NEWTON’S LAWS

Isaac Newton put forth three laws to explain why an object moves or doesn’t move.

NEWTON’S FIRST LAW

Newton’s first law is also referred to as the law of inertia.

First law: 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.

The state of motion of an object is maintained as long as the object is not acted upon by an unbalanced force. All objects resist changes in their state of motion - they tend to "keep on doing what they're doing."

SOME APPLICATIONS

There are many applications of Newton's first law of motion. Consider some of your experiences in an automobile. Have you ever observed the behavior of coffee in a coffee cup filled to the rim while starting a car from rest or while bringing a car to rest from a state of motion? Coffee "keeps on doing what it is doing." When you accelerate a car from rest, the road provides an unbalanced force on the spinning wheels to push the car forward; yet the coffee (that was at rest) wants to stay at rest.

While the car accelerates forward, the coffee remains in the same position; subsequently, the car accelerates out from under the coffee and the coffee spills in your lap. On the other hand, when braking from a state of motion the coffee continues forward with the same speed and in the same direction, ultimately hitting the windshield or the dash. Coffee in motion stays in motion.

MORE APPLICATION

http://www.physicsclassroom.com/mmedia/newtlaws/cci.cfm

Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.

To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.

Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object that abruptly halts the motion of the skateboard.

INERTIA

Inertia: the resistance an object has to a change in its state of motion.

When an object is moving it is difficult to stop

When an object is at rest, it is difficult to move

INERTIA

Is dependent on mass It is more difficult to stop an adult on a swing

than it is a child. Mass is that quantity that is solely

dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion.

Galileo, a premier scientist in the seventeenth century, developed the concept of inertia. Galileo reasoned that moving objects eventually stop because of a force called friction.

By using a pair of inclined planes and a ball (assuming friction was eliminated), Galileo observed that a ball which rolled down one incline would roll up another incline to the same height at which it started. Galileo's basic thought process was...

Initial height should equal final height however …Since that would not be possible here, Galileo, by inductive reasoning, stated that the ball would continue in motion at a constant speed in a straight line forever (trying to achieve the initial height).

NEWTON’S SECOND LAW

When a net force acts on an object, it accelerates in the direct of the net force. The acceleration varies directly to the net force on it and inversely to its mass.

Fnet= net force (N) m= mass (kg) a= acceleration m/s2

What is the acceleration of a 1600 kg racecar if the net force was 4500 N?

Example: Two crates - one large and heavy, the

other small and light rest on a smooth level surface. If you push with force F on either the large or small crate, is the contact force between the two crates:

A) the same regardless if you push on the large or small crate;

B) larger when you push on the small crate;

C) larger when you push on the larger crate?

REASONING Since the same force pushed on the crates, you may reason

that the contact force is the same in both cases. It is not. The crates do have the same acceleration in either case. The net force acts on the total mass (m1 + m2) so the same acceleration, a, results.

To find the contact force between the crates we must examine each individual crate. Note: Newton's 2nd Law must be satisfied for each of the crates, just as it is for the entire system (both crates).

When the external force is applied to the small crate, the only force acting on the large crate (mass m1) is the contact force; therefore, the contact force must have a magnitude equal to m1a. In the second case, the only force acting on the small box (mass m2) is the contact force, and so the magnitude of the contact force is m2a. Since m1 is greater than m2, it follows that the force of contact is larger when you push on the small box (m1a) than when you push on the large box (m2a)

The answer to the question above is (b), the contact force is larger when you push on the small crate.

NEWTON’S THIRD LAW

When one object exerts a force on a second object, the second exerts a

force on the first that is equal in magnitude but opposite in direction. These forces are called action-reaction forces. For every action force there exists a

reaction force that is equal in magnitude but opposite in direction.

FORCES

Is a vector quantity         Magnitude = Size + Unit

One Newton is the amount of force required to give a 1 kg mass an acceleration of 1.

Direction "Down" is used as the direction gravity pulls.

TYPES OF FORCES

Applied Force (Fa) A force which is applied to an object by

another object. When Shelly is pushing the

wheelbarrow across the yard, there is an applied force acting on the wheelbarrow. The applied force is the force exerted on the wheelbarrow by Shelly.

Force of Gravity (Fg) The force which the earth, moon, and

other massive bodies attracts an object toward itself. The gravitational force of the Sun on the Earth holds the Earth in its orbit.

On Earth, all objects experience a "downward" force of gravity.

The force of gravity on an object is always equal to the weight of the object. m = mass (kg) g = acceleration due to gravity (m/s/s)

Normal Force (FN) Can be called the support force. It is exerted on an object which is in

contact with another stable object. On a level surface, the normal force is

equal in magnitude but opposite in direction to the force of gravity.

The normal force is perpendicular to the surface at which the object is on.

Friction Force (Ff) The force exerted by a surface as an object

moves across it ( sliding friction) or makes an effort to move across it ( static friction).

This force is opposite to the motion of the object.

The force of friction is equal in magnitude but opposite in direction to the applied force when the object is moving with constant velocity.

Friction depends on the nature of the two surfaces interacting and the force pushing the two surfaces together (FN).

µ= coefficient of friction ( tables can be found in various textbooks)

FREE BODY DIAGRAMS

Used to analyze situations involving more than one force acting on an object. The object is represented as a square. The forces acting on the object are drawn

from its center. The size and direction of the vector

represents the size and direction of the force.

Include a scale and reference coordinates. Vector addition can then be used to

determine the net force.

FREE BODY DIAGRAMS

Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

The size of the arrow in a free-body diagram reflects the magnitude of the force. The direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labelled to indicate the exact type of force.

FINDING THE TOTAL FORCE