JC Science: Force, Work and Power

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Force, Work and Power

Force

A force is something that causes an object to change velocity or

accelerate.

The newton (symbol N) is the unit of

force.Forces can be measured using a

spring-balance (the proper name is a newton-meter).

Friction

Friction is a force that opposes motion

between two objects in contact.

Friction can be a nuisance (e.g. it

slows things down when you don’t want it to when driving a car or riding a bike).Friction can also be useful (e.g. brakes

on a bike, and to enable us to walk).

Experiment:

To

demonstrate friction and how a lubricant can be used to reduced friction.

Attach a spring balance to a wooden

block and use it to pull the block over the bench. Note the reading on the

spring-balance.

Now put some oil or grease under the

block and repeat. Notice that the force required to pull the block is much

less.

Hooke’s

Law

The extension of a stretched string is directly

proportional to the force that is stretching it

This

means that if the force on the string doubles so will the extension; if the

force triples then the extension will also triple, etc.

Experiment: To investigate the

relationship between the extension of a spring and the force that is stretching

it.

1. Use a newton-meter to stretch a spring and note the extension and

the force used.

2. Repeat for lot of different forces.

3. Plot a graph of force used against extension.

4. Result: Your

graph should be a straight line.

5. Conclusion: The graph is a

straight line through the origin which shows that the extension is directly

proportional to the applied for

{Strictly speaking stating Hooke’s Law

is not on the syllabus but it did get asked by mistake one year so it’s best to

learn it just in case it happens again. You do however have to know the

experiment.}

Weight and Mass

Mass is a measure of the amount of matter (“stuff”) in something.

The

weight of

an object is a measure of the gravitational pull on it.

Weight is a force (due to gravity)

The mass of an object doesn’t change

from one place to the next because it still has the name amount of stuff inside

(the same number of atoms), but the weight of the object will be much less on a

planet smaller than Earth (or on the moon) while it will weigh much more (it

will be much ‘heavier’) on a bigger planet. In fact if you could go to one of

the bigger planets you would not even be able to stand up because you weigh so

much!

Maths Problems

Weight (in newtons) = Mass (in

kilograms) × 10

e.g. a student of mass 60 kg

has a weight of 600 N.

Note: There are 1000 grams in a kilogram

(kg).

A little more on Mass and Weight

There is a very important and often misunderstood difference between mass

and weight.

Mass is the amount of matter present in the object. It doesn’t change, i.e.

the amount of ‘stuff’ in your phone doesn’t change, regardless of where you

bring it (even if you bring it to the moon it will still have the same number

of atoms in it.)

Another way to look at it is to compare weight, which is caused by

gravity, to magnetism.

Bringing a magnet from one place to another won’t change the amount of

stuff (mass) in it, but if you bring the south end of a magnet closer to the

north pole of another magnet it will experience a greater attraction to the

north pole. It will appear to be heavier. The force of attraction between two

magnets depends partly on the strength of the two magnets. Similarly the force

of gravitational attraction between two objects depends partly on the mass of

the two objects. ‘Weight’ is our shorthand way of talking about gravitational

attraction.

So the gravitational attraction between you and the Earth is greater

than the force of attraction between you and the moon.

Weight is the force with which the object is attracted to the Earth (or other

planet) by gravity.

Weight

(in newtons) = Mass (in kilograms) × 10

The

number 10 is an indication of the size of the force of gravity. It would be too

much of a coincidence for it to be exactly 10. It turns out to be about 9.8,

and because the force of gravity is less on the moon than on the Earth, this

number is smaller for the moon (it’s about 1.6).

Energy,

Work and Power

Energy is the ability to

do work

Energy

The unit of energy is also the joule.

Work done = Force

× distance

Work

The unit of work is the joule (J).

Power

Power is the rate at which work is done

The unit of power is the watt (W)