Penerangan
Introduction to Forces
A force is a push or a pull that acts upon an object as a result of interaction with another object. Forces are vector quantities, meaning they have both a magnitude, or size, and a direction. When forces act on an object, they can cause changes in the speed, shape, or direction of movement of that object. Forces are measured in units called newtons, named after the physicist Sir Isaac Newton, using an instrument called a newton meter or force meter.
In scientific diagrams, forces are represented by arrows. The direction of the arrow shows the direction in which the force is acting, and the length of the arrow typically represents the magnitude of the force. Multiple forces can act on a single object at the same time. The combined effect of all the forces acting on an object is known as the resultant force. Determining the resultant force involves adding forces that act in the same direction and subtracting forces that act in opposite directions.
Balanced and Unbalanced Forces
When two or more forces act on an object, their combined effect determines whether the motion of the object changes. If the opposing forces acting on an object are equal in size but opposite in direction, they are described as balanced forces. When forces are balanced, the resultant force acting on the object is zero newtons. According to the laws of motion, if an object is stationary and balanced forces act upon it, the object remains stationary. If the object is already moving at a constant speed in a straight line, it continues to move at that exact same speed and in that exact same direction.
Conversely, when the forces acting on an object are not equal in size, they are unbalanced forces. An unbalanced force results in a non-zero resultant force. When an unbalanced force acts on an object, it causes a change in the object's motion. This change can manifest as an increase in speed, a decrease in speed, or a change in the direction of travel. For example, when a cyclist pedals harder, the forward driving force becomes greater than the resistive forces of friction and air resistance, creating an unbalanced force that causes the bicycle to accelerate. If the cyclist stops pedalling, air resistance and friction become the dominant unbalanced forces, causing the bicycle to decelerate and eventually stop.
Turning Forces and Moments
Forces do not only cause objects to move in straight lines; they can also cause objects to rotate about a fixed point, pivot, or fulcrum. A turning force is known as a moment. The turning effect, or moment of a force, depends on two distinct factors: the size of the force applied and the perpendicular distance from the pivot to the line of action of the force.
The magnitude of a moment is calculated using a specific mathematical relationship. The standard unit for a moment is the newton-metre, abbreviated as Nm. To increase the turning effect without increasing the applied force, the distance from the pivot can be increased. This principle explains why door handles are placed as far away from the door hinges as possible, making it much easier to open the heavy door with a relatively small push.
Calculating Moments
The calculation of a turning force follows a strict mathematical formula. The formula states that the moment equals the force multiplied by the perpendicular distance from the pivot. In algebraic terms, this is expressed as moment equals force times distance. When using this formula in scientific calculations, the force must be measured in newtons, and the distance from the pivot must be measured in metres.
For a balanced system, such as a seesaw, the principle of moments applies. The principle of moments states that when an object is in equilibrium, the total clockwise moment about any pivot is equal to the total anticlockwise moment about that same pivot. This means a smaller force acting at a greater distance from the pivot can balance a larger force acting closer to the pivot, which is the foundational principle behind simple machines like levers, crowbars, and spanners.
