Physics Class: Newton's Laws of Motion

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Questions and Answers

What is the effect of acceleration of the center of mass (COM) on ground reaction force (GRF)?

It decreases GRF regardless of mass.

It does not affect GRF.

It affects GRF depending on both mass and the direction of acceleration. (correct)

It increases GRF regardless of mass.

Which of the following statements best describes Newton's Third Law of Motion?

An object at rest tends to stay at rest due to inertia.

The acceleration of an object is directly proportional to the net force acting on it.

For every action, there is an equal and opposite reaction. (correct)

An object in motion stays in motion unless acted upon by an external force.

Which of the following is a component of ground reaction force (GRF)?

Rotational force

Tensile force

Vertical force (correct)

Frictional force

How is weight calculated on the Moon for an object with a mass of 100 kg?

Weight = 100 * (9.81/6) Signup and view all the answers

During running, what happens to the vertical ground reaction force (vGRF) and horizontal ground reaction force (hGRF) at mid-stance?

vGRF is smaller, and hGRF propels the runner forward. Signup and view all the answers

What does the moment of inertia represent in the context of Newton's Laws?

The resistance of a body to rotation about a given axis. Signup and view all the answers

What is the key factor that affects the extent to which a segment contributes to ground reaction force during movement?

The mass and acceleration of the center of mass Signup and view all the answers

When running, which forces act during the initial contact phase?

Both upward vGRF and backward hGRF act to decelerate downward motion. Signup and view all the answers

What does Newton's 1st Law of Motion primarily describe?

The concept of inertia in the absence of external forces Signup and view all the answers

According to Newton's 2nd Law of Motion, what is the formula that relates force, mass, and acceleration?

F = ma Signup and view all the answers

What is the SI unit of force as stated in Newton’s 2nd Law?

Newton (N) Signup and view all the answers

In the context of mass and weight, what does weight measure?

The gravitational pull on an object Signup and view all the answers

What happens to an object's motion if external forces are absent according to the first law of motion?

The object's velocity remains constant Signup and view all the answers

What is the primary distinction between mass and weight?

Mass indicates the difficulty of acceleration, weight measures gravitational force Signup and view all the answers

When calculating the weight of a 100 kg object on the moon, given that moon's gravity is 1/6th of Earth's, what would be its weight?

16.3 N Signup and view all the answers

Which statement about Newton's laws is true?

The second law applies uniformly to all objects regardless of their mass Signup and view all the answers

What is the center of mass (COM) of a body?

A point that can lie outside the body depending on its shape Signup and view all the answers

Which condition results in a turning moment (T) of zero?

When the force acts through the axis of rotation Signup and view all the answers

What primarily causes a turning moment in a somersault?

Applying a force that does not act through the center of mass Signup and view all the answers

What does the moment arm represent in the context of angular motion?

The distance from the center of mass to the point of force application Signup and view all the answers

During flight, about which point does a rigid body typically rotate?

The center of mass of the body Signup and view all the answers

What is the primary factor influencing moment of inertia?

The distribution of mass Signup and view all the answers

When two objects have the same mass, which factor determines which is easier to swing?

The distance of mass from the axis of rotation Signup and view all the answers

Which equation represents the relationship between angular momentum, moment of inertia, and angular velocity?

$H = I * w$ Signup and view all the answers

What happens to angular velocity if moment of inertia is increased, assuming no external moments act on the body?

Angular velocity decreases Signup and view all the answers

Which law states that if no external moments act on a body, the angular momentum remains constant?

Newton's First Law of Angular Motion Signup and view all the answers

In the context of angular motion, what units are commonly used for moment?

Newton-meters Signup and view all the answers

According to Newton's Third Law of Angular Motion, what do bodies exert on each other?

Equal and opposite moments Signup and view all the answers

What is the result of applying a moment to an object in terms of its angular acceleration?

It produces angular acceleration proportional to the moment Signup and view all the answers

Study Notes

Weight on Earth and Moon

Weight is calculated using the formula Weight = mass (m) * acceleration due to gravity (g).
Weight on Earth for a mass of 100 kg is 981 N (100 kg * 9.81 m/s²).
Weight on the Moon for the same mass is 163.5 N (100 kg * (9.81 m/s² / 6)) due to reduced gravitational pull.

Newton’s Third Law of Motion

Action-reaction principle: for every action, there is an equal and opposite reaction.
Forces exerted by two objects on each other are equal in magnitude and opposite in direction.

Ground Reaction Force (GRF)

GRF is the reaction force provided by a surface during movement, derived from Newton’s third law.
Measured using a force platform; consists of vertical, forward-backward, and side-to-side components.
The contribution of a body segment to GRF depends on mass and the acceleration of the center of mass (COM).
Estimates: trunk and head contribute 50%, legs 17% each, arms 5% each to the GRF.

GRF During Running

During running, initial contact generates upward vGRF and backward hGRF.
Mid-stance and toe-off involve reduced vGRF and hGRF aiding in forward propulsion.
GRF counters the downward motion of the runner throughout the gait cycle.

Newton’s Laws: Angular Motion

Moment of Inertia (I) reflects a body's resistance to rotational motion and depends on mass distribution from the axis of rotation.
Units for moment of inertia are kg*m².
Moment of inertia is influenced more by how mass is distributed than by mass alone.

Angular Momentum

Angular momentum (H) is calculated as the product of moment of inertia and angular velocity (w).
Formulated as H = I * w.

Newton’s Laws: Angular

Law I: Angular momentum remains constant unless acted upon by an external moment.
Law II: Angular acceleration is proportional to the moment causing it (T = I * a).
Law III: Bodies exert equal and opposite moments on one another, crucial for understanding biomechanics in physical activities.

Kinetics and Linear Motion

Kinetics involves forces causing movement and forms the basis of understanding human motion.
Newton’s linear laws include:
- Law I: Inertia - a body in motion stays in motion unless acted upon.
- Law II: Acceleration - directly proportional to force and inversely proportional to mass (F = ma).

Mass vs. Weight

Mass reflects the quantity of matter, measured in kg; weight is the gravitational force acting on a mass (Weight = m * g).
Gravity accelerates bodies towards the Earth at 9.81 m/s², with weight varying on different celestial bodies (e.g., Moon’s gravity is 1/6th of Earth’s).
Example calculation of weight on Earth (981 N) vs. Moon (163.5 N) for a 100 kg block.

Whole Body Center of Mass (COM)

Center of Mass indicates where mass is evenly distributed, influencing rotational dynamics.
Path of a body in flight can be traced by tracking the COM.
Rotational motion occurs about axes that pass through the COM.

Somersaulting and Angular Motion

Applying a force that does not act through the COM creates a turning moment (T) essential for somersaulting.
The moment arm (d) contributes to the turning moment; if d = 0, T = 0, indicating no torque produced.

Key Topics Summary

Covered principles of kinetics, linear motion, ground reaction force, and angular laws.
Highlighted significance of moment of inertia, angular momentum, and the center of mass in understanding biomechanics and physical movement.

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Description

Test your understanding of Newton's Laws of Motion with this quiz focused on weight calculations and the action-reaction principle. Explore how weight differs between Earth and the Moon, and apply Newton's third law in various scenarios. Perfect for physics students aiming to solidify their grasp on these fundamental concepts.