Why are door handles near the side of a door instead of the middle? And why does the door rotate when you push or pull on it?

So far we've covered linear kinematics and linear dynamics (linear motion and forces). We've also covered rotational kinematics. Now let's talk about rotational dynamics - what causes things to rotate.

Imagine you're using a wrench to loosen a bolt that's stuck. You push on the wrench with 100 N of force but the bolt doesn't turn. Then you try a longer wrench and push on the handle with the same 100 N of force, and the bolt begins to turn. What changed? You applied the same force to both wrenches, so it's not a certain amount of force that was needed to turn the bolt but a certain amount of something else: torque.

Torque is what causes things to rotate. You could think of torque as a rotational force. When you use a wrench, you're applying a torque to the bolt to rotate it. When you push on a door, you're applying a torque to the door to rotate it. And when you're riding a bike or driving a car, you (or the engine) are applying a torque to the wheels to make them rotate so you move forward.

What's the difference between a force and the torque that's produced by that force? The torque depends on the amount of force applied, the direction of the force, and the distance between the force and the axis of rotation. In the earlier example, you were able to loosen the bolt because you used a longer wrench - the force was applied at a greater distance from the bolt so more torque was produced. The bolt didn't know how much force you applied or what size wrench you used, it only experienced the torque (the combination of the force and distance).

And just like force, torque is a vector. That means it has a magnitude and a direction. Like rotational motion, torque basically has 2 possible directions: clockwise and counterclockwise.

In the next lesson on rotational dynamics, we'll learn how to add torques together to get a net torque, and how to apply Newton's laws to rotational motion.

Multiple-Choice Questions

Answers

- Torque

• Khan Academy - Introduction to torque
• Khan Academy - Finding torque for angled forces
• Michel van Biezen - How to Calculate a Torque (Method 1)
• Michel van Biezen - How to Calculate a Torque (Method 2)

- Torque Vector and Direction

• Michel van Biezen - Direction of Torque
• Matt Anderson - Torque is a vector
• Matt Anderson - Torque Cross Product

• Michel van Biezen - How to Calculate a Torque
• Organic Chemistry Tutor - Torque, Lever Arm, Moment of Force

• Engineering Explained - Horsepower vs Torque - A Simple Explanation

• Torque balancer

Other Resources

Why are door handles near the side of a door instead of the middle? And why does the door rotate when you push or pull on it?

So far we've covered linear kinematics and linear dynamics (linear motion and forces). We've also covered rotational kinematics. Now let's talk about rotational dynamics - what causes things to rotate.

Imagine you're using a wrench to loosen a bolt that's stuck. You push on the wrench with 100 N of force but the bolt doesn't turn. Then you try a longer wrench and push on the handle with the same 100 N of force, and the bolt begins to turn. What changed? You applied the same force to both wrenches, so it's not a certain amount of force that was needed to turn the bolt but a certain amount of something else: torque.

Torque is what causes things to rotate. You could think of torque as a rotational force. When you use a wrench, you're applying a torque to the bolt to rotate it. When you push on a door, you're applying a torque to the door to rotate it. And when you're riding a bike or driving a car, you (or the engine) are applying a torque to the wheels to make them rotate so you move forward.

What's the difference between a force and the torque that's produced by that force? The torque depends on the amount of force applied, the direction of the force, and the distance between the force and the axis of rotation. In the earlier example, you were able to loosen the bolt because you used a longer wrench - the force was applied at a greater distance from the bolt so more torque was produced. The bolt didn't know how much force you applied or what size wrench you used, it only experienced the torque (the combination of the force and distance).

And just like force, torque is a vector. That means it has a magnitude and a direction. Like rotational motion, torque basically has 2 possible directions: clockwise and counterclockwise.

In the next lesson on rotational dynamics, we'll learn how to add torques together to get a net torque, and how to apply Newton's laws to rotational motion.