Forces Chapter 4

Motion Acceleration describes a change in velocity of an object Velocity describes a change in the object’s position What causes acceleration in the first place? Why do things move? What is the cause of motion?

Sir Isaac Newton 300 years ago he gave us the answer Explained the way in which forces- pushes and pulls- influence motion Summed up his explanations in three laws

Forces A force is a push or a pull on an object The object is the system The world around the object is the environment

Types of Forces Contact force acts on an object only by touching it Long-range force is exerted without contact Examples: magnets, gravity Draw force diagrams

Newton’s Second Law of Motion F = ma (a= Fnet / m) Forces are vectors, so the total force on an object is the vector sum of all forces exerted on the object The vector sum of 2 or more forces is called the net force

Measuring Forces F = ma Where mass is measured in kg and acceleration is measured in m/s2 So Force is measured as (kg x m)/ s2 The SI unit for Force is the newton, (N)

Newton’s First Law of Motion An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on that object is zero (unless acted upon by an outside Force) Also called the law of inertia

Inertia The tendency of an object to resist change If an object is at rest, it tends to remain at rest If it is moving at a constant velocity, it tends to continue moving at that velocity If the net force on an object is 0, it is in equilibrium Either at rest or moving at constant velocity

Causes of motion A net force is something that disturbs a state of equilibrium A change in velocity is caused by an unbalanced force Changes in velocity or acceleration are the result of a net force acting on an object

Types of Forces Friction Normal Spring Tension Thrust Weight

Mass and Weight Mass is how much stuff is in an object Weight is how the acceleration due to the Earth’s gravity pulls on the mass of an object So weight is a force F=ma so the force of gravity is mg 1 lb = 4.448 N & 1 N = 0.225 lb

Causes of friction When two surfaces touch, the high points on each are in contact and temporarily bond When you try to move one of objects, you must break the bonds This is the cause of static friction

As the surfaces move past each other, the electrostatic forces, that caused the bonds, continue to create an attraction between the high points on the moving surfaces and this results in the weaker kinetic friction

Drag and resistance When an object moves through the air or any other fluid, the fluid exerts a friction like force on the moving object. This force depends upon the speed of the motion, size and shape of the object, and the density and kind of fluid

As velocity increases, so does the drag force No net force means no acceleration, so velocity becomes constant The constant velocity that is reached when the drag force equals the force of gravity is terminal velocity

Video- crushing barrel

Periodic Motion There is one position in which the net force is zero- this is in equilibrium When pulled away from equilibrium position, the net force on the system becomes nonzero, and pulls back toward the equilibrium point.

The force that restores the object is directly proportional to the displacement of the object, the motion is simple harmonic motion The time needed to repeat one complete cycle of motion is the period (T) Maximum distance the object moves from equilibrium is amplitude

Drawings The upward force of the spring is directly proportional to the amount stretched- Hooke’s Law

Pendulum P.379 (1-3) A pendulum is also simple harmonic motion Bob is suspended Drawing T= 2¶ √(l/g) Mass does not matter Often used to measure gravity of Earth

Interaction Forces system 1 system 2 system External Forces interactions system 1 system 2

Newton’s Third Law For every action force there is an equal and opposite reaction force. Interaction pair FA on B = -FB on A Page 124 (1-2) practice A P. 132 (1-5) & P. 379 (1-3)