Newton’s Laws

Basic Laws of Biomechanics Sir Isaac Newton developed three laws to explain the relationship between forces acting on a body and the motion of the body.

What is a Force? A force is a push or a pull or a twist. Forces are measured in Newtons. Did you know that forces only exist when objects interact!

What is a Force? A force gives energy to an object. Whenever two objects touch, forces are involved.

What is a Force? A force can cause acceleration, a change in direction or deceleration. A force is NOT required to keep an object in motion Examples: -Drag, Friction, Thrust, Gravity, Weight, Magnestism

Law 1: Law of Inertia An object at rest will remand at rest unless acted upon by some external force. The greater the inertia an object has the greater the force needed to move it.

Objects at rest remain at rest unless acted on by a net force. A lot of inertia! Very little inertia. Since the train is so huge, it is difficult to change its speed. In fact, a large net force is required to change its speed. Since the baby carriage is so small, it is very easy to change its speed. A small net force is required to change its speed.

Objects in motion remain in motion in a straight line (unless acted upon by an outside force). A lot of inertia! Very little inertia. Since the train is so huge, it is difficult to stop it once it is moving. It is difficult to change its speed. In fact, a large net force is required to change its speed. Since the soccer ball is so small, it is very easy to stop it once it is moving. A small force is required to change its speed.

Law 2: Law of Acceleration When a force acts upon a mass, the result is acceleration of that mass. The greater the force, the great the acceleration. The smaller the mass, the greater the acceleration. The mass will accelerate in the direction the force is applied. F = m x a (force) (mass) (acceleration)

Big masses are hard to accelerate Big masses are hard to accelerate. Big masses require big forces to change speed. Small masses are easy to accelerate. Small masses require small forces to change speed

Assume that both steam engines below apply the same amount of force. A heavy train has a difficult time accelerating. Big masses require big forces to change speed. When the same force is applied to a less massive train its acceleration is greater. Small masses require small forces to change speed.

Law 3: Action - Reaction Law For every action, there is an equal and opposite reaction. When we apply a force this is known as an action force. The object we apply the force to, applies a force back, this is a reaction force.

Law 3: Action - Reaction Law These two forces always work in pairs, and are opposite in direction and equal in size.

The forces here are equal and opposite The forces here are equal and opposite. Neither the dog nor its owner pulls with greater force. They pull with the same force in opposite directions

The forces will be equal when the truck crashes into the car. Since the car is smaller, the car will have a greater acceleration.

If forces are always equal and opposite, how can anything move? Here is a famous problem: A horse is pulling on a cart, and the cart pulls back with the same amount of force. If all forces are equal, how can the horse and cart move? Answer: The horse moves because the force he exerts with his hooves is greater than the force of the wagon pulling him back.

If forces are always equal and opposite, how can anything move? What forces act on the cart? The horse pulls it forward, and there is a backward force from the ground: friction. If the horses' pull exceeds the friction of the cart, it will accelerate. Acceleration will occur if one force pair (push of ground/push of horse) is greater than another force pair (friction/pull of cart).

If forces are always equal and opposite, how can anything move? Example 2: If the person's friction forces against the floor are greater than the refrigerator's friction forces, the fridge will accelerate.