why is a counter clockwise positive and clockwise negative torque?

That is an arbitrary convention that we follow. Questions should normally give you the direction that is considered positive though, like how it was given here.

Which mass is the mass used in the equation T = F x r? Is force acceleration times the *mass of the person pushing* or is it the *mass of the object being pushed*?

The mass of the object being pushed, because it is the object that is being focused on within the system. The weight of the person pushing the object is irrelevant

so im stuck home because of c-19. and studying applied physics for my 3rd trimester is this what i start with to study it.

I would think probably not. Im currently in AP Physics 1 and I would say it would be a good idea to start with 2d kinematics

This was great and helpful, but quick question regarding equilibrium and torque. So the equilibrium here seemed to be of an object with a pivot point but what about examples like a hanging sign... I find this incredibly difficult to think about because let's say you have a hanging sign that sticks out from a wall via a rod both with respective masses and a cable from a building holding it. You are trying to calculate the tension in the cable. You have to find the tension of this static equilibrium case -- I know that you have to balance the torques though they add up to 0, but I guess I am just confused about how to find the tension Force from the torques.

Think about it this way (I am going to assume that we know where the cable is attached to the sign): Both the rod and sign have mass. So that means that they both have weight. This weight is a force that acts on their centers of mass. This means there is a torque on the sign because of gravity. There are two torques on the sign through gravity due to the rod's weight and the sign's weight, treat these like two different torques and add them together to get the total torque due to gravity. Now, since you know that the system is in equilibrium, this means that force of tension must exert a torque equal in magnitude to the total torque due to gravity. You can create an equation from this: Tension * lever arm = Torque due to gravity. Now just divide by the lever arm and you have your tension. Note: In this example I assumed that the cable is perpendicular to the sign, if the cable is not perpendicular, you have to find the component of the tension force that is perpendicular to the sign using some trigonometry and replace the tension in the equation with this. Hope this helps!

Main content

Course: AP®︎/College Physics 1 > Unit 7

Lesson 3: Torque and equilibrium

Torque and equilibrium review

Google Classroom

Review the concept of torque and how it is affected by the applied force and lever arm.

Key terms

Term (symbol)	Meaning
Torque ( $τ$ ‍)	Measure of the twisting action caused by a force that can cause an object to rotate about an axis. Vector quantity with SI units of $N \cdot m$ ‍.
Net torque ( $Σ τ$ ‍)	Sum of all the torques on a system
Balanced system	When the net torque on a system is zero
Lever arm	Perpendicular distance from the axis of rotation to where the force is applied. Vector quantity with SI units of $m$ ‍.
Pivot point	Point that an object rotates around. Sometimes called the fulcrum or rotational axis.

Equations

Equation	Symbol breakdown	Meaning in words
$τ = r F \sin θ = r_{⊥} F$ ‍	$τ$ ‍ is torque, $F$ ‍ is applied force, $r$ ‍ is the radius from the axis of rotation to the location where the force is exerted, and $θ$ ‍ is the angle between $F$ ‍ and $r$ ‍ when these vectors are placed tail to tail.	Torque is proportional to both the lever arm and the force component perpendicular to the lever arm.

How to visualize the torque equation

A wrench produces a torque on a nut if a force is applied to it correctly (see Figure 1). The equation for torque is:

τ = r F \sin θ

To produce a torque, the force

F

must be applied at some distance

r

away from the pivot point. Since only the perpendicular component

F_{⊥}

produces torque, the equation includes

\sin θ

(see Figure 2 below).

The magnitude of the torque depends on:

Applied force $F$ ‍: Larger forces increase torque.
Radius $r$ ‍: Increasing the radius increases the torque.
Angle between the force and lever arm $θ$ ‍: Directing a force perpendicular to the lever arm increases the torque.

An applied force can result in zero torque if there is no lever arm or the applied force is parallel to the lever arm (see Figure 3 and 4 below).

How to determine the direction of torque

The direction of rotation can be clockwise (cw) or counterclockwise (ccw). These terms refer to the movement of hands on a clock (see Figure 5). In physics, the counterclockwise direction is defined as positive and clockwise is negative for rotational variables.

For example, a torque that rotates an object counterclockwise is a positive torque (see figure 6 below).

A torque that rotates an object clockwise is a negative torque (see figure 7 below).

Learn more

For deeper explanations of torque, see our video about torque and equilibrium.

