Force and Laws of Motion

The One Thing

Force changes an object’s motion, direction, speed, or shape, while inertia resists changes in motion. Newton’s three laws describe how forces, mass, acceleration, momentum, and interactions are related.

Reactions, Processes and Experiments

Key Terms

• Force: A push or pull that may change the motion, direction, speed, or shape of an object.
• Balanced forces: Forces whose resultant is zero; they do not change the state of rest or uniform motion.
• Unbalanced forces: Forces whose resultant is not zero; they produce acceleration or a change in motion.
• Net force: The vector sum of all forces acting on an object.
• Inertia: The tendency of an object to resist any change in its state of rest or motion.
• Inertia of rest: The tendency of a stationary object to remain at rest unless an external force acts on it.
• Inertia of motion: The tendency of a moving object to continue moving with the same speed and direction.
• Inertia of direction: The tendency of an object to continue moving in the same direction.
• Mass: The quantity of matter in an object and a measure of its inertia.
• Newton's First Law of Motion: An object remains at rest or moves with uniform speed in a straight line unless acted upon by an unbalanced external force.
• Newton's Second Law of Motion: The acceleration of an object is directly proportional to the net force and inversely proportional to its mass, in the direction of the force.
• Newton's Third Law of Motion: For every action, there is an equal and opposite reaction; the two forces act on different objects.
• Momentum: The quantity of motion possessed by an object, equal to the product of its mass and velocity.
• Impulse: The product of force and the time for which it acts; it equals the change in momentum.
• Conservation of momentum: In an isolated system, the total momentum remains constant when no external unbalanced force acts.
• Newton (N): The SI unit of force.
• Kilogram (kg): The SI unit of mass.
• Metre per second squared (m/s²): The SI unit of acceleration.
• Kilogram metre per second (kg m/s): The SI unit of momentum.

Easily Confused

• Balanced forces and unbalanced forces: Balanced forces have a zero resultant and do not change motion; unbalanced forces have a non-zero resultant and produce acceleration or another change in motion.
• Mass and momentum: Mass measures the quantity of matter and inertia, whereas momentum depends on both mass and velocity.
• Inertia of rest and inertia of motion: Inertia of rest concerns remaining stationary; inertia of motion concerns continuing to move.
• Inertia of motion and inertia of direction: Inertia of motion resists changes in speed, whereas inertia of direction resists changes in the direction of motion.
• Force and momentum: Force changes momentum, whereas momentum is the quantity of motion already possessed by an object.
• Newton’s First Law and Newton’s Second Law: The First Law describes motion when the net force is zero; the Second Law relates non-zero net force to acceleration through F = ma.
• Action–reaction forces and balanced forces: Action–reaction forces act on different objects and therefore do not cancel; balanced forces act on the same object and have a zero resultant.
• Impulse and force: Impulse is the product of force and time, J = FΔt, whereas force is the rate of change of momentum, F = Δp/Δt.

What Gets Asked

• Definitions and units: Questions may ask for the definitions of force, inertia, momentum, impulse, or net force, together with units such as N, kg, m/s², and kg m/s. Marks are lost by confusing mass with momentum or omitting the direction of vector quantities.
• Newton’s laws: Questions may require a statement or application of Newton’s First, Second, or Third Law. Common errors include stating that continuous force is needed to maintain uniform motion, or claiming that action–reaction forces cancel because they act on different objects.
• Numerical applications: Questions may require use of F = ma, F = Δp/Δt, p = mv, or J = FΔt = Δp. Marks are lost by using mass instead of momentum, omitting units, or failing to account for direction.
• Collisions and explosions: Questions may ask students to apply conservation of momentum before and after an interaction. The essential condition is that the system is isolated or external forces are negligible.
• Seat-belt applications: Questions may ask why seat belts reduce injury. The required explanation is that they increase the time over which momentum changes, thereby reducing the average force.
• Force comparisons: Questions may compare the acceleration of objects with different masses under the same force. The smaller mass has greater acceleration, while the larger mass has greater inertia.