Course

AS Physics: Forces and Energy (9702)

Name: AS Physics: Forces and Energy (9702)
Price: 1799 BDT

9702 topics 3, 4 and 5. Newton’s laws, free-body diagrams and moments, then the two conserved quantities that answer a question without tracking every force.

37 lessons3h 44mEnglish

What you'll learn

Draw a free-body diagram that is complete, which is most of the marks on a forces question
Say why mass and weight are different quantities, and use each in the right place
Apply conservation of momentum to a collision, and tell elastic from inelastic by the energy
Take moments about a sensible point, and choose that point to make the algebra vanish
Decide when energy is the quicker route than force, and justify the choice

Requirements

AS topic 1: resolving a vector into components, and adding vectors
AS topic 2: the equations of motion, and reading a gradient off a graph
Able to rearrange an equation with several symbols in it
Basic trigonometry, including the component of a weight along a slope

Course content

37 lessons · 3h 44m

3.1-3.3 · Dynamics

What a force actually does, why mass is not weight, and why momentum is conserved in a collision when kinetic energy is not. The three laws of Newton, then the momentum rules used in the rest of mechanics.

0/11

What keeps a moving object moving?

A force is not what keeps an object moving. It is what changes the motion.

Preview6m 2.

Force sets acceleration, not speed

Tilt a ramp and watch which quantity actually follows the force.

Preview7m

Why mass is not weight

Add one kilogram to each side of a pulley and the motion still changes.

Why you feel heavy in a lift

A scale reads the push of the floor, and the direction of travel is not the question.

Every force is one half of a pair

If the cart pulls back just as hard, how does the horse ever move?

Momentum: how hard something is to stop

A heavy slow cart and a light fast one can be equally hard to stop.

The total momentum does not change

Run any collision on the track and one number comes out the same.

Which collisions keep their kinetic energy?

Momentum is conserved in every collision. Kinetic energy separates the two types.

Force is the rate of change of momentum

The egg arrives with the same momentum however you catch it.

10.

Impulse is the area under a force-time graph

Real forces are never constant, so read the area instead of the height.

11.

Exam question: the ball that bounces back

Seven marks, three parts, no options: one rebound, and the sign that decides the lot.

4.1-4.3 · Forces, density and pressure

Make a force turn something, then balance the turns. After that the fluid itself becomes the physics: density, pressure with depth and upthrust explain both floating and terminal velocity.

0/11

The turning effect of a force

Same push, two places on the door, and only one of them opens it easily.

Preview6m 2.

A couple turns without pushing

Two forces that cancel completely and still spin the thing they act on.

Preview5m

The principle of moments

Set the unknown weight so the clockwise and anticlockwise turns match.

Where an object keeps its weight

One point where the whole weight acts, and the reason a bus conductor asks you to sit.

Three forces that close a triangle

If nothing moves, the arrows drawn head to tail have to come back to the start.

Same mass, different size

The one number that separates lead from feathers, and the unit trap that follows it.

Pressure grows with depth

A wide roof tank and a narrow pipe, filled to the same depth. Same pressure at the bottom.

Why anything floats

A steel bolt sinks, a steel launch does not, and the water pushed aside explains both.

Why a falling body stops speeding up

Weight does not change on the way down, so something else must have grown to match it.

10.

Beyond the syllabus: where the pipe narrows

For BUET and HSC candidates: continuity and Bernoulli in a Venturi tube, where the narrow part has the lowest pressure.

11m

11.

One exam question, end to end

Nine marks, three parts, no options: weight, submerged volume, then what upthrust always equals.

5.1-5.2 · Work, energy and power

Energy is counted, and the count always balances. Work as a component and as an area, where ½mv² and mgh come from, what friction does with the missing joules, and power as the rate of doing work.

0/12

Only part of a force does work

Pull a case with a rope at an angle, and only part of that force moves it.

Preview6m 2.

Work is the area under the graph

A spring pulls back harder the further you stretch it, so W = Fs no longer applies.

Preview7m

Where ½mv² comes from

Same brakes, twice the speed, and the braking distance is four times longer.

Why mgh is only a near-Earth rule

mgh holds while g is constant. Find the height where it stops being safe to use.

Lifting something that is not a point

Stack six bricks and each one climbs a different height, so mgh has no h. One rule covers bricks, a hanging chain and a well being pumped dry.

The total energy never moves

Release a skater and watch one bar fall exactly as fast as the other rises.

Friction spreads energy, it does not destroy it

Every pass is lower than the last, and the missing joules are still in the total.

An energy budget: how many planks?

Each plank charges the same joules. Double the speed and the bullet gets through four times as many, not twice as many.

Efficiency is a ratio, not an amount

Guess how much of the energy a filament bulb sends out as light, then watch the split.

10.

Power is work per second

Two cranes, the same beam, different times. Only one of them needs a bigger motor.

11.

Power when the force is moving

A car at steady speed gains no kinetic energy, so find where the 18 kW is going.

12.

One exam question, end to end

Eight marks, four parts, no options: power in, kinetic energy out, then the missing 105 kJ.

About the creator

8bitlesson

Cambridge O Level, A Level and university courses, taught through interactive labs.

Mentora builds the sciences and mathematics as things you can move rather than videos you watch. Cambridge O Level and A Level mathematics, physics, chemistry and accounting, a primary mathematics track, and first-year university computer science. Every lesson works a real exam question end to end.