To check your understanding and work toward mastering these concepts, check out our exercises:

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asaifpyp
Posted 5 years ago. Direct link to asaifpyp's post “why is a counter clockwis...”
why is a counter clockwise positive and clockwise negative torque?
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(9 votes)
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- V T
  Posted 5 years ago. Direct link to V T's post “That is an arbitrary conv...”
  That is an arbitrary convention that we follow. Questions should normally give you the direction that is considered positive though, like how it was given here.
  Comment on V T's post “That is an arbitrary conv...”
  (14 votes)
Ursula Power
Posted 5 years ago. Direct link to Ursula Power's post “Which mass is the mass us...”
Which mass is the mass used in the equation T = F x r?
Is force acceleration times the mass of the person pushing or is it the mass of the object being pushed?
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(5 votes)
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- leathchr19
  Posted 5 years ago. Direct link to leathchr19's post “The mass of the object be...”
  The mass of the object being pushed, because it is the object that is being focused on within the system. The weight of the person pushing the object is irrelevant
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  (7 votes)
stephen.w.elsbury
Posted 4 years ago. Direct link to stephen.w.elsbury's post “so im stuck home because ...”
so im stuck home because of c-19. and studying applied physics for my 3rd trimester is this what i start with to study it.
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(2 votes)
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- Mathew Brown
  Posted 4 years ago. Direct link to Mathew Brown's post “I would think probably no...”
  I would think probably not. Im currently in AP Physics 1 and I would say it would be a good idea to start with 2d kinematics
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  (5 votes)
nc5pande
Posted 5 years ago. Direct link to nc5pande's post “Can we balance torque on ...”
Can we balance torque on a point which is in accelerated motion?
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(4 votes)
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sunitachand
Posted 4 years ago. Direct link to sunitachand's post “This was great and helpfu...”
This was great and helpful, but quick question regarding equilibrium and torque. So the equilibrium here seemed to be of an object with a pivot point but what about examples like a hanging sign... I find this incredibly difficult to think about because let's say you have a hanging sign that sticks out from a wall via a rod both with respective masses and a cable from a building holding it. You are trying to calculate the tension in the cable.

You have to find the tension of this static equilibrium case -- I know that you have to balance the torques though they add up to 0, but I guess I am just confused about how to find the tension Force from the torques.
Button navigates to signup pageButton navigates to signup page
(1 vote)
Answer
- obiwan kenobi
  Posted 4 years ago. Direct link to obiwan kenobi's post “Think about it this way (...”
  Think about it this way (I am going to assume that we know where the cable is attached to the sign):
  
  Both the rod and sign have mass. So that means that they both have weight. This weight is a force that acts on their centers of mass. This means there is a torque on the sign because of gravity. There are two torques on the sign through gravity due to the rod's weight and the sign's weight, treat these like two different torques and add them together to get the total torque due to gravity. Now, since you know that the system is in equilibrium, this means that force of tension must exert a torque equal in magnitude to the total torque due to gravity. You can create an equation from this:
  
  Tension * lever arm = Torque due to gravity.
  
  Now just divide by the lever arm and you have your tension.
  
  Note: In this example I assumed that the cable is perpendicular to the sign, if the cable is not perpendicular, you have to find the component of the tension force that is perpendicular to the sign using some trigonometry and replace the tension in the equation with this. Hope this helps!
  Button navigates to signup page
  (4 votes)
sunitachand
Posted 4 years ago. Direct link to sunitachand's post “Also is there a video exp...”
Also is there a video explaining what lever arm is or why we use it. I do not understand why there is the formula that torque=lever arm*force
why is this used? what is its advantage of finding torque this way?
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(2 votes)
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- obiwan kenobi
  Posted 4 years ago. Direct link to obiwan kenobi's post “This is the definition of...”
  This is the definition of torque. If you know the magnitude of the force being applied and the distance from the axis of rotation it is applied, then by multiplying the two, you now know the torque. It is the distance from where the force is applied to the axis of rotation that we refer to as the lever arm. Hope this helps!
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  (1 vote)
vehl achari
Posted 5 years ago. Direct link to vehl achari's post “do we have to follow the ...”
do we have to follow the clockwise and counter clockwise direction?
our teacher down here in fiji says use both as positive.................
can somone plz reply to this and tell me wat i have to do.....plz
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(1 vote)
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- esrivishagan
  Posted 5 years ago. Direct link to esrivishagan's post “If u use both as positive...”
  If u use both as positive, sometimes you cannot tell what direction the torque is applied, which would be difficult for more advanced sums. It definitely is better to follow clockwise and counterclockwise direction, but consult with your teacher first
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  (1 vote)
Nizel Jem Salinas
Posted 3 years ago. Direct link to Nizel Jem Salinas's post “how do you determine the ...”
how do you determine the direction of torque when it is parallel to pivot point?
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EthanChacon
Posted 4 years ago. Direct link to EthanChacon's post “what are 5 methods to mea...”
what are 5 methods to measure forces?
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