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 – woman 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 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)
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,
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. 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
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?
-
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
21. Are mass and weight the same thing?
- no, mass and weight are very different
Force in Mechanical System
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.
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
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 = 2.205 lbs
- 1 kilogram = 9.8 Newtons
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
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
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