Physics -- Torque and Simple Machines

Physics -- Torque and Simple Machines

Chapter 7 Section 4 Notes

Center of Mass

• An object in rotational motion will spin around a point called the "center of mass".

• The center of mass is not always the center of an object.

• The center of mass is where an object can be balanced.

Multiple Choice

Where will the center of mass be located on the baseball bat?

• A

• B

• C

• D

Where will the center of mass be located on the baseball bat?

Answer: Position C

Rotational Motion

• Objects can have both rotational and linear motion.

• These types of motion can be analyzed separately.

Multiple Select

Determine whether each of the following situations involves a combination of both rotational motion and linear motion.

• a baseball dropped from the roof of a house

• a baseball rolling toward third base

• a pinwheel in the wind

• a door swinging open

Answers

• Incorrect, a baseball dropped from the roof of a house may have linear motion but it may not have rotational motion.

• Correct, a baseball rolling toward third base will have rotational motion and linear motion.

• Incorrect, a pinwheel in the wind will have only rotational motion.

• Correct, a door swinging open will have rotational motion near the hinge and linear motion near the door's outside edge.

The Magnitude of a Torque

• Imagine a cat trying to leave a house by pushing perpendicularly on a cat-flap door.

• The door is free to rotate around a line that passes through the hinge called the "axis of rotation".

The Magnitude of a Torque

• When the cat pushes at the bottom edge of the door with a force that is perpendicular to the door, the door opens.

• The ability of a force to rotate an object around some axis is measured by a quantity called torque.

Multiple Choice

What quantity describes the ability of a force to rotate an object?

• rotational motion

• linear motion

• torque

• gravitational pull

What quantity describes the ability of a force to rotate an object?

• Answer: torque

Torque depends on the force and the lever arm.

• A force must be applied perpendicular the the lever arm.

• The lever arm is the perpendicular distance from the axis of rotation to a line drawn along the direction of the force.

The lever arm depends on the angle.

• Forces do not have to be perpendicular to cause object's to rotate.

• When the angle is less the 90°, the object will still rotate, but not as easily.

Torque

• The symbol for torque is the Greek letter tau (τ)

• (F) refers to force in Newtons

• (d sin θ) is the perpendicular distance from the axis of rotation to a line drawn along the direction of the force.

• The SI unit for torque is N·m

The sign of Torque

• The sign of torque resulting from a force is positive if the rotation is counterclockwise.

• The sign is negative if the rotation is clockwise.

Multiple Choice

What force is required to produce a 1.4 N·m torque when applied to a door at a 60.0° angle and 0.40 m from the hinge?

• 7.0 N

• 0.25 N

• 4.0 N

• 3.5 N

Answer:

4.0 N

Multiple Choice

A worker hanging 65.0° from the vane of a windmill exerts an 8.25 x 10³ N·m torque. If the worker weighs 587 N, what is the vane's length?

• 1.55 m

• 15.5 m

• 0.078 m

• 10.6 m

A worker hanging 65.0° from the vane of a windmill exerts an 8.25 x 10³ N·m torque. If the worker weighs 587 N, what is the vane's length?

• Answer: 15.5 m

Assignment:

Questions 1-3 on page 248