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Torque

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Torque in Everyday Life

When we hear the term ‘torque’ brought up, it’s most often in the context of automobiles. Torque is one of the terms commonly thrown around to describe how powerful a car is, but what exactly does it mean?

In a car, torque is the force that pistons put on the crankshaft, causing it and the wheels to turn.

– Damien Howard

More general:

Torque is a force that causes an object to rotate about an axis.

torque-piston-crankshaft

Here we see the piston and crankshaft motion in an internal combustion engine (like in an automobile.) Image from Wikipedia.

four-stroke-engine-gif

While often considered an automotive term, torque is actually a general physics term that has many applications.

Torque is defined as a twisting force that tends to cause rotation.

We call the point where the object rotates the axis of rotation.

You use torque every day without realizing it. You apply torque three times when you simply open a locked door: (1) Turning the key, (2) turning the doorknob, and (3) pushing the door open so it swings on its hinges are all methods of applying a torque.

– Damien Howard, Holt McDougal Physics Chapter 4: Forces and the Laws of Motion

Biophysics

Torque is important when studying physiology (the study of how cells and organs interact to form a whole being, including skeletons, muscles, organs, etc.)

In this lesson we learn about Forces and Torques in Muscles and Joints.

Many body motions require or take advantage of torque, for instance:

A cheetah’s tail provides an excellent example of torque and angular momentum in action. With a simple clockwise flick of the tail, the cheetah’s body (in a response which conserves angular momentum) rolls in the anti-clockwise direction (and conversely).
This enables the cheetah to position its body mid-flight so that it is ready to turn the instant its feet make contact with the ground.

http://physicsfootnotes.com/physics-of-the-cheetahs-tail/

cheetah-tail-angular-momentum-torque
l

http://physicsfootnotes.com/physics-of-the-cheetahs-tail/

Torque: Introductory physics

Chapter 8, Rotational Motion, Section 2, Rotational dynamics

A lever arm is… (p.209)

figure 5

lever-arm-torque-opening-a-door-glencoe-physics-chap-8

text

torque-axis-of-rotation-lever-arm-glencoe-physics-chap-8

Torque is defined as a force that tends to cause rotation. It changes the rotational motion of an object.

We call the point where the object rotates the axis of rotation.

Torque = τ  (Greek letter tau)

τ = [Force applied] x [lever arm]

τ = F·r

The amount of torque applied depends on the angle, θ, so then:

τ = F·r·sin(θ)

lever arm = perpendicular distance from the axis of rotation to the line of action of the force.

Finding the net torque

net-torque-on-pencils

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net-torque-example-seesaw

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PhET labs

PhET lab: Torque

PhET lab: Balancing act

PhET lab: Balancing act (HTML 5 app)

External resources

Open Stax College Physics: Statics and torque

Rotational dynamics: SparkNotes

Learning Standards

Mass High School Technology/Engineering

HS-ETS1-2. Break a complex real-world problem into smaller, more manageable problems that each can be solved using scientific and engineering principles.

HS-ETS4-1(MA). Research and describe various ways that humans use energy and power
systems to harness resources to accomplish tasks effectively and efficiently.

SAT Physics test

Torque and equilibrium

“When SAT II Physics tests you on equilibrium, it will usually present you with a system where more than one torque is acting upon an object, and will tell you that the object is not rotating. That means that the net torque acting on the object is zero, so that the sum of all torques acting in the clockwise direction is equal to the sum of all torques acting in the counterclockwise direction.” – SparkNotes Rotational dynamics

AP Physics

Enduring Understanding 3F: A force exerted on an object can cause a torque on that object.

Enduring Understanding 4D: A net torque exerted on a system by other objects or systems will change the angular momentum of the system.

A Framework for K-12 Science Education, National Research Council

PS2.A: FORCES AND MOTION: How can one predict an object’s continued motion, changes in motion, or stability?

Interactions of an object with another object can be explained and predicted using the concept of forces, which can cause a change in motion of one or both of the interacting objects… At the macroscale, the motion of an object subject to forces is governed by Newton’s second law of motion… An understanding of the forces between objects is important for describing how their motions change, as well as for predicting stability or instability in systems at any scale.

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