Motion and Force Chapter 11.3 Notes

Force In science, force is any action that can change the state of motion of an object There are four fundamental forces found in nature—these forces are the force of gravity, the electromagnetic force, the strong nuclear force, and the weak nuclear force. Strong and weak nuclear forces are only effective over very short distances Gravity you feel everyday Friction and other everyday forces are a result of the electromagnetic force 1 Newton = 1kg·m/s 2

Force Force is a vector—it has both magnitude and direction The SI unit for force is the Newton One Newton will cause a 1 kg object to experience an acceleration of 1 m/s 2

Forces vary in strength Strong nuclear force: holds protons and neutrons together in the nucleus, and it is the strongest of all forces If the distance is greater than the size of the nucleus, the force is not effective Gravitational and electromagnetic forces are effective over long distances Electromagnetic force is weaker than the strong nuclear Gravity is even weaker than the electromagnetic force

Contact vs. Field Forces Contact forces are when there is actually contact between two objects—you push a cart and it moves, you catch a baseball and it stops moving Field forces do not require that objects touch each other— the attraction of gravity, the attraction between the north and south pole of two magnets Both contact and field forces can cause an object to start moving or stop moving

Net Force Net force is the combination of all forces that are acting on an object Whenever there is a net force acting on an object, the object accelerates in the direction of the net force Unbalanced forces occur when there is a net force—different forces acting on an object and one is greater than the other If the net force acting on an object is zero, then the object will not accelerate Balanced forces occur when the net force is zero—an object at rest will not start moving and there is no change to an object that is already in motion

Unbalanced vs. Balanced Forces

Force of Friction The force of friction always opposes that of motion Friction occurs because the surface of any object is rough— when two rough surfaces are rubbed together, heat is created Static friction is the friction between surfaces that are not moving; kinetic friction is the friction between moving surfaces Static friction is greater than kinetic friction because the force needed to make an object start moving is greater than the force required to keep it moving

Kinetic Friction There are different kinds of kinetic friction because the type of friction depends on the motion and nature of the objects Sliding friction occurs when objects slide past each other Rolling friction occurs when a rounded object rolls over a flat surface Rolling friction is usually less than sliding friction

Friction and Motion Without friction, car tires couldn’t push against the ground to move the car forward, brakes wouldn’t stop the car, and you couldn’t even grip the steering wheel to turn the car Without friction you would slip and fall every time you tried to walk Therefore, friction is necessary for many everyday tasks Some friction is helpful, but there is some friction that is not wanted

Unwanted vs. Helpful Friction We may want to reduce friction by using low-friction materials, such as nonstick coatings on cooking pans Lubricants such as motor oil, wax, and grease are also used to lower friction We can increase friction by making surfaces rougher— scattering sand on icy roads prevents slipping, baseball players might wear textured gloves to increase the friction between their hands and the bat

Gravitational Force The force of gravity is equal to an objects weight The gravitational force can be calculated by multiplying an object’s mass by gravity (10 N/kg on Earth) F grav =mass · g

Normal Force A normal force exists between two solid objects when their surfaces are pressed together due to other forces acting on one or both objects It is always at right angles from the surfaces of the interacting objects For example, if a book is resting on a surface, then the surface is exerting an upward force on the book in order to support the weight of the book

Tension A tension force occurs when a non-slack rope, wire, cord, or similar device pulls on another object The tension force always points in the direction of the pull