Unit IV Part B-Forces and Newton’s Laws

Forces and Newton’s Laws Dynamics is the branch of physics that deals with the interaction between forces and matter. Vectors are used to represent forces in various problems. Sir Issac Newton described 3 “laws” which explain the interaction between forces and matter.

Force Facts An applied force will cause a change in motion of an object Objects at rest either have no forces acting on them, or have equal forces acting on them (equilibrium).

Types of Forces- ranked from weakest to strongest. 1. The gravitational Force: a force of attraction between all objects (ex. You and desk, desk and pencil, two planets). The magnitude of this force increases as the mass of the object increases. Also called “the weak force” Any and all objects exert this force Difficult to feel this force, unless objects are huge.

2. The Magnetic Force- caused when substances with organized electrons (magnets) come into contact The force may be attractive or repulsive Relatively easy to “feel” this force (think of bringing magnets near each other)

3. The Electric Force- aka “the mechanical force” or “the frictional force” This force occurs when two different substances interact. The electrons on each substances surface repel, causing a “pull” or force You will really “feel” this force (especially friction) Much is done to overcome this force.

4. The Nuclear Force- “The big daddy of forces”, responsible for holding the nucleus of an atom together, “the strong force” Very, very strong, but you won’t feel it, as it works only over teeny, tiny short distances When you break apart an atom however, you WILL feel it- as it will cause a nuclear explosion (basis for atomic bombs)

Newton’s Laws and Forces- The law of inertia 1st law- Isaac Newton discovered 3 basic principles which relate motion and forces. He studied the work of Galileo, who first observed friction and wondered what would happen if a ball rolled on a frictionless surface Newton’s first law was born from further observations and experiments It reads “an object continues its state of rest, or in motion, unless acted upon by an external force.” This law defines the function of a force-It is to change the state of motion of an object

2nd Law This law describes what happens to an objects rate of motion when a force acts on it. It reads “when an unbalanced force acts on an object, the object will be accelerated. The acceleration will be in the same direction as the force. It will vary inversely with the mass of the object.” Mathematically we can reduce Newton’s second law to 2 equations

a = Fnet/m Fnet = net force m= mass a= acceleration Mass is in kg, acceleration is m/s2, so the units for the force are kg * m/s2. This is called a Newton (N), in honor of our hero.

Sample Problem An object of mass 10 kg is accelerated at a rate of 26 m/s2. What force is acting on the object? a=Fnet m 26 m/s2= Fnet 10 Kg Fnet= 260 N

The Second Equation The second equation involves the acceleration due to gravity (g) which causes a downward force on objects. This downward force is called the “weight”.

More 2nd Equation This tells you that weight and mass are not the same. Mass is the amount of matter an object has. It DOES NOT change by where you are. (You have the same mass on the earth or moon) Weight will depend on the planets “g”. So, you will have a different weight on the earth and the moon

Weight Equation g= Fg m The equation to calculate the weight is: The units for Fg (weight) are Newton's Fg= force due to gravity (weight) m= mass g= 9.81 m/s2

Sample Problem What is the mass of an object that weighs 856N? g= Fg 9.81 m/s2= 856N m=88 kg

Newton’s 3rd Law Newton’s 3rd law describes what are known as “action” and “reaction” forces. It reads “Every Force has an equal and opposite force acting on it”.

Real Life As an example, think of a book on a desk. The book is at rest. Actually there are two forces at work here, an action force and a reaction one. The action force- the book on the table The reaction force- the table on the book This pair is equal in magnitude, but they are in the opposite directions.