2.2 Center of Mass

50 question-and-answer flashcards reviewing forces, torques, Newton’s laws, mass vs. weight, center of mass, moment arms, and internal/external forces.

What does Newton’s First Law (Law of Inertia) state?

An object remains at rest or in uniform motion in a straight line unless acted upon by an unbalanced external force.

According to Newton’s First Law, what changes the motion of an object at rest or in uniform motion?

An unbalanced external force.

State the translational form of Newton’s Second Law.

Force equals mass times acceleration (F = m a).

How does Newton’s Second Law relate acceleration to the applied force?

Acceleration is in the same line as the force and directly proportional to it.

What is Newton’s Third Law (Law of Reaction)?

For every action there is an equal and opposite reaction.

Write the rotational analogue of F = m a.

Torque (τ) = mass moment of inertia (I) × angular acceleration (α).

Define torque (moment).

A rotational force produced when a force acts at a distance from an axis of rotation.

What is the most common external force acting on the human body?

Gravity.

What is the second most common external force encountered during activities such as walking?

Ground reaction force.

What is the most common internal force in the body?

Muscle‐generated force.

Name four characteristics used to fully describe a force vector.

Magnitude, direction, point of application, and spatial orientation.

Differentiate mass and weight.

Mass is the amount of matter (kg); weight is the force on that mass due to gravity (N) and equals m g.

What numerical value is used for gravitational acceleration on Earth?

9.8 m/s².

Define center of mass (COM).

The point where an object’s mass is evenly distributed in all directions.

Where is the center of mass located in a perfectly symmetrical ball?

At the geometric center of the ball.

How does mass distribution affect the COM in an irregular object like a baseball bat?

The COM shifts toward the heavier (thicker) end of the bat.

Under what condition will an object balance without tipping?

When it is supported directly under its center of gravity/center of mass.

What is a joint reaction force?

An internal force acting at a joint axis that is opposite in direction to the muscle force.

Define moment arm.

The perpendicular distance from a force’s line of action to the axis of rotation.

How is internal torque calculated?

Muscle (internal) force × internal moment arm.

How is external torque due to gravity calculated?

Gravitational force × external moment arm.

List the steps for finding an external moment arm of a limb segment.

1) Locate the segment’s center of mass; 2) draw the gravity line downward; 3) measure the perpendicular distance from this line to the joint axis of interest.

What happens to the triceps’ internal moment arm as the elbow moves from 90° flexion toward extension?

The internal moment arm increases because the perpendicular distance grows.

Which type of motion is described as linear: translation or rotation?

Translation.

What two terms are interchangeable with torque in biomechanics?

Moment and rotational force.