA aAx mB : ΣFBx = mB aBx
ΣFAy = mA aAy ΣFBy = mB aBy
Step 4:
Refer to your diagram to sum the forces for each object as instructed by Sir Isaac Newton. For this example:
ΣFAx = mA aAx = T − Ff ΣFBx = mB aBx = 0.0 N
(mB : No forces acting in the horizontal direction.)
ΣFAy = mA aAy = FN − mA g = 0.0 N ΣFBy = mB aBy = T − mB g
(aAy : Constant velocity means this acceleration equals (Note that since mA and mB are connected and mov-
0.0 m/s2 ) ing together, T is the same for each object, and aAx =
−aBy .)
Step 5:
Write the known quantities. Write the question.
Always write equations algebraically first, then insert the known values.
Now do the math! That’s all there is to it.
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