85.1 Newtons first law of motion and the concept of force 3 The causes of acceleration of a system--The first law of motion states that zero total force exists when the system has a constant velocity; a nonzero total force on the system causes its acceleration d≠0分Fom≠0 @Inertial reference frames- -the reference frames in which Newton's first law is valid 85.1 Newton's first law of motion and the concept of force 2. The concept of force Force is a vector quantity-the vector sum of all the forces on a system is the force Ftotal on the system. 2 The measurement of force and the unit of force--N(Newton)($ 5.4) 3. The fundamental forces of nature Gravitational force Electromagnetic force Strong force Weak force2 4Inertial reference frames—the reference frames in which Newton’s first law is valid 3 The causes of acceleration of a system--The first law of motion states that zero total force exists when the system has a constant velocity; a nonzero total force on the system causes its acceleration. 0 0 a ≠ ⇔ Ftotal ≠ r r §5.1 Newton’s first law of motion and the concept of force 2. The concept of force 1force is a vector quantity—the vector sum of all the forces on a system is the force on the system. Ftotal r 2 The measurement of force and the unit of force-- N (Newton) (§5.4) §5.1 Newton’s first law of motion and the concept of force 3. The fundamental forces of nature Gravitational force Electromagnetic force Strong force Weak force