The concept of lever
Definition of lever: a hard rod can rotate around a fixed point O under the action of force. This hard stick is a lever.
Five elements of leverage
1, fulcrum: 2, power; 3. resistance; 4. Power arm; 5. Resistance arm
Let's explain in detail what the five elements are.
(1) fulcrum: the point o around which the lever can rotate;
(2) Force: the force f1that makes the lever rotate;
(3) Resistance: the force F2 that hinders the rotation of the lever;
(4) Force arm: the distance from fulcrum O to force F 1 action line L1;
(5) Resistance arm: the distance L2 from fulcrum O to the line of action of force F2;
As shown in the above figure, the force F is the power and the gravity of the object is the resistance. The experiment shows that when the lever is in equilibrium, its power F 1, power arm L 1, resistance F2 and resistance arm L2 meet the following requirements: f1l1= f2l2;
Lever balance condition
When the rotation of force and resistance cancel each other out, the lever will be in a state of balance, which is called lever balance, but lever balance is not the balance of force.
When the lever is stationary or rotating at a constant speed, it is called lever balance.
Equal arm lever
According to the lever equilibrium condition F 1L 1=F2L2, when L 1=L2, f1= F2;
At this time, strength and resistance are equal. Such a lever is called an equal arm lever.
Compared with labor-saving bars and labor-saving bars, equal arm bars do not save labor and distance.
Common labor-saving levers are: balance and crown block.
labor saving lever
If the power arm of a lever is larger than the resistance arm, the lever is a labor-saving lever.
According to the lever balance condition F 1L 1=F2L2:
F 1=F2L2/L 1
Obviously, the power arm is larger than the resistance arm, that is, L1>; L2，f 1 >； Second generation
At this time, power is less than resistance.
Hard lever
If the power arm is smaller than the resistance arm, then the lever is a laborious lever. This can also be proved by mathematical relations.
According to the lever balance condition F 1L 1=F2L2:
F 1=F2L2/L 1
Obviously, the power arm is smaller than the resistance arm, that is, when L 1 < L2 and F 1 < F2.
Application of lever
In life, we can see tools that use the lever principle everywhere, as shown below.
Equal arm lever
The balance uses the principle of equal arm lever, that is, the power arm is equal to the resistance arm, to measure the mass of the object. Because the power arm is equal to the resistance arm, the gravity in all the left and right trays is equal. According to the relationship between gravity and mass (G=mg), the measured mass is equal to the mass of the weight plus the mass of the marching code.
labor saving lever
Prying a stone with a crowbar is a typical labor-saving lever. In essence, we hope to lift heavy stones with less power.
Although the labor-saving lever saves labor, it also consumes distance.
Hard lever
Fishing with a fishing rod is a laborious lever.
practise
As shown in the figure, the fulcrum of a light lever is at point O, two hook codes are hung on the left A, and after an object is hung on the right B, the lever is balanced in a horizontal position. If you hang another hook code under A, you must move the object and hang it at C to balance the lever horizontally. If BC=5cm, AO= 15cm, and the mass m of each hook code is 50g, find the length of OB and the mass m of the object.
Solution:
(1) When two hook codes are hung on the left side A and one object is hung on the right side B:
∫ Lever level balance,
∴2mg×OA=Mg×OB，
Namely: 100g× 15cm=M×OB, ﹣ ﹣ ①.
(2) When a hook code is hung under A, move the object to C,
∫ Lever level balance,
∴3mg×OA=Mg×OC=Mg×(OB+BC)，
150g× 15cm=m×(ob+5cm)﹣﹣﹣﹣﹣②
(3) Simultaneous equations from ① ②:
OB= 10cm，M= 150g。
Answer: The length of OB is10cm, and the mass of the object is150g.
When analyzing the problem of lever balance, we should not judge it only by the magnitude of force, but must consider it from the basic knowledge, so as to solve the problem rationally, avoid solving the problem by subjective feeling and be familiar with the conditions of lever balance.