If the three sides of an obtuse triangle are a + 1, a + 2 and a + 3, the value range of a is __

If the three sides of an obtuse triangle are a + 1, a + 2 and a + 3, the value range of a is __

∵ the length of three sides of obtuse triangle is a + 1, a + 2, a + 3,
The angle of a + 3 pair is an obtuse angle, set as α，
∴cos α= (a+1)2+(a+2)2−(a+3)2
2(a+1)(a+2)=a−2
2(a+1)＜0，
Solution: - 1 ＜ a ＜ 2,
From a + 1 + A + 2 > A + 3, the solution is: a > 0,
Then the value range of a is 0 < a < 2
So the answer is: 0 < a < 2

It is known that the three sides of an obtuse triangle are a, a + 1 and a + 2 respectively, in which the maximum internal angle does not exceed 120 °, and the value range of the real number a is obtained

∵ the length of three sides of a triangle is a, a + 1 and a + 2 respectively, ∵ a + (a + 1) > A + 2, and the solution is a > 1; ∵ the triangle is an obtuse triangle, ∵ A2 + (a + 1) 2 ＜ (a + 2) 2, the solution is - 1 ＜ a ＜ 3; Therefore, it can be obtained that 1 < a < 3. And ∵ the maximum internal angle does not exceed 120 °, ∵ A2 + (a + 1) 2 − (a + 2) 22a (a + 1) ≥ - 1

Can two objects produce friction without contact? What are the conditions for friction?

To generate friction between objects, the following three conditions must be met:
First, objects contact and squeeze each other
Second, the contact surface is not smooth
Third, there is a trend of relative motion or relative motion between objects
Elastic force is the force produced by the deformation of objects. Here, it refers to the interaction force when objects contact and squeeze each other

Friction is everywhere. The conditions for friction are: two objects should contact and squeeze each other; Between two objects Forget your textbook,

, the conditions of friction are: 1. Two objects should contact and squeeze each other; 2. Relative motion or tendency of relative motion between two objects

1. Conditions for friction: ① two objects must be; ② Two objects do; ③ The contact surface is 2. The direction of friction is always the same as 3. The point of action of friction is always at the of the object

1 two objects must be in contact. 2 two objects make relative motion. 3 the contact surface is rough (this roughness should be smooth relative to the theory, personal understanding)
2. The direction of friction is always opposite to the motion of an object
3. The point of action of friction is always on the contact surface of an object

I think the action point of friction should be on the contact surface. Why draw the action point in the middle of the object? Please explain the reasons, Can it be painted on the contact surface?

This is the concept of physical particle, that is, the object is regarded as a point with constant mass but neglected shape, which is convenient for hand analysis
Yes, but it's hard to draw on the contact surface for force analysis, because for the force analysis of the same object, the three elements of the force (size, direction and action point) tell us that the force on the object can be easily analyzed only at one point. For example, if the object is subjected to gravity, the action point is at the center of gravity rather than the contact surface. At this time, if there is friction, there will be more than one action point of the force, and the synthesis of the force will be difficult, It will be inconvenient to solve the problem