Physics

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Motion, gravitation, work-energy, fluids and levers

⏱ 6 min read🧩 5 question types🎯 18 practice Q

The idea in one minute

Motion, force, gravity, energy, fluids and levers make up this subtopic. Exams ask which law or principle explains an everyday event. Learn each law with one example, plus the key numbers for gravity and energy.

Motion in simple words

A body is in motion when its position changes with time.

Distance is the total path covered. It is a scalar.
Displacement is the straight-line gap from start to end, with direction. It is a vector.
Speed is distance ÷ time. Velocity is displacement ÷ time.
Acceleration is the rate of change of velocity. Its unit is m/s².

Walk once round a circular park. The distance is large but the displacement is zero.

Newton's three laws

Law	Statement	Everyday example
First (inertia)	A body stays at rest or in uniform motion unless an outside force acts.	Passengers jerk forward when a bus brakes. Dust leaves a beaten carpet.
Second	Force = mass × acceleration, $F = ma$ . It is also the rate of change of momentum.	A cricketer pulls his hands back while catching.
Third	Every action has an equal and opposite reaction.	Rocket, gun recoil, swimming, walking.

Momentum is $p = mv$ . It stays constant when no outside force acts. That is why a gun recoils when it fires.

More on the first law: mass measures inertia. Passengers fall forward when a moving bus brakes (inertia of motion). They fall backward when a stopped bus starts suddenly (inertia of rest).

Tip: Pulling the hands back while catching a ball increases the time of impact. So the force on the hands is smaller. Cars use air bags and crumple zones for the same reason.

Gravitation

Every two masses attract each other. Newton's law is $F = \dfrac{Gm_1m_2}{r^2}$ , with $G = 6.67 \times 10^{-11}$ N m² kg⁻². Near the Earth this pull gives $g = 9.8$ m/s².

g is greatest at the poles and least at the equator.
g falls with height and with depth. It is zero at the Earth's centre.
Mass never changes. Weight is $mg$ and changes with place.
On the Moon, weight is about $\dfrac{1}{6}$ of the Earth value. A 60 kg person still has a mass of 60 kg.
Escape velocity from Earth is about 11.2 km/s. Orbital velocity near the surface is about 7.9 km/s.
A geostationary satellite orbits about 36,000 km above the equator with a period of 24 hours.
Astronauts in orbit feel weightless because they are in free fall with the spacecraft.

Watch: Mass is in kilogram and stays fixed. Weight is in newton and changes.

Work, energy and power

Work is $W = Fs\cos\theta$ . It is zero if there is no displacement or if the force is at right angles to the motion. A satellite in circular orbit is an example of zero work.

Kinetic energy is $\tfrac{1}{2}mv^2$ . Double the speed and KE becomes 4 times.
Potential energy is $mgh$ .
Power is work ÷ time. Its unit is the watt.

Energy only changes form. A falling stone turns PE into KE.

Device	Energy change
Electric motor	Electrical to mechanical
Dynamo or generator	Mechanical to electrical
Battery	Chemical to electrical
Solar cell	Light to electrical
Microphone	Sound to electrical
Loudspeaker	Electrical to sound

Fluids

Pressure is force ÷ area. A knife is sharp because a small area gives high pressure.

Principle	Idea	Use
Pascal's law	Pressure on an enclosed liquid spreads equally	Hydraulic lift, brakes, press
Archimedes' principle	Upthrust = weight of liquid displaced	Ships, hydrometer, lactometer, submarines
Bernoulli's principle	Faster flow means lower pressure	Aeroplane lift, roofs blown off, atomiser
Surface tension	The surface acts like a stretched skin	Round drops, insects on water
Capillarity	Liquid rises in thin tubes	Lamp wick, blotting paper, plant xylem
Viscosity	Internal friction of a fluid	Honey flows slowly

Ice floats because its density (about 0.92 g/cm³) is less than water's. A body floats if its density is less than the liquid's. A ship floats because its shape displaces a lot of water. Soap and kerosene lower surface tension. Kerosene on ponds kills mosquito larvae. Liquid viscosity falls when heated.

Levers

A lever has a fulcrum, a load and an effort. The class depends on which one sits in the middle.

Class	In the middle	Examples
Class 1	Fulcrum	Seesaw, scissors, pliers, crowbar, beam balance
Class 2	Load	Nutcracker, wheelbarrow, bottle opener, lemon squeezer
Class 3	Effort	Tongs, forceps, tweezers, fishing rod, human forearm, broom

Rule: Fulcrum, Load, Effort in the middle gives class 1, 2, 3. Class 2 always gives mechanical advantage. Class 3 gives speed and control.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common3 practice Q

Everyday event and Newton's law

How to spot it:

A daily-life story (bus braking, rocket, gun recoil, cricketer catching, carpet beating) with the laws as options.

Method

If a body resists a change in rest or motion, choose the first law.
If force is reduced by taking more time, or the question uses F = ma, choose the second law.
If you push one way and move the other way (rocket, recoil, swimming), choose the third law.

Why it works:

Each law explains one kind of effect, and the story tells you which.

Try this

When a carpet is beaten with a stick, the dust comes out. This is due to:

Show solution

The carpet moves suddenly.
The dust tends to stay at rest.
That is inertia of rest.

