5.2 - Newton’s laws

Newton’s three laws of motion tell us how objects move when they are acted on by net forces. The first law describes constant motion when there is no net force, the second law describes the acceleration of an object when it experiences a nonzero net force, and the third law describes how every action force has an equal but opposite reaction force. Read the text aloud
Newton’s first law and friction
Newton’s first law of motion says that an object at rest or in motion remains in its identical state of rest or motion unless acted upon by a net force. Since inertia is the property of mass that resists change in motion, the first law is sometimes referred to as the law of inertia. Read the text aloud
It may be intuitive that an object at rest stays at rest without the action of a net force. But the first law additionally says that an object in motion remains in the identical state of motion unless acted upon by a net force. In real life, objects in motion slow down and eventually stop unless pushed or pulled constantly. They do not keep moving in their identical state of motion. How is this fact reconciled with the first law? Read the text aloud
Newton's first law
The first law is not violated because friction creates another force whenever motion occurs in the real world. Friction is a “catch-all” word used to describe any resistive force that may be caused by motion and always acts to resist motion. When a box slides across the floor, the sliding motion between the box and the floor results in a friction force that acts on the box to slow it down. The reason you must continually push a box to keep it moving at constant speed is to counteract friction. A box moving with constant velocity has zero net force acting on it because your applied pushing force cancels the force of friction. Read the text aloud
Effect of friction when applying Newton's first law
The first law tells you how to approach problems that involve forces and motion. If things are at rest, and stay at rest, then the first law requires that the net force must be zero. This means that you can use the condition of equilibrium to find unknown forces. The same is true for situations of constant velocity. Constant velocity is a condition of force equilibrium. If the net force is not zero, then we need Newton’s second law. Read the text aloud

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