Chapter 4 - newton"s second law of motion. Any push or pull exerted on an object, measured in newtons either past another object with which it is in contact or through fluid. To increase the acceleration of an object, you must increase the net force acting on it. Acceleration ~ net force (~ -> is directly proportional ) Caused by irregularities in the surfaces in mutual contact. The direction of the friction force is always in the direction that opposes motion. Static friction is somewhat greater than sliding friction. Friction can also occur in liquids and gases. Acceleration sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way. Ex. if the mass is doubled, the weight is also doubled. The acceleration is inversely proportional to the mass. The quantity of space an object occupies.