Final answer:
An object at rest begins to move when the applied force exceeds static friction. Once moving, kinetic friction is lower, making it easier to maintain motion. Newton's first law explains that net force is zero for constant velocity and positive for acceleration.
Step-by-step explanation:
When a stationary object begins to accelerate due to applied forces, it transitions from static friction to kinetic friction. Initially, static friction prevents the object from moving, increasing until it is equal to the applied force. Once the applied force exceeds static friction, the object starts to move and kinetic friction takes over, which is usually less than static friction. If the object, such as a crate, receives additional mass, the required force to overcome static friction increases. Conversely, reducing friction (e.g., by oiling the surface) makes it easier to start and maintain motion. Furthermore, according to Newton's first law, an object at rest stays at rest and an object in motion maintains its velocity unless acted upon by a net external force. This means for an object moving at a constant velocity, such as a car, the net horizontal force, including friction, must be zero, thus no acceleration occurs. However, if the car is accelerating, the net horizontal force must be greater than the frictional forces.