Chapter 4- Newton’s laws of Motion. Kinematics the study of how objects move Dynamics the study of why objects move Force a push or a pull; symbol is F ; SI unit is the Newton, or N One Newton is the force necessary to cause a one kilogram mass to accelerate at the rate of 1 m/s 2
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an attractive or a repulsive force between charged particles; when charged particles are in motion, they produce magnetic forces on each other; the magnitude of the force is directly proportional to the magnitudes of the charges and inversely proportional to the square of the distance between the two charges
an attractive force between the particles in the nucleus; it is the strongest of the four forces, but only acts over very small distances (does not obey inverse square law for distances)
a force involved in the radioactive decay of some nuclei (in the 1960’s, Weinberg theorized the existence of the electroweak force, combing the electromagnetic and the weak nuclear force)
To work free-body diagram problems:
1. Weight – a measure of the gravitational force acting on an object; direction is down (toward the earth’s center); symbol is Fg
It is easy to see that the force of gravity acts on an object when it is falling. When an object is at rest on a surface, the gravitational force still continues to act. According to the second law, only a net force would cause the motion of the object to change. Since the object remains at rest, the net force acting on it must be zero. Another force (the normal force) balances the gravitational force. This is not an example of an action/reaction pair of forces! Both forces (gravitational and normal) act on the same object. Action/reaction pairs of forces act on different objects.
the force exerted by a surface to support an object; symbol is FN; direction is always upward; when an object rests on a horizontal surface, the normal force equals the object’s weight; when an object is being pushed or pulled by a horizontal force, the normal force equals the object’s weight. When a push or a pull is something other than horizontal, a free-body diagram is used to determine the normal force (taking into account the vertical component of the push or the pull). A normal force is a "response" force. In other words, a surface responds to a weight resting on it by acting to oppose it to the extent necessary to "cancel out" these forces. For our purposes, normal forces only exist on a surface (if the object is in the air, there is no normal force). The normal force is not an action/reaction pair of forces with the weight.
Returning to our weight example ... An upward force (the normal force) is exerted by the table on the object. The reaction to this is the downward force exerted by the object on the table. Since these forces act on different objects, they are action/reaction forces described by Newton's Third Law. What is the reaction force acting in response to the gravitational force? The object exerts a gravitational force on the earth!
The object rests on the table and is in the Earth's gravitational field. The Earth exerts a downward gravitational force on the object equal to the object's weight. The object exerts an equal gravitational force on the Earth. The downward gravitational force exerted by the Earth and the upward gravitational force exerted by the object represent an action/reaction pair of forces. The table exerts an upward force on the object. The object exerts an equal downward force on the table. These also represent an action/reaction pair of forces. An action/reaction pair of forces never act on the same object. Action/reaction pair of forces represent the mutal interaction of two bodies.
A man hitches a horse to a cart. The horse refuses to pull the cart, telling the man, "No matter how hard I pull, the cart pulls back with an equal force, and it will be impossible for me to ever pull the cart!"
4. Applied force act vertically. – the push or pull that "you" use to move an object; symbol is Fapp
6. Tension act vertically. - a force usually associated with a rope or a cable; it is a "response" force. In other words, if one pulls on a rope, the rope "fights back" by resisting being stretched. If the rope has negligible mass, the force exerted at one end is transmitted undiminished to each adjacent piece of rope along its entire length to the other end. Note: ropes, cords, and string can only pull. They cannot push because they bend.
7. Forces involved in springs act vertically.: Most mass/spring systems obey a simple relationship between force and displacement (x), known as Hooke's Law. The restoring force (F) is proportional to the displacement (x). The spring constant (k) is the proportionality constant. Felastic = -kx
Forces on an Inclined Plane act vertically.
Inclined plane problems are easier to work if one chooses the direction parallel to the incline surface as your x-axis. The y-axis is then perpendicular to the incline surface. If an object slides down an incline, three forces act on it: the normal force, the frictional force, and the gravitational force (or the object's weight).