Unit V – Newton’s laws of motion Laws of motion, free body diagrams, friction, gravitation and mass vs. weight Unit V – Newton’s laws of motion

Newton’s first law Newton’s 3 laws of motion. Newton’s first law of motion is called law of inertia. It states an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Newton’s First Law Two parts to law – one applies to object at rest and the other applies to objects in motion. State of motion stays the same UNLESS acted upon by an unbalanced force.

Newton’s first law Example: A dish filled with water and you were walking around an oval track trying to complete a lap in the least amount of time. The water will tend to spill from the container when: The container is at rest and you attempt to move it. The container is in motion and you attempt to stop it. The container is moving in one direction and you attempt to change its direction.

Newton’s first law Applications of 1st law: Car and coffee in a cup inside the car. Braking automobile and you have your seat belt on. Break a brick painlessly on someone’s hand without hurting the person’s hand. Headrests in cars prevent whiplash injuries

Inertia and Mass Inertia is the tendency of an object to resist changes in their state of motion. Galileo developed the concept of inertia. He reasoned that moving objects will stop because of a force called friction. Newton built on Galileo’s ideas. Newton’s first law – a force is NOT needed to keep an object in motion.

Inertia and mass A book sliding on a table does not come to rest because of an absence of a force. The book comes to rest because there is a force there – FRICTION. The tendency of an object to resist changes in motion varies with the mass of the object. Mass is a measure of the amount of inertia an object has.

Inertia and mass The more massive an object, then the more the object resists the change in motion. The less mass an object has, then the least resistant it is to changes in motion.

Inertia and mass A 2 kg object is moving horizonatally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction? Mac and Tosh are arguing in the cafeteria. Mac says that if he flings his Jello with a greater speed it will have greater inertia. Tosh says that inertia does not depend on speed but mass. Who is right and why?

State of motion What does state of motion mean? It is defined by the object’s velocity Velocity is speed with direction Inertia then means the tendency of an object to resist changes in its velocity. Object at rest – 0 m/s

State of motion An object that is not changing its velocity has an acceleration of 0 m/s^2 Inertia also means the tendency of an object to resist accelerations.

Balanced & Unbalanced Forces What is an unbalanced force? What are the two forces acting upon a book that is resting on a tabletop? Earth’s gravitational pull – downward force Push of table on book (normal force) – upward force The two forces are equal and acting in opposite directions

Balanced and Unbalanced Forces A person standing on the ground. What are the two forces? Since these two forces are equal magnitude and direction, they are balanced and at equilibrium. Now consider a book sliding across a table from left to right (Do not worry about how it started moving).

Balanced and Unbalanced forces Gravity pulling down and table pushing up balance each other. No force to balance the force of friction as the book slides to the right. This is an unbalanced force. Book changes its state of motion. Book not in equilibrium, thus accelerates. Unbalanced forces cause accelertation.

Balanced and Unbalanced forces How to tell what is balanced or unbalanced? Analyze the situation. Determine if the individual forces are equal in magnitude and opposite direction – then it is balanced. If the individual forces are NOT equal in magnitude and opposite direction then it is acted upon by an unbalanced force.