Force in Mechanical Force in Mechanical SystemsSystems

Force in Mechanical SystemForce in Mechanical System1. How do we measure forces?1. How do we measure forces?

- The English unit for force is - The English unit for force is the pound(lb.)the pound(lb.)

- The metric unit for force is - The metric unit for force is the newton(N) the newton(N)

Force in Mechanical SystemForce in Mechanical System

2. What is a balanced force?2. What is a balanced force?- forces that are equal - forces that are equal

in size and opposite in in size and opposite in directiondirection

- example – two men - example – two men pushing on a fencepushing on a fence

Force in Mechanical SystemsForce in Mechanical Systems

3. What is an unbalanced force?3. What is an unbalanced force?

- force that unequal in either size - force that unequal in either size or directionor direction

- causes movement- causes movement

- example – man pushing lawn - example – man pushing lawn mower, the mower movesmower, the mower moves

Force in Mechanical SystemForce in Mechanical System

4. What is a net force?4. What is a net force?

- the force that is leftover when all - the force that is leftover when all forces are taken into considerationforces are taken into consideration

5. A man pulls a rope with 5 N of 5. A man pulls a rope with 5 N of force to the right. A second man force to the right. A second man pulls with 15 N of force to the left. pulls with 15 N of force to the left. What is the net force on the rope?What is the net force on the rope?

- 10 N to the left- 10 N to the left

Force in Mechanical SystemForce in Mechanical System6. Are the forces on the rope 6. Are the forces on the rope

balanced or unbalanced?balanced or unbalanced?

- unbalanced- unbalanced

7. Will the rope move?7. Will the rope move?

- Yes, because an unbalanced - Yes, because an unbalanced force causes motionforce causes motion

Force in Mechanical SystemForce in Mechanical System8. When is a body at 8. When is a body at

equilibrium?equilibrium?- when all forces - when all forces are equalare equal- the body is at rest- the body is at rest

9. What will happened to a body if it 9. What will happened to a body if it is at rest and balanced forces are is at rest and balanced forces are applied to it?applied to it?- it will stay at rest (Newton’s First - it will stay at rest (Newton’s First Law of Motion)Law of Motion)

Force in Mechanical SystemForce in Mechanical System

10. If a body is at 10. If a body is at rest and rest and unbalanced forces unbalanced forces (net force) act on it, (net force) act on it, what will happen.what will happen.

- the body will be - the body will be accelerated in the accelerated in the direction of the direction of the force (Newton’s force (Newton’s Second Law of Second Law of Motion)Motion)

Force in Mechanical SystemForce in Mechanical System11. If you push down a 11. If you push down a

trampoline with 10 N of trampoline with 10 N of force, what how much force force, what how much force will push back on you?will push back on you?

- 10 N of force will push you - 10 N of force will push you upup

- Newton’s Third Law of - Newton’s Third Law of Motion – for every action, Motion – for every action, there is an equal and there is an equal and opposite reactionopposite reaction

Force in Mechanical SystemForce in Mechanical System

12. What two factors determine 12. What two factors determine the effect of a force?the effect of a force?

- the strength of the force- the strength of the force

- direction of the force- direction of the force

13. What is the strength of the 13. What is the strength of the force called?force called?

- Magnitude- Magnitude

Force in Mechanical SystemForce in Mechanical System

14. What is the difference between 14. What is the difference between vectors and scalars?vectors and scalars?- a vector is a physical quantity - a vector is a physical quantity that has both a magnitude and that has both a magnitude and directiondirection

- forces, velocity, acceleration- forces, velocity, acceleration- a scalar quantity only has a - a scalar quantity only has a magnitudemagnitude

- temperature, pressure, mass- temperature, pressure, mass

Force in Mechanical SystemForce in Mechanical System

15. How can we represent forces?15. How can we represent forces?- all vectors can be represented - all vectors can be represented by arrowsby arrows- the arrow length represents the - the arrow length represents the magnitude of the forcemagnitude of the force- the arrow heading represents - the arrow heading represents the direction of the forcethe direction of the force

TailTail Head Head

Force in Mechanical SystemForce in Mechanical System

16. Consider the example – A 16. Consider the example – A worker pulls a heavy cart with a worker pulls a heavy cart with a force of 30 pounds at an angle of force of 30 pounds at an angle of 3030oo above the floor. Draw out this above the floor. Draw out this vector.vector.

30 lbs

30o

Force in Mechanical SystemForce in Mechanical System

17. How do we add forces that act 17. How do we add forces that act along the same line?along the same line?

- if the forces act in the direction, - if the forces act in the direction, we add magnitude of the forceswe add magnitude of the forces

- if the forces act in opposite - if the forces act in opposite directions, we subtract the directions, we subtract the magnitude of the forcesmagnitude of the forces

Force in Mechanical SystemForce in Mechanical System18. Example – Five people are 18. Example – Five people are

playing tug of war. Three people playing tug of war. Three people on one side pull against two on one side pull against two people on the other side. The people on the other side. The three each pull with 50 pounds of three each pull with 50 pounds of force to the left. The two pull with force to the left. The two pull with 70 pounds of force each to the 70 pounds of force each to the right. What is the net force? right. What is the net force? (magnitude and direction)(magnitude and direction)- 10 pounds of force to the left - 10 pounds of force to the left (net force)(net force)

Force in Mechanical SystemForce in Mechanical System19. What would happen if the two 19. What would happen if the two

people pulled with a force of 75 people pulled with a force of 75 pounds to the right?pounds to the right?

