Forces and Motion

    When we hear the words "Force" or "Motion" one name should come to our minds.  That name, Sir Isaac Newton.  In the late 1600s, Sir Isaac Newton studied how objects moved when acted on by gravity and other forces. Newton's studies led to his discovery of the three basic laws of motion.

Key terms related to Force and Motion

Non-contact forces: Forces exerted by an agent which is not in contact with the receiver.

Gravitational force:    Normally understood in this unit as the force or pull exerted by the Earth on an object.

Weight:    The gravitational force which the Earth exerts on an object.

Newton (N):    The SI unit for force, approximately equal to the gravitational force of the Earth on a 100g mass at the Earth's surface.

Inertia:    The tendency of objects at rest to remain at rest and objects in motion to stay in motion at constant speed and in the same direction.

Accelerate:    To move faster or gain speed.

Terminal speed:    The speed at which the downward pull of gravity is balanced by the equal and upward opposing force of air resistance for a falling object.

Friction:     The resistance to motion of things that are touching. Friction is a force that slows down moving things. When two things rub together, like your bike tires and the road, friction acts between them to slow you down. That's why you eventually stop rolling when you stop pedaling. Rough things make more friction than smooth things. Riding on sandpaper would slow you down more than riding on ice.

Buoyancy:    Makes things float. Boats, ducks, surfboards, even your nine-year-old brother, are all buoyant. When you throw your brother into a pool, his body pushes away some of the pool water. Scientists say the water is displaced. Your brother floats because the water he displaced weighs the same as his body. All things float for the same reason -- the amount of water a thing displaces weighs the same as the thing itself.

Force:    Everything needs a push or pull -- a force -- to make them move or stop. Cause of motion or change.
 

Motion:    An act or instance of moving a body or its parts.

*    A force is a push or pull exerted by one object upon another.

*    A force alters the shape or the motion of an object.

*    A force may be represented by an arrow since it has both direction and size.

*    More than one force may be exerted on an object at one time.

*    There are many types of forces, including buoyant, magnetic, electrical, gravitational, elastic, and frictional.

*    Falling is motion resulting from the gravitational force of the Earth acting upon an object.

*    The gravitational force of the Earth upon an object is called the weight of the object.

*    The moon remains in orbit around the Earth due to the Earth's gravitational force.

*    Every object in the universe exerts an attractive force on every other object.

*    The size of the gravitational force which one object puts on another depends upon the masses of the objects and the distance between them.

*    The amount of matter in an object is called its mass.

*    Theories are often based upon reasoning that may not be easily tested or often not tested until years later.

*    The weight of an object changes with its location in the universe, but its mass remains constant.

*    The weight of an object on the moon is 1/6 its weight on the Earth.

*    A force is measured by measuring the size of its effect on a receiver.

*    Because "elastic" materials are distorted by forces predictably, they can be used to make forces measurers.

*    A spring can be used to measure the size of the gravitational force.

*    A spring stretches uniformly when increasing masses are added to it.

*    The unit of force, the newton, is approximately the weight of 100g at the Earth's surface.

*    The unit for mass is grams.

*    Although the terms mass and weight are often used interchangeably, mass is different from weight.

*    Weight is usually measured with a spring scale, but mass is measured with equal-arm balance.

*    The SI base unit for mass is the kilogram (kg), and for weight it is the newton (N).

*    To find the mass of an object, the gravitational force on it is usually compared with the gravitational force on a known standard mass, using  an equal-arm balance.

*    The mass of an object is constant throughout the universe, whereas its weight in relation to another given object varies with the mass of the two objects and the distance between them.

*    Friction is caused by one object moving against another.

*    Frictional forces slow down objects in motion an oppose the setting of objects at rest into motion.

*    Sliding friction is less than starting friction.

*    Rolling friction is less than sliding friction.

*    The force needed to lift an object is greater than that needed to drag the object.

*    The force of friction is dependent upon the weight of an object and the types of surfaces which are rubbing together.  It is independent of surface area for a constant weight.

*    Friction results from the interlocking of opposing surfaces as they move over each other.

*    Friction may be reduced by smoothing opposing surfaces, by lubrication, an by separating opposing surfaces with ball or roller bearings.

*    Friction is useful wherever motion between two surfaces is not desired.

*    Friction may be harmful when it makes motion more difficult, causes heat, or causes the surface to wear.

*    Fluids offer friction to objects moving through them, but this friction is less than that between two solid surfaces moving over each one another.

*    An object at rest with no forces on it or with balanced forces on it will remain at rest.

*    An object in motion with no forces on it or with balanced forces on it will continue in steady motion in a straight line.

*    An unbalanced force will cause an object at rest to start moving and an object in motion to speed up, slow down, or change direction.

*    The inertia of an object is its tendency when at rest to remain at rest, and when in motion to remain in motion in the same direction and at the same speed.

*    Inertia causes objects to behave in surprising ways, which makes some people think incorrectly that inertia is a force.

*    If object A Places a force on object B (an action), then object B places a force back on object A ( a reaction), which is equal in size but opposite in direction.

*    This can be expressed in the form of the equation :
Force of A on B (one direction) = Force of B on A (opposite direction).

*    In many cases, both the action and reaction forces produce or affect the motion of the objects upon which the forces are applied.