1. Force in Mechanical Systems

2. Force in Mechanical System
1. How do we measure forces?
- The English unit for force is the pound(lb.)
- The metric unit for force is the newton(N)

3. Force in Mechanical System
2. What is a balanced force? - forces that are equal in size and opposite in direction - example – two men pushing on a fence

4. Force in Mechanical Systems
3. What is an unbalanced force?
- force that unequal in either size or direction
- causes movement
- example – man pushing lawn mower, the mower moves

5. Force in Mechanical System
4. What is a net force?
- the force that is leftover when all forces are taken into consideration
5. A man pulls a rope with 5 N of force to the right. A second man pulls with 15 N of force to the left. What is the net force on the rope?
- 10 N to the left

6. Force in Mechanical System
6. Are the forces on the rope balanced or unbalanced?
- unbalanced
7. Will the rope move?
- Yes, because an unbalanced force causes motion

7. Force in Mechanical System
8. When is a body at equilibrium?
- when all forces are equal
- the body is at rest
9. What will happened to a body if it is at rest and balanced forces are applied to it?
- it will stay at rest (Newton’s First Law of Motion)

8. Force in Mechanical System
10. If a body is at rest and unbalanced forces (net force) act on it, what will happen.
- the body will be accelerated in the direction of the force (Newton’s Second Law of Motion)

9. Force in Mechanical System
11. If you push down a trampoline with 10 N of force, what how much force will push back on you?
- 10 N of force will push you up
- Newton’s Third Law of Motion – for every action, there is an equal and opposite reaction

10. Force in Mechanical System
12. What two factors determine the effect of a force?
- the strength of the force
- direction of the force
13. What is the strength of the force called?
- Magnitude

11. Force in Mechanical System
14. What is the difference between vectors and scalars?
- a vector is a physical quantity that has both a magnitude and direction
- forces, velocity, acceleration
- a scalar quantity only has a magnitude
- temperature, pressure, mass

12. Force in Mechanical System
15. How can we represent forces?
- all vectors can be represented by arrows
- the arrow length represents the magnitude of the force
- the arrow heading represents the direction of the force
Tail Head

13. Force in Mechanical System
16. Consider the example – A worker pulls a heavy cart with a force of 30 pounds at an angle of 30o above the floor. Draw out this vector.
30 lbs
30o

14. Force in Mechanical System
17. How do we add forces that act along the same line?
- if the forces act in the direction, we add magnitude of the forces
- if the forces act in opposite directions, we subtract the magnitude of the forces

15. Force in Mechanical System
18. Example – Five people are playing tug of war. Three people on one side pull against two people on the other side. The three each pull with 50 pounds of force to the left. The two pull with 70 pounds of force each to the right. What is the net force? (magnitude and direction)
- 10 pounds of force to the left (net force)

16. Force in Mechanical System
19. What would happen if the two people pulled with a force of 75 pounds to the right?
- the rope would not move because the forces are balanced

17. Force in Mechanical System
20. How do we add forces that act at an angle?
- you cannot add forces when they act at angles
- you can solve them graphically
21. What is the overall force when multiple forces are acting on an object?
- a resultant force
- the resultant always acts in a direction between the directions of the two forces

18. Force in Mechanical System
22. Example – Two people pull on a boat with ropes that form a right angle. The person on the right of the boat pulls with 40 pounds of force. The person in front of the boat pulls with 30 pounds of force. What is the resultant force of the boat?

19. Force in Mechanical System
- Step One – Draw the 40 lb force
- Choose scale for the line
- 1 cm = 10 lb
- use a ruler
- Step Two – Add the 30 lb force to the 40 lb force
- ALWAYS add the tail of the second vector to the head of the first vector

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-Step Three – Draw the resultant force
- connect tail of the first vector to the head of the second vector
-Step Four – Determine the magnitude and direction of the resultant
- use ruler and protractor

21. Force in Mechanical System
23. Are mass and weight the same thing?
- no, mass and weight are very different

22. Force in Mechanical System
24. What is the difference between weight and mass?
-Mass – the amount of matter in an object
- measured in pound mass (lbm) or grams
-Weight – measure of the Earth’s gravitational pull
- measured in pounds (lb) or newtons

23. Force in Mechanical System
25. Can mass change?
- no, mass does not change
26. Can weight change?
-yes, weight can change as gravity changes
-example –a person weighs less on the moon than on Earth

24. Force in Mechanical System
27. What are other common units?
SI English
Length meter (m) foot (ft)
Time second (s) second (s)
28. Mass & weight conversions
1 pound = 4.45 Newtons
1 kilogram weighs 9.8 Newtons

25. Force in Mechanical System
29. What is torque?
- the effect of a force applied on a body at some distance from the axis of rotation of that body
- torque causes rotation
30. What are two ways an object can rotate?
- clockwise and counterclockwise

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31. When is torque used?
- opening a paint can, opening a door, using a wrench to turn a nut, turning gears

27. Force in Mechanical System
32. How is torque measured?
- lb·ft (English)
- N·m (SI)
33. How is torque calculated?
- T = (F)(L)
- torque = applied force x lever arm
- applied force  lbs. or N
- lever arm  ft or m