# 9.5 Simple machines  (Page 3/4)

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An ordinary pulley has an MA of 1; it only changes the direction of the force and not its magnitude. Combinations of pulleys, such as those illustrated in [link] , are used to multiply force. If the pulleys are friction-free, then the force output is approximately an integral multiple of the tension in the cable. The number of cables pulling directly upward on the system of interest, as illustrated in the figures given below, is approximately the MA of the pulley system. Since each attachment applies an external force in approximately the same direction as the others, they add, producing a total force that is nearly an integral multiple of the input force $T$ .

## Section summary

• Simple machines are devices that can be used to multiply or augment a force that we apply – often at the expense of a distance through which we have to apply the force.
• The ratio of output to input forces for any simple machine is called its mechanical advantage
• A few simple machines are the lever, nail puller, wheelbarrow, crank, etc.

## Conceptual questions

Scissors are like a double-lever system. Which of the simple machines in [link] and [link] is most analogous to scissors?

Suppose you pull a nail at a constant rate using a nail puller as shown in [link] . Is the nail puller in equilibrium? What if you pull the nail with some acceleration – is the nail puller in equilibrium then? In which case is the force applied to the nail puller larger and why?

Why are the forces exerted on the outside world by the limbs of our bodies usually much smaller than the forces exerted by muscles inside the body?

Explain why the forces in our joints are several times larger than the forces we exert on the outside world with our limbs. Can these forces be even greater than muscle forces (see previous Question)?

## Problems&Exercises

What is the mechanical advantage of a nail puller—similar to the one shown in [link] —where you exert a force $\text{45 cm}$ from the pivot and the nail is $\text{1.8 cm}$ on the other side? What minimum force must you exert to apply a force of $\text{1250 N}$ to the nail?

25

50 N

Suppose you needed to raise a 250-kg mower a distance of 6.0 cm above the ground to change a tire. If you had a 2.0-m long lever, where would you place the fulcrum if your force was limited to 300 N?

a) What is the mechanical advantage of a wheelbarrow, such as the one in [link] , if the center of gravity of the wheelbarrow and its load has a perpendicular lever arm of 5.50 cm, while the hands have a perpendicular lever arm of 1.02 m? (b) What upward force should you exert to support the wheelbarrow and its load if their combined mass is 55.0 kg? (c) What force does the wheel exert on the ground?

a) $\text{MA}=\text{18}\text{.}5$

b) ${F}_{\text{i}}=\text{29.1 N}$

c) 510 N downward

A typical car has an axle with $1\text{.}\text{10 cm}$ radius driving a tire with a radius of $\text{27}\text{.5 cm}$ . What is its mechanical advantage assuming the very simplified model in [link] (b)?

What force does the nail puller in [link] exert on the supporting surface? The nail puller has a mass of 2.10 kg.

$1\text{.}\text{3}×{\text{10}}^{3}\phantom{\rule{0.25em}{0ex}}\text{N}$

If you used an ideal pulley of the type shown in [link] (a) to support a car engine of mass $\text{115 kg}$ , (a) What would be the tension in the rope? (b) What force must the ceiling supply, assuming you pull straight down on the rope? Neglect the pulley system’s mass.

Repeat [link] for the pulley shown in [link] (c), assuming you pull straight up on the rope. The pulley system’s mass is $\text{7.00 kg}$ .

a) $T=\text{299 N}$

b) 897 N upward

An American traveler in New Zealand carries a transformer to convert New Zealand’s standard 240 V to 120 V so that she can use some small appliances on her trip.
What is the ratio of turns in the primary and secondary coils of her transformer?
nkombo
How electric lines and equipotential surface are mutually perpendicular?
The potential difference between any two points on the surface is zero that implies È.Ŕ=0, Where R is the distance between two different points &E= Electric field intensity. From which we have cos þ =0, where þ is the angle between the directions of field and distance line, as E andR are zero. Thus
sorry..E and R are non zero...
By how much leeway (both percentage and mass) would you have in the selection of the mass of the object in the previous problem if you did not wish the new period to be greater than 2.01 s or less than 1.99 s?
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why no diagrams
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Fayyaz
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it's a natural phenomena
Hassan
sap
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Benedine
A 20MH coil has a resistance of 50 ohms and us connected in series with a capacitor to a 520MV supply
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it is the science which we used in our daily life
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Physics is the branch of science that deals with the study of matter and the interactions it undergoes with energy
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it is branch of science which deals with study of happening in the human life
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A 20MH coil has a resistance of 50 ohms and is connected in series with a capacitor to a 250MV supply if the circuit is to resonate at 100KHZ, Determine 1: the capacitance of the capacitor 2: the working voltage of the circuit, given that pie =3.142
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Physics is the branch of science that deals with the study of matter and the interactions it undergoes with energy
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Yeah true ilwith d help of Adiabatic
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Physics? Is a branch of science dealing with matter in relation to energy.
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Physic... Is a purging medicine, which stimulates evacuation of the bowels.
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are you asking for qualities or quantities?
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Yes the Universe itself
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sports shoes are designed in such a way they are gripped well with your feet and their bases have and high friction surfaces, Thong shoes are for comfort, these are easily removed and light weight. these are usually low friction surfaces but in wet conditions they offer greater friction.
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thong sleepers are usually used in restrooms.
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yeah that was a mistake
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if it flows in three dimensions does it mean if an object theoretically moves beyond the speed of light it won't experience time
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but if an object moves beyond the speed of light time stops right for it
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yes it always flow from past to future.
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