Grade 09 Physics: How Forces Affect Motion

Vocabulary flashcards covering core concepts, laws, and experimental scenarios from the Grade 09 Physics question bank on 'How Forces Affect Motion'.

Balanced Forces

Forces acting simultaneously on an object whose resultant net force is zero ($F_{\text{net}} = 0$). They cannot set a stationary body into motion or change the speed or state of motion of a body, but they can change its shape and size.

Unbalanced Forces

Forces acting simultaneously on a body that produce an acceleration, thereby changing its state of rest or of uniform motion in a straight line, with a net force not equal to zero ($F_{\text{net}} \neq 0$).

Net Force in Opposing and Co-directional Systems

In this force diagram, equal opposing vertical forces ($25\,\text{N}$ upward and $25\,\text{N}$ downward) cancel out, while horizontal forces acting in the same direction ($15\,\text{N}$ and $5\,\text{N}$) add together to give a non-zero net force ($F_{\text{net}} \neq 0$), classifying them as unbalanced forces.

Inertia

The natural tendency of an object to resist any change in its state of motion, causing undisturbed objects to stay at rest or to keep moving with the same velocity.

Mass (Measure of Inertia)

The quantitative measure of a body's inertia; bodies with greater mass possess greater inertia and require a proportionally larger force to alter their state of motion.

Coin–Card–Glass Tumbler Demonstration

An experimental demonstration of the inertia of rest; when the stiff card covering an empty tumbler is flicked rapidly, the card moves away while the coin falls straight into the glass because of its inertia to stay at rest.

Newton's First Law of Motion

The law stating that an object will continue in its state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force; also known as the law of inertia.

Momentum

A quantitative measure of the motion of an object defined as the product of its mass and velocity ($p = m \times v$); at any given instant, it depends on mass and velocity but is independent of acceleration.

Newton's Second Law of Motion

The physical law providing the measurement of force, stating that the rate of change of momentum of a body is directly proportional to the applied unbalanced force ($F = m \times a$).

Acceleration–Force Graph

A linear graph illustrating Newton's second law, demonstrating that the acceleration of an object increases proportionally with the applied net force ($a \propto F$) and decreases for an object of larger mass under the same force.

Rate of Change of Momentum and Impact Force

The principle that the impact force experienced by an object depends inversely on the time duration over which its momentum changes; extending the duration (e.g., pulling hands back while catching a ball or landing on sand/cushioning) decreases the force exerted.

Internal Forces in a Connected System

Forces that act between constituent parts within a defined multi-body system (such as the tension $T$ between Box 1 and Box 2); when analyzing the whole system together, these forces act in equal and opposite directions and do not affect the total system acceleration.

Newton's Third Law of Motion

The law stating that every action has an equal and opposite reaction; these forces always act simultaneously on two different interacting objects and therefore never cancel each other out.

Action-Reaction Tension Demonstration

An experimental setup demonstrating that whether a person pulls on a rope attached to a stationary wall or an elephant, the scale measures an identical pulling force ($600\,\text{N}$ in both cases) because the pulling force exerted equals the reaction force exerted back on the scale.

Lawn Water Sprinkler Rotation

The rotational motion of a lawn sprinkler caused by Newton's third law of motion; as water is pushed out of the nozzles in one direction, an equal and opposite reaction force pushes backward on the arms, causing them to rotate.

Rocket Propulsion and Orbital Motion

A mechanism where a rocket lifts off by expelling exhaust gases downward to create an upward reaction force (Newton's third law); once established in circular orbit, inertia keeps the satellite moving forward while gravitational force acts as the centripetal force to curve its path.

Weightlessness in an Astronaut

A physical condition experienced by an astronaut orbiting Earth inside a satellite, arising because both the astronaut and the satellite are in a continuous state of free fall toward Earth.

Recoil of a Cannon

The backward motion of a gun or cannon following firing, explained by Newton's third law of motion: the forward force propelling the bullet or projectile produces an equal and opposite reaction force sending the gun backward.

Warehouse Conveyor Dilemma (Single System Acceleration)

A scenario illustrating that two connected platforms of masses $m_1 = 60\,\text{kg}$ and $m_2 = 40\,\text{kg}$ pulled by an external force of $F = 200\,\text{N}$ accelerate together as a single combined mass ($m_{\text{total}} = 100\,\text{kg}$) with uniform acceleration ($a = 2\,\text{m/s}^2$), since internal cable tension does not alter their common acceleration.