chapter 2 相互作用

Universe Gravitational law
2、The two spheres pictured below have equal densities and are subject only their mutual gravitational attraction . Which of the following quantities must have the same magnitude for both spheres ? (A) acceleration (B) velocity (C) kinetic energy (D) Displacement from the center of mass (E) Gravitational force
Some types of Forces
(C) W
F
W
v
Some types of Forces
A ball of mass is suspended from string of unequal length as show above .The tension T1 and T2 in the strings must satisfy which of the follow relations? (A)T1=T2 (B)T1>T2 (C)T1<T2 (D)T1+T2=mg (E)T1-T2=mg
Force and Newton’s Laws
Brendon Hu
Some types of Forces
1、Weight
W=mg FN 2、normal force
3、Tension
mg
Some types of Forces
4、friction force
Static friction
Slide friction
(A) 8 newton to the left
(B) 8 newton to the right
(C) 10 newton to the left
(D) 12 newton to the right (E) 20 newton to the left
20N
3kg
2kg
Newton’s Law of motion
F=ma
Example 2 . A box of mass 40kg is pushed in a straight line across a horizontal floor by an 80N force . If the force is kinetic friction acting on the box has a magnitude of 60N , what is acceleration of line box. (A) 0.25m/s2 (B) 0.5m/s2 (C) 1.0m/s2 (D) 2.0m/s2 (E) 3.5m/s2
Newton’s Law of motion
(2) Newton’s second Law of motion The acceleration of an object is directly proportional to the net force acting on it , and inversely proportional to its mass . The direction of acceleration is in the direction of the net force acting on the object .
T1 m
T2
Newton’s Law of motion
(1) Newton’s first Law of motion Every object continues in its state of rest , or of uniform velocity in a straight line , as long as no net force acts on it . Inertia refers to how much an object resists a change in its velocity and is only measured by mass .
(1)all of the system’s mass were concentrated there ,and (2)all external forces were applied there.
Center of mass
The center of mass of a uniform wire , bent in the shape shown below , is located closest to point
A rope of negligible mass supports a block that weight 30N , as show below . The breaking strength of the rope is 50N .The largest acceleration that can be given to the block by pulling up it with the rope without breaking the rope is mostly nearly
Newton’s Law of motion
(3) Newton’s third Law of motion For every force exerted by one object an another , there is another force equal in magnitude and opposite in direction that is exerted back by the second object on the first . When one body exerts a force on a second body , the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
L
L
A
B C D E
Universe Gravitational law（万有引力定律）
By such reasoning , Newton came to the conclusion that any two objects in universe exert gravitational attraction on each other , with the force having a universal form : m1 F1 F2 m2Newt源自n’s Law of motion
Two blocks are pushed along a horizontal frictionless surface by a force of 20 newton to the right , as shown below . The force that the 2-kilogram block exerts on the 3kilogram block is
G
universal gravitational constant
Universe Gravitational law
1、The gravitational force between two objects is F . If one of the mass is doubled while the distance between them is also doubled , the new gravitational force between them will be equal to (A) F (B) 2F (C) F/2 (D) 4F (E) F/4
Example 1 .Inertia can be best described as the (A) force that keeps an object in motion with constant velocity (B) force that keeps an object at rest (C) force that overcomes friction (D) property responsible for an object’s resistance to change in motion (E) property responsible for slowing down an object
（A）6m/s2 （B）6.7m/s2 （C）10m/s2 （D）15m/s2 （E）16.7m/s2
Weight=30N
Newton’s Law of motion
Center of mass
The center of mass is a point that moves as through