Newton’s First Law of Motion Physics – 3rd Six Weeks

Introduction Sir Isaac Newton Isaac Newton was a British scientist who lived from 1642 – 1727. He is responsible for many innovations in the realms of math and science, most notably defining the laws of motion that bear his name and inventing calculus. His laws were first published in his 1687 book Principia

Newton’s First Law of Motion The law states: an object in motion tends to stay in motion at a constant velocity unless acted upon by another object or force; an object at rest tends to stay at rest unless acted upon by another object or force. An object will accelerate in the direction of an unbalanced force Examples: you continue to move forward during a car crash until the airbag, the road, or a tree stops you. Billiard balls will sit calmly on the table until hit by the cue ball.

Inertia is defined as the tendency of an object to resist change in motion. Newton’s 1st Law of Motion is also known as the “Law of Inertia”. The greater the mass of an object, the greater the amount of force is necessary to change its motion.

Forces A force is a push or pull upon an object resulting from the object's interaction with another object. Forces only exist as a result of an interaction. The metric unit for force is the Newton (N). Force is a vector quantity Contact force is defined as the force exerted when two physical objects come in direct contact with each other. Action-at-a-distance forces are those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation.  Action-at-a-distance forces, such as gravitation and electromagnetic forces, can exert themselves even across the empty vacuum of space.

Net Force The Net Force is the sum of all of the forces that are acting on an object. When forces are in the same direction, the net force is the sum of those forces. When forces are in opposite directions, the net force is the difference of those forces. If the Net Force = 0, then there is no acceleration. Remember, that even though acceleration may = 0, the object could still be moving. An object will accelerate in the direction of the Net Force – without a Net Force there is no acceleration. In a vacuum (when falling), net force = force of gravity.

Types of Forces: 1) Forces that oppose motion

Friction The force of friction is a force that resists motion when two objects are in contact. Friction is symbolized as Ffrict If you look at the surfaces of all objects, there are tiny bumps and ridges. Those microscopic peaks and valleys catch on one another when two objects are moving past each other.

Types of Friction There are two basic forms of friction: kinetic and static. Static friction is a force that resists the movement of two objects against one another when the objects are initially at rest A simple example is a wooden block sitting on a ramp — a force needs to be applied to make the block slide down the ramp. Another term, kinetic friction, applies to the force that opposes objects that are already moving against one another

Example of Static vs. Kinetic Friction Static Friction Kinetic Friction Pull on a block with a spring scale until just before the block moves. Note the reading on the spring scale. Pull the block slowly across the table. Compare the spring scale readings. The top picture shows static friction, just before the block moves. The bottom picture shows kinetic friction, while the block is in motion.

Types of Friction Continued. Centripetal force of a car In a curve is the rolling friction Between the wheels & the road There are several types of kinetic friction: the most common are sliding, rolling friction, and fluid friction. Sliding friction is that force which opposes the motion of one surface that is sliding past another Rolling friction is that force which opposes the motion of one surface that is rolling past another. As less surface area touches, the force of friction decreases The strength of the types of friction from strongest to weakest is static, sliding, rolling, and then fluid friction

Fluid Friction and Viscosity Fluid Friction is the weakest friction, and is the force that opposes motion between a fluid and another surface (or even another fluid) The resistance to flow is called viscosity. Thicker fluids tend to be more viscous than others and viscosity varies with temperature, the hotter it is, the less viscous a fluid is and vice versa

Types of Forces: Normal and Applied Forces

Friction, Motion, and First Law Applied Force – Any force applied to an object by another person or object and is symbolized as Fapp Recall that Newton’s First Law implies that an object will accelerate in the direction of an unbalanced force. Therefore, if a force is applied that is greater than the friction that opposes it, then the object will accelerate in the direction of that applied force since Net Force > Zero

The Normal Force Currently you are sitting down or standing. The ground experiences a force pulling you down. The normal force is perpendicular to the surface and is a reaction to the force(s) holding you down. It is symbolized as Fnorm If you are standing on an incline the normal force would be the reaction force keeping you from sinking into the incline AND it would be perpendicular to the incline's surface. If you were to lean on a wall, it is the normal force that keeps you from puncturing the wall. In order to have normal force, you must have a contact surface touching something.

Normal Force, continued The Normal Force is always perpendicular to the contact surface but not always opposite in direction to the gravitational force. The Normal Force is a force that acts on an object lying on a surface, and it acts in a direction that is perpendicular to the surface. The word “normal” is another word for “perpendicular” Essentially, the normal force is the force required to prevent the item from sinking through the table or other surface – requiring the table to be sturdy enough to deliver that force with out breaking (accelerating). Commonly, the normal force is the same magnitude as the gravitational force (N = mg), however it will be less when the object is on an incline (N < mg). When an object is held up by a rope, a chain, etc. the force supporting it against gravity (or any other force being applied) is called the Tension Force

Additional Important Forces

Gravity The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth. The force of gravity on earth is always equal to the weight of the object. Force of gravity is symbolized as Fgrav Air Resistance The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. Air resistance is symbolized as Fair Tension The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire. Tension force is symbolized as Ftens

Free Body Diagrams

Free Body Diagrams A free body diagram is a picture showing the forces that act on a body. Most importantly it shows the forces' directions without the clutter of drawing the body 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 body is removed or “free” from its environment The word “body” is used to describe any object. But physicists and engineers like to simplify the drawing of the object by drawing a dot (or a box) instead of a detailed picture. Occasionally some simple details are added to further clarify the situation. 

Free Body Diagrams When drawing a free body diagram (FBD) draw the body and external forces only. Don’t draw other bodies or internal forces (the body acting upon itself) Note: if a body ONLY had the force of gravity acting on it, it would be said to be in free fall

Friction & Free Body Diagrams The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it.  Friction opposes the direction of motion (or the intended direction without friction) and is parallel to surface.  If the box is at rest, the acceleration must be zero, therefore the net force must be zero. Remember – an object will accelerate in the direction of the unbalanced force

Friction & Free Body Diagrams continued Friction = Applied Force Friction < Applied Force Friction = Applied Force Note: frictional forces (of all types) point in the opposite direction of motion, (or the intended motion if there were no friction,) and are parallel to the surfaces of contact.

Normal Force, Friction, & a Ramp as the angle of a ramp increases from 0, the Normal Force decreases and the force of friction increases (pointing away from the apparent direction of movement down the incline) At a certain critical point, the incline will be tilted enough that the force of friction reaches a maximum If the normal and force of friction do not balance out gravitational acceleration – the object on the ramp will move down the ramp.

Free Body Diagram: Example 1  A book is at rest on a tabletop. A free-body diagram for this situation looks like this:

Free Body Diagram: Example 2 A girl is suspended motionless from the ceiling by two ropes. A free-body diagram for this situation looks like this:

Free Body Diagram: Example 3 A skydiver is descending with a constant velocity. Consider air resistance. A free-body diagram for this situation looks like this:

Free Body Diagram: Example 4 A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration. Neglect air resistance. A free-body diagram for this situation looks like this:

Free Body Diagram: Example 5 A box sat on a ramp at an angle of θ. A free body diagram of this situation looks like: