UPCAT Physics — Newton's Laws, Dynamics & MomentumRevision Notes

Formulas

Exam Tips

• Always identify whether an object is in equilibrium before applying Newton's First Law
• Remember that 'at rest' and 'constant velocity' are both equilibrium states
• Draw free body diagrams to visualize all forces acting on an object

Key Points

• An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted upon by an unbalanced force
• Inertia is the tendency of objects to resist changes in their state of motion
• More massive objects have greater inertia and are harder to accelerate or decelerate
• In the absence of friction and air resistance, a moving object would continue moving forever
• Examples include a book on a table staying put, or a jeepney continuing forward when brakes are suddenly applied

Definitions

Section Title

Newton's First Law of Motion (Law of Inertia)

Common Mistakes

• Thinking that moving objects always need a force to keep moving
• Confusing mass with weight when discussing inertia
• Forgetting that constant velocity motion also requires zero net force

Formulas

Exam Tips

• Always convert units to SI before calculations (kg, m, s)
• Draw free body diagrams to identify all forces
• Remember that acceleration and net force point in the same direction
• Check if your calculated force makes sense physically

Key Points

• The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass
• Force is a vector quantity with both magnitude and direction
• Acceleration occurs in the same direction as the net force
• SI unit for force is Newton (N), where 1 N = 1 kg⋅m/s²
• This law allows us to calculate unknown forces, masses, or accelerations in motion problems

Definitions

Section Title

Newton's Second Law of Motion (F = ma)

Common Mistakes

• Using weight instead of mass in F = ma calculations
• Forgetting to find the net force when multiple forces act on an object
• Not considering the direction of forces when adding them vectorially

Formulas

Exam Tips

• Always identify which objects the action and reaction forces act upon
• Remember that action-reaction pairs never cancel each other out because they act on different objects
• Use Newton's Third Law to find unknown forces in collision problems

Key Points

• For every action, there is an equal and opposite reaction
• Forces always occur in pairs - you cannot have a single isolated force
• Action and reaction forces act on different objects, never on the same object
• The forces are equal in magnitude but opposite in direction
• Examples include walking (foot pushes ground backward, ground pushes foot forward), rocket propulsion

Definitions

Section Title

Newton's Third Law of Motion (Action-Reaction)

Common Mistakes

• Thinking action-reaction pairs act on the same object
• Believing that larger objects exert larger forces on smaller objects
• Confusing action-reaction pairs with equilibrium force pairs

Formulas

Exam Tips

• Distinguish between static and kinetic friction in problems
• Normal force equals weight only on horizontal surfaces
• Tension forces are particularly important in pulley problems

Key Points

• Gravitational force (weight) always acts downward with magnitude mg
• Normal force acts perpendicular to surfaces and prevents objects from passing through
• Friction force opposes motion and acts parallel to surfaces
• Tension force is transmitted through strings, ropes, and cables
• Applied forces are external pushes or pulls exerted by people or machines
• Air resistance opposes motion through air and increases with speed

Definitions

Section Title

Types of Forces in Dynamics

Common Mistakes

• Using kinetic friction coefficient for stationary objects
• Assuming friction always equals μN rather than being limited by it
• Forgetting that tension is the same throughout a massless rope

Formulas

Exam Tips

• Always define positive direction when working with momentum problems
• Use conservation of momentum for collision problems
• Remember that impulse can reduce injury by extending collision time

Key Points

• Momentum is the product of mass and velocity (p = mv)
• Momentum is a vector quantity with the same direction as velocity
• Impulse is the change in momentum (J = Δp = FΔt)
• Large forces acting for short times can produce the same impulse as small forces acting for long times
• Momentum is conserved in isolated systems with no external forces

Definitions

Section Title

Momentum and Impulse

Common Mistakes

• Forgetting that momentum is a vector - direction matters
• Confusing impulse with force or momentum
• Not considering all objects in the system when applying conservation of momentum

Formulas

Exam Tips

• Always start collision problems by writing conservation of momentum equation
• Check whether kinetic energy is conserved to identify collision type
• Use subscripts consistently to track initial and final states

Key Points

• Elastic collisions conserve both momentum and kinetic energy
• Inelastic collisions conserve momentum but not kinetic energy
• Perfectly inelastic collisions occur when objects stick together after impact
• In all collisions, momentum is conserved if no external forces act
• Real-world collisions are usually somewhere between elastic and perfectly inelastic

Definitions

Section Title

Collision Analysis

Common Mistakes

• Assuming all collisions conserve kinetic energy
• Forgetting to apply conservation of momentum in collision problems
• Not defining coordinate system and sign conventions clearly