Introduction to Dynamics: Forces and Newton's Laws What causes an object's motion to change? What is a Force? What are Newton's 3 Laws of Motion? Physics 1
a When I drop a tennis ball, it accelerates downwards. Why? What causes it to accelerate? Physics 2
Force, F: Any push or pull on an object [basic, general definition] An interaction between 2 objects that may cause an acceleration [better, more specific definition] A Vector quantity; it will always have a directional component, as well as a magnitude > Can be + or - depending on direction (generally, Up and Right are +; Down and Left are -) Measured in units called Newtons (N); for sake of comparison, a weight of 1 pound is about 4.45 N. > A Newton is a derived unit; it is a combination of other units that is given a more convenient name and symbol. > By definition, 1 N = 1 kg*m/s 2. [You can hopefully see that a Newton is a more convenient name and symbol than a kilogramtimes-meter-per-second-squared!] Can be due to actual contact between two objects (e.g., I press a key on the keyboard), or can also act at a distance (e.g., the gravitational force acting on a tennis ball flying through the air). Physics 3
This Physics textbook is resting on a surface. If I leave it alone, will it accelerate all on its own? Physics 4
Newton's 1st Law: Law of Inertia An object will maintain its current state of motion (either at rest, or moving in a straight line at a constant speed) unless it is acted upon by a Net External Force. In simpler terms, an object will NOT accelerate unless it is Forced to do so by a Net External Force. Inertia: The natural property of all matter to resist a change to its motion (i.e., a resistance to acceleration). This is the sensation you feel when you are "pushed" out during a turn in a car, the weirdness you experience when you are riding in an elevator, and the reason that rollercoasters are awesome. Mass: Technically, a measure of an object's Inertia; measured in kg. More mass More Inertia. Also the "amount" of matter that an object is made up of; a product of Density and Volume. This is why a stalled car is hard to push, but an empty shopping cart is easy to push; a car has much more mass than a shopping cart, so it has a much greater resistance to changes in its motion. Physics 5
This Physics textbook is resting on a surface. Are there any Forces acting on it? Physics 6
Some specific Forces: 1. Gravity - Also referred to as an object's weight, this is the force that the Earth exerts on your mass. The Force of Gravity, F g, is equal to an object's Mass (m) times the acceleration due to Gravity (g = 9.8 m/s 2 ). In other words, F g =mg. (Note that weight is NOT the same thing as mass!!! Weight depends on mass, but they are not equal.) 2. Normal - This is the force that is exerted by a surface to an object that is on the surface. The Normal force is always directed Perpendicular to the surface, and serves to try to balance out other forces acting in that direction. The Normal Force is often (but not always) equal to F g. 3. Friction - A challenging force to understand, this is the force that exists between two surfaces in contact that opposes the relative motion of the two surfaces. Sometimes, friction will prevent an object from moving; in other cases, friction makes a moving object stop; in some cases, friction is the force that makes an object move. We will investigate friction in some detail. Physics 7
This Physics textbook is resting on a surface. If I want to accelerate the book, what do I need to do? Physics 8
Newton's 2nd Law: The Net External Force acting on an object is equal to its mass times its acceleration. *The Net force is really the sum of all forces that are acting on an object (remember that Forces are Vectors, and must be added as such). We say External force because the internal forces cannot affect the motion of a system. ΣF = ma Physics 9
Newton's 3rd Law: When two objects interact with each other, they exert forces upon each other that are equal in magnitude and opposite in direction. For example, if box A pushes against box B with a force F AB, then box B pushes back on box A with a force F BA, just as hard, but in the opposite direction. F BA F AB We would say that F BA = -(F AB ) because the forces are equally strong, but they act in opposite directions (left vs. right). In the diagram to the left, a man is punched in the face. What experiences a greater force: The boxer's fist that is punching this man, or the man's face who is receiving the punch? Physics 10
F Fist F Face According to Newton's 3rd Law, the man's Face and the boxer's Fist both experience the same amount of Force! F Face = F Fist ; the magnitudes of the Forces felt by the Face and by the Fist are equal! [Incidentally, this is why a common injury for a boxer is a broken hand; repeatedly bashing an opponent in the skull causes a boxer's fist to absorb enough force to break bones!] Physics 11
*Not drawn to scale F g This man of mass m stands on the surface of the Earth; the Earth exerts a gravitation force on the man, pulling him downwards, with a magnitude F g = mg. In terms of F g, what is the strength of the force that he exerts on the Earth? Is it greater than, less than, or the same as F g? According to Newton's 3rd Law, the Force that he exerts on the Earth would be the exact same strength as the Earth exerts on him! So, he pulls UP on the Earth with the same force F g! Physics 12