Chunk 5 Application of Newton’s Laws

How do Newton’s laws apply to other physics concepts? Newton’s 2nd law defined how applying a net force (FN) can cause an acceleration or change in velocity. (speed up, slow down, change direction) Those forces can be used in many ways; this unit is about some of the ways forces apply to some phenomenon in physics.

Force of Gravitation The force of gravitation works between all objects. We can measure the force that pulls 2 objects together using Newton’s universal law of gravitation. The force of gravity is dependent on two factors: The mass of the two objects The distance between them The force of gravity is directly proportional to mass and inversely proportional to the distance between them squared.

Weight Weight is the term we use for how much force an abject exerts on the ground because of how gravity pulls on mass. Fw = mg Where m is mass (kg) and g is gravity (10 m/s2) Since weight is a force quantity, the unit for weight is Newtons (N) Usually we don’t fall through the things that are supporting us so the opposing force to weight (going up) is the normal force (Fn) that pushes back on us.

Pressure Pressure is a term used to describe how weight is distributed over an area. where Fw is the force (N) the object exerts downward and A is the surface area (cm2) of the object that contacts the ground. Pressure is inversely proportional to area so the smaller the area, the larger the pressure. Pressure is a combined unit (N/cm2) P = Fw A

Friction Friction is a force that occurs anytime 2 surfaces touch and always opposes the applied force. Friction transforms kinetic energy into heat. Since friction is one part of 2 opposing forces, you must subtract it from the applied force to find the net force. FN = Fa – Ff this equation can be rearranged to solve for any of the variables Friction depends mostly on the amount of force pushing the objects together and partially on the texture of the surface area of the two objects.

Types of friction The usual type of friction happens between two solid surfaces and is the interaction between the electron clouds of the atoms the two surfaces have. Sliding (kinetic) friction: the friction experienced when an object is dragged across a surface. Static friction: friction that must be overcome to get things moving. It is greater than sliding friction because of inertia. Friction in fluids: it is affected by surface area. In a fluid, we call it drag and we would look at the surface area of the object in the direction of the applied force. For falling objects in air, it is called air resistance

Air resistance Air resistance is friction for falling objects in air. Instead of being only effected by gravity only we must account for the upward force that opposes the weight of the object and slows the rate with which the object will fall. where R is air resistance. At some point the downward force of weight will be equal with air resistance and the falling object will reach terminal velocity where the object is no longer accelerating toward the earth. (Newton’s first law!) a = FN = Fw – R m m

Air resistance increases with speed and increased cross- sectional area. Video

Circular motion so far we have looked at forces for objects moving in a straight line. What if the path is curved? Uniform circular motion is motion along a circular path in which there is not change in speed, only a change in direction. v Fc Constant velocity tangent to the circular path Constant force toward the center of the circle The inward force toward the center of the circle is called centripetal force. It keeps the ball from flying off tangent to the circle. Holds the object in a circular motion (orbit).

What about centrifugal force? When you spin a bucket of water, the water stays in the bucket, even if it is upside down. Is there a force that is pushing out of the circle to hold it in the bucket? Before we fully understood this phenomenon, we call this force “Centrifugal force”, but we have come to understand that it is the inertial of the water in the bucket that is making the water stay in the bucket. There is no centrifugal force.

the larger the circle, the faster you go Rotational speed We have been calculating speed as distance/time. For a circle, the distance (circumfrance) is less as the circle gets smaller but the rotation will be the same for objects on a disk. Linear speed for a circle = 2πr t the larger the circle, the faster you go