what is the answer of this question？---**Question 29**
**Question Stem:**
A person uses a bar that is 3.0 m long to lift a rock of weight 900 N off the ground. There is a pivot under the bar at 0.50 m from the rock, as shown. The person pushes vertically downwards on the other end of the bar from the rock, as shown. Ignore the weight of the bar.
What is the minimum force needed to lift the rock off the ground?
**Options:**
A 150 N
B 180 N
C 4500 N
D 5400 N
**Chart/Diagram Description:**
Type: Lever diagram (First-class lever)
Main Elements:
- Bar: A straight horizontal line representing the lever.
- Pivot: A triangular shape supporting the bar, positioned between the rock and the person. Labeled "pivot".
- Rock: An irregular shape under one end of the bar. Labeled "rock".
- Person: A simplified drawing of a person applying force to the other end of the bar.
- Forces: A downward arrow labeled "rock" (with handwritten "force") indicating the weight of the rock (load). A downward arrow labeled "force" indicating the applied force (effort).
- Distances:
- Distance from the rock to the pivot is indicated as 0.50 m.
- The total length of the bar is indicated as 3.0 m.
- The distance from the pivot to the point where the person applies force is not directly labeled but can be inferred from the total length and the distance from the rock to the pivot.
**Other Relevant Text:**
M = F x d (Handwritten formula)
**Question 30**
**Question Stem:**
Which energy resource does not have the Sun as its source of energy?
**Options:**
A coal
B geothermal
C hydroelectric
D water waves
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Let's analyze this lever problem step by step. We have a 3 meter long bar acting as a lever to lift a 900 Newton rock. The pivot point is positioned 0.5 meters from the rock. We need to find the minimum force required to lift the rock.
Now let's apply the principle of moments. For a lever in equilibrium, the clockwise moments must equal the anticlockwise moments. The load creates a clockwise moment of 900 Newtons times 0.5 meters. The effort creates an anticlockwise moment of F times 2.5 meters. Setting these equal gives us our equation.
Let's solve this step by step. First, we calculate the load moment: 900 Newtons times 0.5 meters equals 450 Newton-meters. Next, we set up our equation: 450 equals F times 2.5. Finally, we solve for F by dividing 450 by 2.5, which gives us 180 Newtons. Therefore, the minimum force needed is 180 Newtons, which corresponds to option B.
Let's verify our answer. The load moment is 900 Newtons times 0.5 meters, which equals 450 Newton-meters. The effort moment is 180 Newtons times 2.5 meters, which also equals 450 Newton-meters. Since the moments are balanced, our answer is correct. The minimum force needed to lift the rock is 180 Newtons, which corresponds to option B.
To summarize: we solved this lever problem using the principle of moments. With a 900 Newton load at 0.5 meters from the pivot and an effort arm of 2.5 meters, we calculated that the minimum force needed is 180 Newtons. This demonstrates how levers provide mechanical advantage by allowing a smaller force to lift a larger load when applied at a greater distance from the pivot. The correct answer is B: 180 Newtons.