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 – woman 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 happen 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, 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. How do we add forces that act at an angle?
- you cannot add forces when they act at angles
- you can solve them graphically
19. 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

16. Force in Mechanical System
20. 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?

17. 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

18. Force in Mechanical System
-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

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

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22. What is the difference between weight and mass?
-Mass – the amount of matter in an object
- The mass of an object is a measure of the objects inertia.
-Weight – measure of the Earth’s gravitational pull
The weight is a measure of the force exerted on an object by gravity.

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

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

23. Force in Mechanical System
27. 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
28. What are two ways an object can rotate?
- clockwise and counterclockwise

24. Force in Mechanical System
29. When is torque used?
- opening a paint can, opening a door, using a wrench to turn a nut, turning gears

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