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# What are Newton’s Law, Friction, and Kinematics?

The following is Newton’s first law of motion: When a body is at rest, it prefers to stay at rest. Unless forced on by a net external force, a body in motion tends to maintain a steady velocity. (Keep in mind that constant velocity refers to a body moving in a straight path at a constant pace.) This law may appear to contradict your ordinary experience at first look. You’ve surely seen that unless you exert some effort to keep a moving thing going, it will eventually slow down and stop. The key to understanding why, for example, a sliding box slows down (apparently on its own) is to recognise that the box is slowed by a net external force. This box would continue to move at a steady velocity if there was no net external force (as stated in Newton’s first law of motion). What force is exerted on the box to cause it to slow down? Friction is the name for this force. Friction is a force that acts in the opposing direction of motion. Friction is a type of resistance to motion that causes objects to slow down.

Newton’s First Law and Friction

Take a look at an air hockey table. Whenever the air is cut off, the puck only moves a short distance before coming to a halt due to friction. When the air is switched on, however, it elevates the puck slightly, resulting in virtually minimal friction as the puck glides across the surface. This puck glides through with little change in speed since friction is nearly non-existent. The puck would encounter no net external force on a frictionless surface (ignoring air resistance, which is also a form of friction). We can also precisely anticipate how soon items will slow down if we know enough about friction. Now consider a different scenario. A guy uses a force of +50 N to push a box across a floor at a consistent speed. (The positive value indicates that the movement is to the right, as is customary.) What is the frictional force that resists motion? Friction force must be less than 50 N. Why? Newton’s first rule of motion states that any object travelling at constant velocity experiences no net external force, implying that the total of the forces acting on the object must be zero. Fnet= 0 or ΣF=0 is a mathematical expression for no net external force acting on an item. So, if the guy exerts +50 N of force, the friction force must also be +50 N in order for two forces to sum up to zero. Newton’s first law states that the net external force is zero whenever the term at constant velocity is used.

The coefficient of friction and the normal force are the two parameters that determine the friction force. The coefficient of friction is a constant that relies on the characteristics of two surfaces that come into contact with one another. The normal force is the force exerted on an item by a surface in reaction to gravity drawing the thing down. The friction force is expressed in equation form as f=μN, where N is the normal force and is the coefficient of friction. (The normal force is addressed in more detail in the section Newton’s Third Law of Motion, while the coefficient of friction is studied in more detail in another chapter.) Remember from the Force section that a net force operates on the item of interest from outside. It works on the interest system, according to a more exact definition. A system is a collection of one or more items that you select to investigate. It’s critical to characterise the system at the start of a challenge in order to determine which forces are external and should be considered, and which are internal and may be ignored.

Two children, for example, push a third child in a waggon at a consistent speed. The waggon plus the young child, as illustrated in portion (b) of the image, is the system of interest. External pressures are exerted on this system by the two youngsters in the waggon (F1, F2). Two further external forces operating on the system are friction f acting at the axles of the wheels and friction f acting at the surface where the wheels meet the ground. Two additional external pressures act on the system: the system’s weight W knocking down as well as the ground’s normal force N forcing up. Because the waggon is not moving vertically, Newton’s first law states that the weight is balanced by the normal force. Because the waggon is going at a constant speed, the friction force must be equal to the total of the two children’s forces.

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