Transcript Force in Mechanical Systems

Force in Mechanical
Systems
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)
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
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
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
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
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)
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)
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
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
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
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
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
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
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)
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
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
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?
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
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
Force in Mechanical System
23. Are mass and weight the same
thing?
- no, mass and weight are very
different
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
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
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
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
Force in Mechanical System
31. When is torque
used?
- opening a paint
can, opening a door,
using a wrench to
turn a nut, turning
gears
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