Answer

Inertia of rest (Newton's first law)

Type 2very common4 practice Q

Gravitation, weight and satellites

How to spot it:

The question asks about g at poles, equator or centre, weight on the Moon, escape velocity, geostationary orbit or weightlessness.

W = mg,\quad g_{moon}\approx \tfrac{1}{6}g_{earth}

Method

Mass is fixed. Weight changes with g.
g is larger at the poles than at the equator. It falls up and down, and is zero at the centre.
Recall the numbers: g 9.8 m/s², escape velocity 11.2 km/s, geostationary period 24 h at about 36,000 km.

Why it works:

Weight is mass times g, so only g changes from place to place.

Try this

A body has a mass of 30 kg on Earth. What is its mass on the Moon?

Show solution

Mass does not depend on place.
Only the weight falls to about one-sixth.

Answer

30 kg

Type 3very common3 practice Q

Fluid principle and its use

How to spot it:

A device or effect (hydraulic brake, floating ship, aeroplane lift, lamp wick, round raindrop) with principles as options.

Method

Enclosed liquid and multiplied force point to Pascal.
Floating, sinking or upthrust points to Archimedes.
Fast air and low pressure point to Bernoulli.
A skin-like surface points to surface tension. Rise in thin tubes points to capillarity.

Why it works:

Each principle has a small set of classic examples.

Try this

The roof of a hut is blown off in a storm. Which principle explains this?

Show solution

Fast wind moves over the roof.
Fast flow means low pressure above.
Higher pressure inside pushes the roof up.

Answer

Bernoulli's principle

Type 4common2 practice Q

Class of a lever

How to spot it:

A tool (scissors, nutcracker, forceps, wheelbarrow, human arm) is given and 'lever of class ...' is asked.

Method

Find the fulcrum, load and effort on the tool.
See which one is in the middle.
Fulcrum gives class 1, load gives class 2, effort gives class 3.

Why it works:

The position of the middle item fixes the class.

Try this

A bottle opener is a lever of which class?

Show solution

The fulcrum is the edge on the cap top.
Your hand gives the effort at the far end.
The cap (load) is between them.

Answer

Class 2

Type 5common2 practice Q

Work, energy and power basics

How to spot it:

The question says 'If velocity is doubled, KE becomes ...', 'work done when ...', or asks a small power sum.

KE=\tfrac12 mv^2,\quad PE=mgh,\quad P=\frac{W}{t}

Method

KE depends on $v^2$ . A speed multiplied by k gives KE multiplied by $k^2$ .
Work is F × s × cos θ. It is zero if s = 0 or the force is perpendicular to motion.
Power is work ÷ time. For lifting, work is mgh.

Why it works:

The square in KE makes energy grow faster than speed.

Try this

A body's velocity is made three times. Its kinetic energy becomes:

Show solution

KE $\propto v^2$ .
3² = 9.

Answer

9 times

Formula sheet

First equation of motion

v = u + at

v is final speed, u is initial speed, a is acceleration, t is time

Second equation of motion

s = ut + \tfrac{1}{2}at^{2}

s is the distance covered

Third equation of motion

v^{2} = u^{2} + 2as

no time needed

Newton's second law

F = ma = \frac{\Delta p}{\Delta t}

p = mv

Law of gravitation

F = \frac{G m_1 m_2}{r^{2}}

G = 6.67 × 10⁻¹¹ N m² kg⁻²

Kinetic and potential energy

KE = \tfrac{1}{2}mv^{2},\quad PE = mgh

m is mass, v is speed, h is height

Work and power

W = Fs\cos\theta,\quad P = \frac{W}{t}

1 W = 1 J/s

Pressure

P = \frac{F}{A},\quad P_{\text{liquid}} = h\rho g

unit pascal

Shortcuts that save time

⚡ Lever class from the middle item

Whatever sits in the middle decides the class. F is 1, L is 2, E is 3.

Example

A wheelbarrow is which class of lever?

Show solution

The wheel is the fulcrum, at one end.
The handles are the effort, at the other end.
The load sits in the middle.

Answer

Class 2

⚡ Squares in energy

KE depends on $v^2$ . Double the speed and KE is 4 times. Triple it and KE is 9 times. Momentum only doubles when speed doubles.

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Saying mass changes on the Moon.

Mass stays the same. Only weight becomes about one-sixth.

Mistake 02

Thinking g is greatest at the equator.

g is greatest at the poles and least at the equator.

Mistake 03

Explaining rocket motion with the first law.

It is the third law (action and reaction).

Mistake 04

Saying work is done by holding a heavy box still.

There is no displacement, so the work is zero.

Mistake 05

Placing tongs in class 2 because they hold a load.

The effort is in the middle, so they are class 3.

Quick revision

Read this the night before the exam.

Inertia measures mass. F = ma. Action equals reaction (rocket, recoil).
g is maximum at the poles, minimum at the equator and zero at the centre. Moon weight is about 1/6.
Escape velocity is 11.2 km/s. A geostationary satellite has a period of 24 h.
KE ∝ v². Work is zero if there is no displacement or the force is perpendicular to motion.
Pascal gives hydraulics. Archimedes gives floating. Bernoulli gives aeroplane lift.
Lever middle: fulcrum, load, effort gives class 1, 2, 3.

Practice: 18 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 3 min · wrong answers go to your mistake notebook automatically.

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