Goal: Hone the vector nature of force.
A thin wire is stretched horizontally between two walls. If a weight, W,
is hung on the wire, what is true about the tension, T, in the wire?
- T < W
- T = W
- T > W
- The relationship between T and W cannot be determined.
(3); The vertical component of the tension force exerted on the weight
must equal half the gravitational force. For a nearly horizontal wire
this means the tension must exceed the weight.
The intent is to get students to use their intuitive notions of
balancing force to reason how directions of forces change the value of
Questions to Reveal Student Reasoning
How do you determine the tension force? What direction does the tension
force point? In (A) what does the tension force equal? Does it matter
what angle the strings point? Explain.
Have some students try to lift an object with strings straight up and at
an angle. Which is easier.
Set up a demonstration with spring scales.