Year 7-8 Science: Forces and Fields

Comprehensive practice flashcards covering introductory physics concepts from Years 7-8 Science Topic 2: Forces, friction, gravity, mass and weight, magnetism, electromagnets, and electrostatics.

What is the scientific definition of a force?

A force is a push or a pull.

What four changes can forces cause in objects?

Forces can cause objects to begin moving, change speed (accelerate or slow down and stop), change shape, or change temperature.

What is the key distinction between a contact force and a field force?

A contact force acts only when an object is physically touched, whereas a field force can push or pull on objects across space without touching them.

What are the three examples of field forces identified in the unit?

Gravity, electrical force, and magnetic force.

What is the standard unit of force, and whom was it named after?

Force is measured in newtons ($N$), named after the English scientist Sir Isaac Newton.

What laboratory instrument is shown here, which measures pull forces in newtons?

A spring balance.

What is a major mechanical limitation of a standard spring balance when measuring forces?

It can only measure pulls, not pushes, and relies on stretching a spring mechanism.

How did Sir Isaac Newton's explanation of moving objects overturn the beliefs held before his time?

People previously thought it was "natural" for moving objects to slow down and stop without a cause. Newton showed that an object will keep moving indefinitely unless acted upon by an external force such as friction.

What is friction, and in what direction does it always push relative to motion?

Friction is a contact force that always pushes in the opposite direction to the way an object is moving.

Why does a spacecraft in outer space keep coasting along at a constant speed even with its engines turned off?

There is no air in outer space, which means there is no friction to slow the spacecraft down.

What force conditions allow a car on a road to travel forward at a constant speed?

The forward thrust force supplied by the engine to the tyres must be equal in strength to the backward friction force (including air resistance), so the forces cancel out.

What low-friction mechanical device is shown here, used to join a wheel to an axle?

A wheel bearing (or bearing).

Why are modern footballs covered in small surface dimples?

The dimples increase friction between the ball and a player's hands or boots, reducing handling errors in wet conditions.

Why does rubbing your hands together rapidly on a cold day make them feel warmer?

The rubbing creates friction between your hands, and frictional forces often produce an increase in temperature.

What is the universal law governing gravitational force?

Gravitational force attracts every object in the Universe to every other object in the Universe.

What is the physical difference between mass and weight?

Mass is the amount of matter in an object measured in kilograms ($kg$) and stays constant everywhere; weight is the force of gravity acting on that mass, measured in newtons ($N$), which varies depending on location.

How do the mass and weight of a $100\,kg$ astronaut differ between Earth's surface, the Moon, and an orbit in the Space Shuttle?

The mass is always $100\,kg$. The weight is $1,000\,N$ on Earth, $160\,N$ on the Moon, and zero $N$ (weightless) in orbit.

Why does a flat sheet of paper fall slower than a brick on Earth, and what happens when the paper is crumpled into a ball?

The flat sheet of paper is slowed down by air resistance. Crumpling it reduces air resistance, allowing it to fall at the same rate as the brick because all objects fall at the same rate due to gravity in the absence of air resistance.

How did Sir Isaac Newton explain how artificial satellites stay in orbit around the Earth?

By firing horizontally at high speed, the projectile curves downwards at the exact same rate that the Earth curves beneath it, causing it to fall continuously around the Earth without ever hitting the surface.

What is the approximate orbital speed required to orbit the Earth?

About $25,000\,km/hr$.

Why do orbiting astronauts experience weightlessness despite gravity still acting on them?

They are in free fall along with their spacecraft; because both fall at the identical rate, they exert no contact force against their surroundings or scales.

What specific class of metals do magnets attract?

Ferrous metals (metals containing iron, including many steels).

What is the fundamental rule governing interactions between magnetic poles?

Opposite poles attract ($N$ and $S$), and like poles repel ($N$ and $N$, or $S$ and $S$).

Why is the north-pointing end of a compass magnet properly termed the "north-seeking pole"?

Because it seeks out and points toward the Earth's magnetic north pole, meaning it is magnetically an opposite pole attracted to the north.

What two critical benefits does the Earth's magnetic field provide?

It enables directional navigation with compasses and shields the planet from dangerous solar radiation, creating the aurora.

How is an electromagnet constructed, and what is its primary practical advantage?

It is made by wrapping insulated wire (a solenoid) around a soft iron core and passing an electric current through it. Its primary advantage is that its magnetism can be switched on and off instantly.

Which three major technological devices rely directly on electromagnets?

Electric generators, electric motors, and audio speakers.

Which subatomic particles carry electric charges within an atom, and which of them can move to create static electricity?

Protons in the nucleus carry positive ($+ve$) charge, and electrons orbiting the nucleus carry negative ($-ve$) charge. Only negative electrons can move between atoms.

What static charge is developed on a perspex rod when rubbed with silk, and why?

It becomes positively charged because negative electrons are rubbed off the perspex onto the silk.

What occurs when a charged rod touches the uncharged polystyrene ball of an electroscope?

Electrons transfer so the ball acquires the same charge as the rod, causing the ball to be immediately repelled by the rod.

What physical process generates a flash of lightning and the resulting sound of thunder?

Lightning is a spark discharge heating a narrow channel of air until it glows; thunder is the acoustic shock wave produced by the rapid thermal expansion of that heated air channel.

How did Alessandro Volta disprove Luigi Galvani's theory of "animal electricity" in frog legs?

Galvani thought twitching was caused by a supernatural life force, but Volta proved it was caused by a chemical reaction between the metal wires and body fluids stimulating the surviving muscle tissue.

What is the ratio of weight ($N$) to mass ($kg$) represented by "$g$" on Earth's surface?

$g = 10\,N/kg$, meaning weight in newtons on Earth is roughly ten times the mass in kilograms ($\text{Weight} = \text{Mass} \times g$).