Grade 9 Science: Chapter 6 - How Forces Affect Motion

Flashcards covering the fundamental concepts of force, measurement units, and Newton's three laws of motion as presented in the lecture notes.

Force

A physical quantity that can make an object move from rest, change the speed and direction of a moving object, or change its shape; it requires the specification of direction, magnitude, and unit.

newton (N)

The SI unit of force, written with a small 'n' and represented by the symbol N.

Weight

The gravitational force with which the Earth pulls an object towards itself.

Spring balance

A device used to measure the weight of an object or the magnitude of a force in general.

Millinewtons ($10^{-3}\,N$)

The order of the smallest forces humans can directly feel in everyday life, such as a light touch.

Yoctonewtons ($10^{-24}\,N$)

Extremely small forces that scientists can measure in specialized experiments as of 2026.

Balanced forces

Two forces acting on an object that are equal in magnitude but opposite in direction, resulting in no change in the object's state of motion.

Unbalanced forces

Forces applied to an object that are not balanced, causing a non-zero net force that sets the object in motion or changes its velocity.

Net force

The overall force acting on an object; it is the sum of magnitudes for forces in the same direction or the difference between magnitudes for forces in opposite directions.

Force of friction

A force arising between the contact surfaces of objects that acts in a direction opposite to the direction of motion.

Normal force

The force exerted by a surface on which an object is placed, acting in an upward direction perpendicular to that surface.

Inertia

A term used by Isaac Newton to describe the tendency of objects to resist change in their state of rest or uniform motion.

Newton’s first law of motion

An object at rest remains at rest and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.

Newton’s second law of motion

When a net force acts on an object, the object accelerates in the direction of the net force; the magnitude of acceleration is proportional to the net force and inversely proportional to the mass ($F = ma$).

One newton of force

The force that produces an acceleration of $1\,m/s^2$ on an object of mass $1\,kg$.

Acceleration due to the gravitational force ($g$)

The acceleration of an object falling toward Earth under the influence of gravity, valued at approximately $9.8\,m/s^2$.

Momentum

A physical quantity defined as the product of an object's mass and its velocity, acting in the same direction as the velocity.

Newton’s third law of motion

Whenever one object is exerting a force on a second object, the second object is simultaneously exerting an equal and opposite force on the first object.

Internal forces

Forces that act within a system, such as tension in a string connecting two boxes, which cancel out when analyzing the system as a whole.

External forces

Forces that act on a system from the outside, which are the only forces considered when determining the acceleration of a single combined system ($a = \frac{F}{m_1 + m_2}$).

Tension (T)

The pull force exerted by a string or rope that acts between connected objects in a system.