Definition of Force

Definition of Force A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces onlyexist as a result of an interaction. Velocity, Acceleration, Momentum, and Impulse Velocity, in physics, is a vector quantity (it has both magnitude and direction), and is the time rate of change of position (of an object). However, quite often when you read ‘velocity’, what is meant is speed, the magnitude of the velocity vector (speed is a scalar quantity, it has only magnitude). For example: escape velocity (the minimum speed an object needs to escape from a planet, say); note that this can be easily turned into a velocity, by adding ‘in the direction radially out from the center of the planet’, and that this direction is sometimes implied (if not actually stated). Velocity is a vector measurement of the rate and direction of motion or, in other terms, the rate and direction of the change in the position of an object. The scalar (absolute value) magnitude of the velocity vector is the speed of the motion. In calculus terms, velocity is the first derivative of position with respect to time.
The most common way to calculate the constant velocity of an object moving in a straight line is with the formula: r = d / t where 1. r is the rate, or speed (sometimes denoted as v, for velocity, as in this kinematics article) 2. d is the distance moved 3. t is the time it takes to complete the movement
The SI units for velocity are m / s (meters per second).

Acceleration is the rate of change of velocity as a function of time. It is vector. In calculus terms, acceleration is the second derivative of position with respect to time or, alternately, the first derivative of the velocity with respect to time.
The SI units for acceleration are m / s2 (meters per second squared ormeters per second per second).

Momentum is the product of an object's mass and velocity. It is a vector. The SI units of momentum are kg * m/s.
The rate of acceleration of a security's price or volume. The idea of momentum in securities is that their price is more likely to keep moving in the same direction than to change directions. Impulse is defined as a force multiplied by the amount of time it acts over. In calculus terms, the impulse can be calculated as the integral of force with respect to time. Alternately, impulse can be calculated as the difference in momentum between two given instances.
The SI units of impulse are N*s or kg*m/s.

Newton's First Law of Motion: I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
This we recognize as essentially Galileo's concept of inertia, and this is often termed simply the "Law of Inertia". The first law says that an object at rest tends to stay at rest, and an object in motion tends to stay in motion, with the same direction and speed. Motion (or lack of motion) cannot change without an unbalanced force acting. If nothing is happening to you, and nothing does happen, you will never go anywhere. If you're going in a specific direction, unless something happens to you, you will always go in that direction. Forever. You can see good examples of this idea when you see video footage of astronauts. Have you ever noticed that their tools float? They can just place them in space and they stay in one place. There is no interfering force to cause this situation to change. The same is true when they throw objects for the camera. Those objects move in a straight line. If they threw something when doing a spacewalk, that object would continue moving in the same direction and with the same speed unless interfered with; for example, if a planet's gravity pulled on it (Note: This is a really really simple way of descibing a big idea. You will learn all the real details - and math - when you start taking more advanced classes in physics.). Newton's Second Law of Motion: II. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.
This is the most powerful of Newton's three Laws, because it allows quantitative calculations of dynamics: how do velocities change when forces are applied. Notice the fundamental difference between Newton's 2nd Law and the dynamics of Aristotle: according to Newton, a force causes only a change in velocity (an acceleration); it does not maintain the velocity as Aristotle held.
This is sometimes summarized by saying that under Newton, F = ma, but under Aristotle F = mv, where v is the velocity. Thus, according to Aristotle there is only a velocity if there is a force, but according to Newton an object with a certain velocity maintains that velocity unless a force acts on it to cause an acceleration (that is, a change in the velocity). As we have noted earlier in conjunction with the discussion of Galileo, Aristotle's view seems to be more in accord with common sense, but that is because of a failure to appreciate the role played by frictional forces. Once account is taken of all forces acting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that are found to be in accord with the observations. Newton's Third Law of Motion: III. For every action there is an equal and opposite reaction. This law is exemplified by what happens if we step off a boat onto the bank of a lake: as we move in the direction of the shore, the boat tends to move in the opposite direction (leaving us facedown in the water, if we aren't careful!).

