PhysicsLAB Practice Problems
Scalar Dot Products of Two Vectors

Directions: On this worksheet we will be investigating the properties of 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 = (3 meters, 36º) and B = (18 meters, 90º)

What is the dot product of G = 9C4B
Question 5  Given the vectors: D = (10 meters, 126º) and A = (14 meters, 0º)

What is the value of 4D9A
Question 6  Given the vectors: A = (14 meters, 0º) and E = (18 meters, 180º) and

What is the value of 4A4E
Question 7  The work done on an object by a constant force is defined by the formula W = FDr. In this formula, F is an applied force which does NOT change in either magnitude nor in direction, and Dr 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 from point A to point B is equal to the change in the object's KE.

How much would work would be done on a 3-kg mass if F and Dr are defined as: F = 9A and Dr = 4B?
Question 8  How much would the kinetic energy of a 3-kg mass change if F and Dr are defined as: F = 9A and Dr = 4E?
Question 9  How much would the kinetic energy of a 3-kg mass change if F and Dr are defined as: F = 4D and Dr = 4C?


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