As the minus sign in the formula im

As the minus sign in the formula implies, the directions of the lifting force and
weight are always opposite. It does not matter whether you assume up is positive or
negative, as long as you are consistent in assigning appropriate signs for all the
other terms: displacement, velocity, etc. Since the maximum power needed for a
move that involves any lifting will occur during acceleration while the direction of
movement is up against the direction of gravity, it is usually easiest to assume up is
positive. With this, and therefore will be positive, will be negative
which will make positive, and as will be shown in the friction chapter,
will be positive.
EXAMPLE: A wagon weighing 1300 lb rolls up and down a stage raked
at 1″in 12″. What is the lifting force, ?
SOLUTION: The designer’s specification for the rake needs to be converted into a rake angle in degrees.
Assume up is the positive direction, which means is negative. The lifting force is:
EXAMPLE (SI): A platform with a mass of 245 kg rolls up and down a
stage raked 1 in 10. What is the lifting force, ?
SOLUTION: Two conversions need to occur, one to find the rake angle,
one to find the weight in newtons of the given mass:
If up is assumed to be the positive direction, then is negative.
becomes:
Counterweighting
A dead hauled load is the name given to describe a load hauled directly, without
the assistance of counterweight. A stagehand can dead haul maybe 30 to 50 lb
without too much effort, and chain hoistsare dead haul machines rated by their
weight lifting capacity. Since stagehands lack capacity, and chain hoists lack speed,
purpose built dead haul winches are often considered to fly heavy scenery quickly.
The motors needed for these machines can however become prohibitively large.
aFaccel Fw
Flifting
Ffriction
Flifting
θ tan
opposite
adjacent
--------------------1
12
----- - ==
θ 4.76° =
Fw
Flifting Fw θ sin – 1300– ()4.76 sin – ==
Flifting 108lb =
Flifting
θ tan
opposite
adjacent
--------------------1
10
----- - ==so: θ 5.71° =
Fw mg 245 ()9.8 ()2401N == =Fw Flifting
Flifting Fw θ sin – 2401– ()5.71 sin – ==
Flifting 239N

Kết quả (Việt) 1:

As the minus sign in the formula implies, the directions of the lifting force andweight are always opposite. It does not matter whether you assume up is positive ornegative, as long as you are consistent in assigning appropriate signs for all theother terms: displacement, velocity, etc. Since the maximum power needed for amove that involves any lifting will occur during acceleration while the direction ofmovement is up against the direction of gravity, it is usually easiest to assume up ispositive. With this, and therefore will be positive, will be negativewhich will make positive, and as will be shown in the friction chapter,will be positive.EXAMPLE: A wagon weighing 1300 lb rolls up and down a stage rakedat 1″in 12″. What is the lifting force, ?SOLUTION: The designer’s specification for the rake needs to be converted into a rake angle in degrees. Assume up is the positive direction, which means is negative. The lifting force is:EXAMPLE (SI): A platform with a mass of 245 kg rolls up and down astage raked 1 in 10. What is the lifting force, ?SOLUTION: Two conversions need to occur, one to find the rake angle,one to find the weight in newtons of the given mass: If up is assumed to be the positive direction, then is negative. becomes:CounterweightingA dead hauled load is the name given to describe a load hauled directly, withoutthe assistance of counterweight. A stagehand can dead haul maybe 30 to 50 lbmà không cần nỗ lực quá nhiều, và chuỗi hoistsare đường chết máy xếp bởi củatrọng lượng nâng công suất. Kể từ khi stagehands thiếu năng lực, và chuỗi khí thiếu tốc độ,mục đích xây dựng đường chết Tời thường được coi là để bay nặng phong cảnh một cách nhanh chóng.Các động cơ cần thiết cho các máy này tuy nhiên có thể trở nên tốn lớn.aFaccel FwFliftingFfrictionFliftingΘ tanđối diệnliền kề--------------------112----- - ==Θ 4.76° =FwFlifting Fw θ sin ()-1300-4.76 tội lỗi ==Flifting 108lb =FliftingΘ tanđối diệnliền kề--------------------110----== như vậy: θ 5.71° =Fw mg 245 (9.8) 2401N == = Fw FliftingFlifting Fw θ sin ()-2401-5,71 tội lỗi ==Flifting 239N