1st law: "An object in motion will remain in motion unless an external force acts upon it. "Perhaps the hardest of the 3 laws to demonstrate on Earth where friction and gravity are ever present (external forces), but in outer space far away from any planets or stars, an object given an initial push (force) will continue forever in the same direction at a constant speed.
2nd law: "F = ma; acceleration is proportional to force and inversely proportional to the mass of the accelerated object ." You are pushing a box across a frictionless surface, if you want to speed up the box faster (increase in acceleration), you push harder (increase in force). If the box was replaced with a heavier box (increase in mass), you have to push harder (increase in force) to speed it up at the same rate as before.
3rd law: "For every action there is a equal and opposite reaction." Perhaps the most misconceived law by the public, this law does not mean if the earth pulls you down by the virtue of its mass, it also "hold" you up.
This law means the force that is applied has an equal and opposite counterpart. For instance, if you apply a force to a object, the object applies the same force on you. Therefore, the earth applies a gravitational force on you, and you apply an equal and opposite force on the earth! This force is not noticed due to the second law, the earth is approximately 100,000,000,000,000,000,000,000 (1022) times more massive than any of us but the force is still there. Static Equilibrium
The condition of Static Equilibrium depends on two conditions:
1. Translational Equilibrium: sum of all forces = 0 (that is, Fnet = 0).
An object may be rotating, even rotating at a changing rate, but may be in translational equilibrium if the acceleration of the center of mass of the object is still zero.
2. Rotational Equilibrium: sum of all torques about a point on an object = 0; (net torque = 0).
An object may be accelerating in a linear fashion (along a straight line and or even turning at a constant rate; an object in rotational equilibrium will NOT be accelerating in a rotational sense (ie. the angular momentum of an object in rotational equilibrium will be constant).
Static Equilibrium is attained by satisfying BOTH of the above conditions at the same time.
Problem Types:
Objects in Translational Equilibrium:
Objects that are suspended (by contact with other objects or by tension forces) without acceleration are in translational equilibrium. These problems are solved either by resolving forces into components, or if there are 3 forces on the object, by adding the forces in a triangle and solving for the missing value.
Example #1: Find the tension in the horizontal string in the diagram below:
Solution:
tan 55o = mg/FT
FT = (15kg)(9.8m/s2) / tan 55o
FT = 103 N
This solution can also be obtained using components of forces on the object. Two equations can be created from the sum of hoizontal forces = 0, and the sum of vertical forces = 0. From substitution (and a bit of labor), the final answer will be the same as above.
Objects in Rotational Equilibrium:
When an object has forces applied to it at some distance from the center of mass of the object, these forces cause a "turning force" on the object. The actual direction of the "turning force" or Torque, is perpendicular to a plane formed by the force on the object, and the displacement of this force from a chosen pivot point. We will consider Torque as a "turning force" in 2 dimensions. t= F x d sin q the sin q term defines the force as the PERPENDICULAR component of force applied at that point on the lever, or beam.
To find an unknown force on a beam, or lever arm in static equilibrium, set up the equation for the sum of torques applied by all forces on the beam = 0.
Establish the preferred pivot point (place at the most undesireable force location as it makes any torque due to that force equal to zero).
Establish the preferred coordinate axes (along the beam and perpendicular to it). Resolve all NON-perpendicular forces into components to find the parts causing torque on the beam.
Solve for the unknown force using the sum of torques = 0 equation.
Example #2: Find the force of the wall on the ladder in the diagram below:
Solution: Consider the pivot at the base of the ladder.
St = 0
0 = (Fwcos30)d - (Fgcos60)d/2
**since beam is uniform, the c.o.m. is at d/2
Fw = 34 N

A particularly important special case arises when the net force on an object is zero. Objects for which this is true are said to be in translational equilibrium. Since the net force equals zero, the acceleration is also zero. Why is this case important? Look around you; objects in translational equilibrium are everywhere. Notice that being in equilibrium (for short) does not mean motionless.