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4.8
Learning intentions
By the end of this topic, you will be able to: • compare and contrast distance magnifiers and force magnifiers • provide examples of first-, second- and third-class levers • calculate mechanical advantage and effort.
Interactive 4.8A Simple machines
Interactive 4.8B Types of levers
lever a simple machine that reduces the effort needed to do work
fulcrum the turning point of a lever
effort the force used to operate a lever
load (in physics) resisting force
Key ideas
• A lever is a solid rod with a turning point. • Levers provide a mechanical advantage of force or distance. • All levers need an effort (force used), fulcrum (turning point) and a load (the section being moved). • Levers can be divided into first-class, second-class or third-class levers. The ancient Egyptians, Romans and Greeks understood forces very well. They made simple machines that helped them to build the pyramids, fi ght wars and build cities (Figure 1). The simplest machine they used was a lever. A lever can be used to decrease the amount of effort needed to do work. You use levers every day. Scissors, pliers, brooms, shovels, wheelbarrows and can openers are all levers. Levers A lever is a solid rod or bar that is supported at a turning point called a fulcrum . Figure 2 shows the main features of a simple lever – a see-saw. The force used to operate a lever is called the effort , and the resisting force it overcomes is called the load . Load
Fulcrum b a DRAFT Effort
Figure 1 Ancient Egyptians used round logs and rope to haul large blocks of stone when they built the pyramids. Figure 2 a A lever has three features: the fulcrum, effort and load. b A see-saw is an example of a lever.
When one person on a see-saw is pulled down by the Earth, the other person is pushed up. The weight of the two people does not need to be equal for this see-saw lever to work. One person can lift a heavier weight by moving further away from the fulcrum in the middle. In fact, a single person 2m away from the fulcrum can lift two people who are 1m on the other side of the fulcrum.
Calculate the mechanical advantage of a lever that allows an effort of 4 newtons to lift a load of 12 newtons.
Solution
Mechanical advantage
The lever gives you a mechanical advantage. The size of the advantage can be calculated by dividing the size of the load by the size of the effort: Effort = 4 N, load = 12 N
Mechanical advantage =
size of the load size of the effort 12 4 = 3
The mechanical advantage is 3.
Mechanical advantage =
size of the load size of the effort
The magnification of the force comes with a disadvantage. For this type of lever, the distance the effort must move is greater than that moved by the load. Worked example 4.8A shows how to calculate mechanical advantage. Worked example 4.8B shows how to calculate effort.
These levers are called force magnifiers. They can change a weak force into a stronger force. A crowbar can be used to lift a heavy rock even though the rock is heavier than the effort used. There is a disadvantage to force magnifiers. The effort section of the lever must move a greater distance to move the rock a short distance.
Other levers are distance magnifiers. They magnify the distance the effort moves. This means that when the effort moves a short distance, the load will move a long distance. The disadvantage is that the effort force required will need to be larger than the load. An example of this is a tennis racquet. The end of the tennis racquet moves a greater distance (and faster) than the hand holding the racquet. Worked example 4.8B: Calculating effort Calculate the effort required to lift a box of books (6 newtons) with a lever with a mechanical advantage 2. Solution
Load = box of books = 6 N, mechanical advantage = 2 Mechanical advantage =
size of the load size of the effort 2 = 6 N effort Effort = 6 N 2
= 3 N The effort needed is 3 newtons. DRAFT
force magnifier a device that can increase the amount of force available (for example, to shift something); an example is a lever
distance magnifier a lever that changes a strong force that acts over a short distance into a weak force that acts over a longer distance
Figure 3 The trebuchet was a powerful machine used to fling objects such as rocks against enemy defences.
a a
LoadLoad EffortEffort b b b
fi rst-class lever a lever that has its fulcrum between the point of effort and the load
second-class lever a lever that has its load between the point of effort and the fulcrum
third-class lever a lever that has its point of effort between the fulcrum and the load FulcrumFulcrum
Figure 4 a In a first-class lever, the fulcrum is between the load and the effort. b Scissors are an example of a first-class lever.
a b
Effort
Load
Figure 5 a In a second-class lever, the load is between the effort and the fulcrum. b A wheelbarrow is an example of a second-class lever. Types of levers Fulcrum Figure 6 a In a third-class lever, the effort is between the load and the fulcrum. b A tennis racquet is an example of a third-class lever. When a person uses a tennis racquet to hit a ball, the muscles exerting the effort are between the shoulder fulcrum and the ball load. Load Effort Fulcrum a First Nations levers b DRAFT There are three types of levers and they are classifi ed according to the position of the fulcrum (turning point): > First-class lever : the fulcrum is between the effort and the load (EFL) (Figure 4). > Second-class lever : the load is between the effort and the fulcrum (ELF) (Figure 5). > Third-class lever : the effort is between the load and the fulcrum (LEF) (Figure 6). Use FLEE to remember the middle position for each type of lever. F (Fulcrum) – fi rst-class lever L (Load) – second-class lever E (Effort) – third-class lever E (Easy to remember!) Many First Nations peoples understand the advantage of levers in hunting. The word ‘woomera’ comes from the Dharag language and refers to a spear thrower that can launch a spear further and with more force and with more acceleration. A spear is fi tted into the 50–100 cm long lever and held in place with a short peg that connects to the end. The person throwing the spear could then hold the woomera and swing it over their head, similar to a catapult action. This makes the lever arm longer, moving the spear faster, increasing the speed that the spear left the thrower and making the spear more accurate.
Retrieve 1 Defi ne the term ‘lever’. Comprehend 2 Describe how you would identify a fi rst-class lever. Analyse 3 Compare a second-class lever with a third-class lever. 4 Examine Figure 8. a Identify the type of lever that is shown. b Calculate whether a mass of less than 200 kg would lift the load. c Describe how you would reposition the fulcrum so that a mass much less than 200 kg could lift the load.
The type of lever depends on the position of the fulcrum. If the wrist is used to fl ick the spear, then this becomes the fulcrum. If the arm and wrist remain straight, and the motion is like a bowler (in cricket), the fulcrum is located between the thrower’s shoulder blades. Once the spear has left the spear thrower, the unbalanced forces of air resistance cause the spear to slow down and the force from the Earth’s gravity causes the spear to fall. Different First Nations groups developed different styles and shapes of spear throwers. Longer spear throwers are able to increase the speed more than shorter spear throwers. This increase in speed means that the spears used need to be lighter. 4.8 Check your learning 6 Modern cranes use leverage to lift heavy objects (Figure 10). a Identify where the load for this lever is located. b Identify where the effort is located. c Identify where the fulcrum is located. d Identify the class of lever being used. e Identify whether this class of lever is a force magnifi er or a distance magnifi er. Justify your answer.
200 kg
Figure 7 A woomera is a lever. DRAFT
Figure 8 A lever
Apply
5 A crowbar can be used to move a load (Figure 9). a Identify the class of lever used. b Identify whether this class of lever is a force magnifi er or a distance magnifi er. Justify your answer.
Figure 9 A crowbar moving a load Figure 10 A modern crane
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