PhysicsLAB Practice Problems
Scalar Dot Products of Two Vectors

Directions: On this worksheet we will be investigating the properties of the dot products of two vectors. There are two principle ways to calculate the scalar dot product, AB, of two vectors. As the name implies, it is important to notice that the dot product of two vectors does NOT produce a new vector; instead it results in a scalar - that is, a value that only has magnitude or size, not direction. These methods are:
  • AB = |A| |B| cosq

    |A| and |B| represent the magnitudes of vectors A and B, while q is the size of the angle between them when placed tail-to-tail

  • AB = AxBx + AyBy

    Ax and Bx represent the horizontal components of vectors A and B, while Ay and By represent their vertical components
The example vectors displayed in the table below are not drawn to scale; however, they do indicate correct relative directions.


A

B

C

D

Question 1  Would the dot product AC be positive, negative, or zero?
Question 2  Would the dot product AB be positive, negative, or zero?
Question 3  Would the dot product AD be positive, negative, or zero?
Question 4  Given the vectors: C = (2 newtons, 42º) and B = (14 meters, 90º)

What is the dot product of W = 9C2B?

[NOTE: work is defined as the dot product of a force vector, F, with its displacement vector, s.]
Question 5  Given the vectors: D = (11 meters, 132º) and A = (11 meters, 0º)

What is the value of G = 6D9A
Question 6  Another application for the scalar dot product is BA which determines the number of magnetic flux lines, ϕ, measured in webers, that pass through a given cross-sectional area. Suppose the magnetic field is given by the vector B = (11 Teslas, 0º) and the cross-sectional area vector is given by A = (28 m2, 180º).

What is the magnitude of the flux (or field lines) passing through the specified area?

[NOTE: a positive sign means that the flux lines are exiting surface, A, while a negative sign means that the flux lines are entering surface A.]
Question 7  As stated earlier, the work done on an object by a constant force is defined by the formula W = Fs. In this formula, F is an applied force which does NOT change in either magnitude nor in direction, and s is the length of the path along which this force is exerted. The work-kinetic energy theorem states that the net work done by one or more forces acting on an object as it moves between two positions is equal to the change in the object's KE.

How much would work would be done on a 2-kg mass if F and s are defined as: F = 9A and s = 2B where A = (11 newtons, 0º) and B = (14 meters, 90º)?
Question 8  How much would the kinetic energy of a 2-kg mass change if F and s are defined as: F = 9C and s = 6E where C = (2 newtons, 42º) and E = (14 newtons, 222º) ?

[NOTE: when work is positive the object's KE is increasing; while negative work means that the object's KE is decreasing.]
Question 9  How much would the kinetic energy of a 2-kg mass change if F and s are defined as: F = 9C and s = -6E?


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