Vectors Practice Problems for AP Physics B & C

Review the following concepts if necessary:

• A Bit About Vectors for AP Physics B & C
• Adding Vectors for AP Physics B & C

Problems

1. A canoe is paddled due north across a lake at 2.0 m/s relative to still water. The current in the lake flows toward the east; its speed is 0.5 m/s. Which of the following vectors best represents the velocity of the canoe relative to shore?





2. Force vector A has magnitude 27.0 N and is direction 74° from the vertical, as shown above. Which of the following are the horizontal and vertical components of vector A?



3. Which of the following is a scalar quantity?

Solutions

1. B - To solve, add the northward 2.0 m/s velocity vector to the eastward 0.5 m/s vector.  These vectors are at right angles to one another, so the magnitude of the resultant is given by the Pythagorean theorem. You don't have a calculator on the multiple choice section, though, so you'll have to be clever.  There's only one answer that makes sense!  The hypothenuse of a right triangle has to be bigger than either leg, but less than the algebraic sum of the legs.  Only B, 2.1 m/s, meets this criterion.

2. A - Again, with no calcultor, you cannot just plug into the calculator (though if you could, careful: the horizontal component of A is 27.0 N cos 16º is the angle from the horizontal.)  Answer B and E are wrong because the vertical component is bigger than the horizontal component, which doesn't make any sense based on the diagram.  Choice C is wrong because the horizontal component is bigger than the magnitude of vector itself - ridiculous!  Same problem with choice D, where the horizontal component is equal to the magnitude of the vector.  Answer must be A.

3. D - A scalar has no direction.  All forces have direction, including weight (which is the force of gravity).  Mass is just a measure of how much stuff is contained in an object, and thus has no direction.