- the rope would not move because - the rope would not move because the forces are balancedthe forces are balanced

Force in Mechanical SystemForce in Mechanical System20. How do we add forces that act at an 20. How do we add forces that act at an

angle?angle?

- you cannot add forces when they act - you cannot add forces when they act at anglesat angles

- you can solve them graphically- you can solve them graphically

21. What is the overall force when 21. What is the overall force when multiple forces are acting on an multiple forces are acting on an object?object?

- a resultant force- a resultant force

- the resultant always acts in a - the resultant always acts in a direction between the directions of the direction between the directions of the two forcestwo forces

Force in Mechanical SystemForce in Mechanical System22. Example – Two people pull on 22. Example – Two people pull on

a boat with ropes that form a a boat with ropes that form a right angle. The person on the right angle. The person on the right of the boat pulls with 40 right of the boat pulls with 40 pounds of force. The person in pounds of force. The person in front of the boat pulls with 30 front of the boat pulls with 30 pounds of force. What is the pounds of force. What is the resultant force of the boat?resultant force of the boat?

Force in Mechanical SystemForce in Mechanical System- Step One – Draw the 40 lb force- Step One – Draw the 40 lb force

- Choose scale for the line- Choose scale for the line- 1 cm = 10 lb - 1 cm = 10 lb - use a ruler- use a ruler

- Step Two – Add the 30 lb force to - Step Two – Add the 30 lb force to the 40 lb forcethe 40 lb force

- ALWAYS add the tail of the - ALWAYS add the tail of the secondsecond vector to the head of the vector to the head of the first vectorfirst vector

Force in Mechanical SystemForce in Mechanical System

-Step Three – Draw the resultant -Step Three – Draw the resultant forceforce

- connect tail of the first vector - connect tail of the first vector to to the head of the second vectorthe head of the second vector

-Step Four – Determine the -Step Four – Determine the magnitude and direction of the magnitude and direction of the resultantresultant

- use ruler and protractor- use ruler and protractor

Force in Mechanical SystemForce in Mechanical System

23. Are mass and weight the same 23. Are mass and weight the same thing?thing?

- no, mass and weight are - no, mass and weight are very differentvery different

Force in Mechanical SystemForce in Mechanical System24. What is the difference 24. What is the difference

between weight and mass?between weight and mass? -Mass – the amount of matter in -Mass – the amount of matter in an objectan object

- measured in pound mass - measured in pound mass (lbm) or grams(lbm) or grams-Weight – measure of the Earth’s -Weight – measure of the Earth’s gravitational pullgravitational pull

- measured in pounds (lb) or - measured in pounds (lb) or newtonsnewtons

Force in Mechanical SystemForce in Mechanical System25. Can mass change?25. Can mass change?

- no, mass does not - no, mass does not changechange

26. Can weight change?26. Can weight change?

-yes, weight can change -yes, weight can change as gravity changesas gravity changes

-example –a person -example –a person weighs less on the moon weighs less on the moon than on Earththan on Earth

Force in Mechanical SystemForce in Mechanical System27. What are other common units?27. What are other common units?

SISI EnglishEnglish

LengthLength meter (m) meter (m) foot (ft) foot (ft)

TimeTime second (s)second (s) second (s) second (s)

28. Mass & weight conversions28. Mass & weight conversions- 1 pound = 4.45 Newtons1 pound = 4.45 Newtons- 1 kilogram weighs 9.8 Newtons1 kilogram weighs 9.8 Newtons

Force in Mechanical SystemForce in Mechanical System29. What is torque?29. What is torque?

- the effect of a force applied - the effect of a force applied on a body at some distance from on a body at some distance from the axis of rotation of that bodythe axis of rotation of that body

- torque causes rotation- torque causes rotation

30. What are two ways an object can 30. What are two ways an object can rotate?rotate?

- clockwise and counterclockwise- clockwise and counterclockwise

Force in Mechanical SystemForce in Mechanical System

31. When is torque 31. When is torque used?used?

- opening a paint - opening a paint can, opening a can, opening a door, using a door, using a wrench to turn a wrench to turn a nut, turning gearsnut, turning gears

Force in Mechanical SystemForce in Mechanical System32. How is torque measured?32. How is torque measured?

- lb- lb·ft (English) ·ft (English)

- N·m (SI)- N·m (SI)

33. How is torque calculated?33. How is torque calculated?

- T = (F)(L)- T = (F)(L)

- torque = applied force x lever - torque = applied force x lever armarm

- applied force - applied force lbs. or N lbs. or N

- lever arm - lever arm ft or m ft or m