What is meant by a resultant force?

The single force that would have the same effect as all the forces acting on an object combined.

A car travels at a steady 25 m/s along a straight road. What can be said about the forces acting on it?

They must be balanced, so the resultant force on the car is zero.

What three changes to an object's motion can a resultant force produce?

It can make the object speed up, slow down, or change direction.

A spacecraft far from any star switches off its engines. What happens to its motion, and why?

It keeps travelling at a constant velocity in a straight line, because no resultant force acts on it.

Why does a book left alone on a table stay exactly where it is?

The downward pull of gravity is matched by the upward push of the table, so there is no resultant force to move it.

OCR GCSE Physics A J249

Electricity

Topic overview Charge, and producing static electricity Transfer of electrons, and electric fields Current as a rate of flow of charge Charge, current and time Series and parallel circuits, and circuit symbols Current, potential difference and resistance Resistors whose resistance changes, and investigating them Linear and non-linear circuit elements from graphs Net resistance in series and parallel Circuits for measurement, and electrical power

Energy

Topic overview Conservation of energy, and energy stores Energy changes on heating, and calculating them Energy of a moving body, a spring and a raised object Dissipation, and energy transfer in domestic devices Power ratings, efficiency, and increasing it Reducing unwanted energy transfer, and the rate of cooling

Forces

Topic overview Measuring distance and time, and calculating speed Converting units, and rates Vectors and scalars in motion Distance-time and velocity-time graphs Average speed, and the equations of uniform motion How objects interact, and forces as vectors Newton's first law Resolving forces, and free body diagrams Resultant forces, and balanced forces Newton's second law, and inertia Momentum, and force as a change in momentum Work done, and stored energy Power, and Newton's third law Circular motion and changing velocity Stretching, bending and compressing; elastic and plastic Force and extension, linear and non-linear The spring constant, and work done in stretching Gravitational fields, weight and free fall Calculations with weight and gravitational field strength Turning forces and the moment of a force Levers and gears Pressure in fluids, and the force on a surface

Global Challenges

Topic overview Typical speeds and everyday accelerations Reaction time, stopping distance and the dangers of large decelerations Estimating stopping distances and the forces in a collision Energy resources, and trends in their use The national grid, and step-up and step-down transformers Why the national grid is efficient, and the transformer equation Mains electricity: a.c., d.c. and the domestic supply Live, neutral and earth wires, and electrical danger Red-shift and the Big Bang The formation of the Sun, and radiation from bodies The solar system Circular orbits and stable orbits Radiation and the temperature of a body, and seismic waves and sonar

Magnetism and Magnetic Fields

Topic overview Permanent and induced magnets, and magnetic poles Magnetic fields, and the dipping compass The magnetic effect of a current, and solenoids The motor effect and Fleming's left-hand rule The force on a conductor, and how a motor turns Induced potential, alternators and dynamos Transformers and the turns ratio The microphone

Matter

Topic overview How the atomic model changed, and the structure of the atom The size of atoms, and defining density Density of the states of matter, and density calculations Conservation of mass in changes of state Heating a system, and specific heat capacity Calculations with specific heat capacity and latent heat Gas molecules, temperature and pressure Compressing a gas, and changing its volume Doing work on a gas Atmospheric pressure and why it varies with height Floating, sinking and pressure in a liquid

Radioactivity

Topic overview Protons, neutrons and isotopes Unstable nuclei and the emissions they give out Balancing nuclear equations Electron energy levels, and radiation from atoms Half-life and the randomness of decay Penetration of alpha, beta and gamma Contamination and irradiation, and half-life as a hazard Uses of nuclear radiation Nuclear fission and fusion

Waves in Matter

Topic overview Amplitude, wavelength, frequency and period The wave equation Transverse and longitudinal waves Sound crossing a boundary; reflection, transmission and absorption Converting sound to vibrations, and the range of hearing Modelling waves with ripples and sound Electromagnetic waves as transverse, and energy transfer Frequency and wavelength across the spectrum What our eyes detect, and light as an electromagnetic wave Uses of electromagnetic waves Hazards of ultraviolet, X-rays and gamma rays Exploring structures with waves, and producing radio waves Absorption, transmission, refraction and reflection Ray diagrams for reflection and refraction How colour arises

Forces

Newton's first law

2 things to know
Paper 3

Overview

What happens to an object with no resultant force on it, what a resultant force can change, and why spacecraft, books, cyclists and sliding loads behave as they do.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State Newton's first law, define a resultant force, and explain why a book stays on a table but a coasting cyclist slows down.

Key term

State the law that describes what happens to an object when no resultant force acts on it.

Questions from this subtopic

Each one with the model answer underneath.

1
State the law that describes what happens to an object when no resultant force acts on it.
Model answer
An object stays at rest, or keeps moving at a constant velocity, unless a resultant force acts on it.
2
What is meant by a resultant force?
Model answer
The single force that would have the same effect as all the forces acting on an object combined.
3
A car travels at a steady 25 m/s along a straight road. What can be said about the forces acting on it?
Model answer
They must be balanced, so the resultant force on the car is zero.
4
What three changes to an object's motion can a resultant force produce?
Model answer
It can make the object speed up, slow down, or change direction.
5
A spacecraft far from any star switches off its engines. What happens to its motion, and why?
Model answer
It keeps travelling at a constant velocity in a straight line, because no resultant force acts on it.
6
Why does a book left alone on a table stay exactly where it is?
Model answer
The downward pull of gravity is matched by the upward push of the table, so there is no resultant force to move it.

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