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Net Force
Objects are often under the influence of multiple forces at any given time. For example, when you throw a ball, the ball is experiencing the push from your hand, the pull of gravity, and the drag of air resistance (friction). If we add these forces together based on their strength and direction, the resulting force is called the net force. When the forces exerted on an object cancel each other out, for example, equal forces in opposite directions, the net force is zero. Imagine a game of tug-o-war where both teams are pulling with the same force; the force in one direction will balance out the force in the opposite direction. If the net force is zero, we say there are balanced forces. However, if one or more forces are stronger than the others, then the net force is non-zero and we say there are unbalanced forces. When an object is experiencing unbalanced forces, the strength and direction of the net force determines the motion of the object.
MOTION
In order to determine if an object is in motion or not, we must use reference points. A reference point can be any other object or place, the relative position of which is compared to the object to see if it is changing over time. For example, when you are riding in a car you may use a tree by the side of the road as a reference point to judge whether you are moving. If your position is changing relative to the tree, you will perceive motion. If your position relative to the tree is not changing, you will conclude that you are motionless. Therefore, motion can be defined as a change in position over time relative to a reference point.
Speed is often used to describe the motion of an object. It is calculated by dividing the total distance an object has travelled by the time it took to travel that distance. Speed can be graphed as a function of distance vs. time, where distance is the y-variable and time is the x-variable. When graphed this way, the slope of the line represents the speed of the object; the steeper the line, the faster the speed.
People often use the term velocity as a synonym for speed, however, it has a different meaning in physics. Velocity is the speed of an object with respect to its direction. So, two objects can be travelling at equal speeds, but if they are headed in different directions they have different velocities.
If the motion of an object is changing, we say that it is accelerating. We tend to think of acceleration as an object speeding up; however, acceleration refers to any change in an objects motion, whether it be speeding up (positive acceleration), slowing down (negative acceleration), or even changing direction.
Click HERE for more content on Newton's 3 Laws of Motion
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