
# 8.5.1: Dot Product (Exercise)


section 8.5 exercises

Two vectors are described by their magnitude and direction in standard position. Find the dot product of the vectors.

1. Magnitude: 6, Direction: $$45^{\circ}$$; Magnitude: 10, Direction: $$120^{\circ}$$

2. Magnitude: 8, Direction: $$220^{\circ}$$; Magnitude: 7, Direction: $$305^{\circ}$$

Find the dot product of each pair of vectors.

3. $$\langle 0, 4 \rangle; \langle -3, 0 \rangle$$

4. $$\langle 6, 5 \rangle; \langle 3, 7 \rangle$$

5. $$\langle -2, 1 \rangle; \langle -10, 13 \rangle$$

6. $$\langle 2, -5 \rangle; \langle 8, -4 \rangle$$

Find the angle between the vectors

7. $$\langle 0, 4 \rangle; \langle -3, 0 \rangle$$

8. $$\langle 6, 5 \rangle; \langle 3, 7 \rangle$$

9. $$\langle 2, 4 \rangle; \langle 1, -3 \rangle$$

10. $$\langle -4, 1 \rangle; \langle 8, -2 \rangle$$

11. $$\langle 4, 2 \rangle; \langle 8, 4 \rangle$$

12. $$\langle 5, 3 \rangle; \langle -6, 10 \rangle$$

13. Find a value for $$k$$ so that $$\langle 2, 7 \rangle$$ and $$\langle k, 4 \rangle$$ will be orthogonal.

14. Find a value for $$k$$ so that $$\langle -3, 5 \rangle$$ and $$\langle 2, k \rangle$$ will be orthogonal.

15. Find the magnitude of the projection of $$\langle 8, -4 \rangle$$ onto $$\langle 1, -3 \rangle$$.

16. Find the magnitude of the projection of $$\langle 2, 7 \rangle$$ onto $$\langle 4, 5 \rangle$$.

17. Find the projection of $$\langle -6, 10 \rangle$$ onto $$\langle 1, -3 \rangle$$.

18. Find the projection of $$\langle 0, 4 \rangle$$ onto $$\langle 3, 7 \rangle$$.

19. A scientist needs to determine the angle of reflection when a laser hits a mirror. The picture shows the vector representing the laser beam, and a vector that is orthogonal to the mirror. Find the acute angle between these, the angle of reflection.

20. A triangle has coordinates at $$A$$: (1,4), $$B$$: (2,7), and $$C$$: (4,2). Find the angle at point $$B$$.

21. A boat is trapped behind a log lying parallel to the dock. It only requires 10 pounds of force to pull the boat directly towards you, but because of the log, you'll have to pull at a $$45^{\circ}$$ angle. How much force will you have to pull with? (We're going to assume that the log is very slimy and doesn't contribute any additional resistance.)

22. A large boulder needs to be dragged to a new position. If pulled directly horizontally, the boulder would require 400 pounds of pulling force to move. We need to pull the boulder using a rope tied to the back of a large truck, forming a $$15^{\circ}$$ angle from the ground. How much force will the truck need to pull with?

23. Find the work done against gravity by pushing a 20 pound cart 10 feet up a ramp that is $$10^{\circ}$$ above horizontal. Assume there is no friction, so the only force is 20 pounds downwards due to gravity.

24. Find the work done against gravity by pushing a 30 pound cart 15 feet up a ramp that is $$8^{\circ}$$ above horizontal. Assume there is no friction, so the only force is 30 pounds downwards due to gravity.

25. An object is pulled to the top of a 40 foot ramp that forms a $$10^{\circ}$$ angle with the ground. It is pulled by rope exerting a force of 120 pounds at a $$35^{\circ}$$ angle relative to the ground. Find the work done.

26. An object is pulled to the top of a 30 foot ramp that forms a $$20^{\circ}$$ angle with the ground. It is pulled by rope exerting a force of 80 pounds at a $$30^{\circ}$$ angle relative to the ground. Find the work done.

1. $$6 \cdot 10 \cdot \cos(75^{\circ}) = 15.529$$

3. (0)(-3) + (4)(0) = 0

5. (-2)(-10) + (1)(13) = 33\)

7. $$\cos^{-1} (\dfrac{0}{\sqrt{4}\sqrt{3}}) = 90^{\circ}$$

9. $$\cos^(-1)(\dfrac{(2)(1) + (4)(-3)}{\sqrt{2^2 + 4^2}\sqrt{1^2 + (-3)^2}}) = 135^{\circ}$$

11. $$\cos^(-1)(\dfrac{(4)(8) + (2)(4)}{\sqrt{4^2 + 8^2}\sqrt{2^2 + 4^2}}) = 0^{\circ}$$

13. $$(2)(k) + (7)(4) = 0, k = -14$$

15. $$\dfrac{(8)(1) + (-4)(-3)}{\sqrt{1^2 + (-3)^2}} = 6.325$$

17. $$(\dfrac{(-6)(1) + (10)(-3)}{\sqrt{1^2 + (-3)^2}}^2) \langle 1, -3 \rangle = \langle -3.6, 10.8 \rangle$$

19. The vectors are $$\langle 2, 3 \rangle$$ and $$\langle -5, -2 \rangle$$. The acute angle between the vectors is $$34.509^{\circ}$$

21. 14.142 pounds

23. $$\langle 10\cos(10^{\circ}), 10\sin(10^{\circ}) \rangle \cdot \langle 0, -20 \rangle$$, so 34.7296 ft-lbs

25. $$40 \cdot 120 \cdot \cos(25^{\circ}) = 4350.277$$ ft-lbs