My 9 year old is studying science and is on the section with fixed pulleys and movable pulleys. He got a handout to fill out with the questions and pictures, but both my wife and I (science university degrees) are either going crazy or something weird is going on. There are some simple definitions that I seem to have either lost my mind or they are too simplistic or something is not defined properly.
The handout says “The force needed to move the load is called the load force.”
In my mind, that’s not true… they mixed up load force and effort force. If you have a pulley system depending on the kind of mechanical advantage you have, it is the effort force that is what is needed to move the load. To me, the load force is the force exerted by the load on the main cable (absent any pulleys or at the point of connection to the machine)… i.e. the weight of the load itself. Or do they mean both are true? In that case yes load force is needed to move the load (without a simple machine) and effort force is also able to move the load (through the mechanical advantage of the simple machine). In that case the definition should point out that it is the machine that makes the effort force differ from the load force required to move the load.
Then they say the following regarding a movable pulley… the kind that is anchored to the ceiling on one end, the cable “slings” down and up around the pulley like a “U”, and the centre of the pulley is attached to the actual load…
“The pulley is attached to the load, the rope attached to the structure, makes lifting load easier because rope attached to the structure takes half the weight of the load. You only use half the effort force that you would have needed with a fixed pulley”
Ok that makes sense, for a 100 N load force you essentially have 2-tethers on it (rope coming out from each side of pulley holding the load) each supporting 50 N. The structure takes 50 N (rope attached to ceiling) and you exert 50 N force to hold the other tether up. If it were attached to the ceiling as well, both ceiling hooks would feel 50 N.
However, I don’t like this explanation to kids as I’d rather they see generally that the force also is related to distance and really has to do with work conservation. The above is a static example of force distribution. However, ultimately the setup means that for every 10 cm you move the effort rope, the load only travels 5 cm against the load force. The mechanical advantage is that the distance x force is equal on both load and effort sides. You can make your effort less by moving a greater distance with less effort.
This analogy is better translated to many other examples including torque wrenches, levers, gears, ramps, and many other simple machines. The above explanation of the ceiling taking “half the weight” makes no sense to my 9 year old and ultimately nothing about the fact that the load is travelling a half the distance (for each pull of the “effort” rope) is mentioned anywhere.
So back to my kid’s science homework…
Questions are 1. What is a load force? 2. What is an effort force? 3. How much effort force is needed to lift 300 kg weight in fixed pulley system, 4. …movable pulley system? 5. In a movable pulley system, where does the other effort come from?
So for 3. and 4. it is 300 kg and 150 kg but I am already cringing since they are mixing up mass and weight and force units. I am over-thinking this?
The 1. and 2. questions don’t seem to be properly addressed in the handout or switched/confusing definitions (see above). The effort force is what you put into the system to move the load. Imagine a black box where you have no idea what kind of mechanical advantage you have set up… all you have is a load on one end, and a place to apply effort force on the other end. The load force would be the direct amount of force the load applies to the machine or black box (first connection to the black box). Load force depends on the load alone (it’s weight), effort force is what you need to move the load, regardless of it’s weight as it depends on the machine (mechanical advantage) between the effort and the load.
Finally last question I am guessing they mean the “other effort” comes from the supporting structure? Your effort force is 50 N and the “other” ceiling effort of 50 N is what allows you to hold up a 100 N load force?
Thank you all for your thoughts on this primary school handout and please let me know if I am just over-thinking this or is this just going to confuse my kids and they will have to unlearn this so they can understand it properly later on? I want to teach it properly in a way that can be generalized to all types of machines and my kids understand the fundamentals properly, not just memorize a bunch of stupid examples that give half-arsed explanations that confuse them.