UPCAT Physics — Newton's Laws, Dynamics & MomentumFlash Cards

State Newton's First Law of Motion and explain what it means by inertia.

Newton's First Law: 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 an object to resist changes in its state of motion. The greater the mass, the greater the inertia. Example: A book on a table remains stationary until pushed; a moving car continues forward when brakes are applied due to inertia.

A 5 kg box is pushed with an acceleration of 3 m/s². Calculate the net force applied.

Formula: F = ma Step 1: Identify given values m = 5 kg a = 3 m/s² Step 2: Apply Newton's Second Law F = ma = 5 kg × 3 m/s² = 15 N Answer: The net force is 15 N

Why does a heavier object require more force to achieve the same acceleration as a lighter object?

According to Newton's Second Law (F = ma), force is directly proportional to both mass and acceleration. For the same acceleration, a heavier object (greater mass) requires proportionally more force. This is why it's harder to push a loaded cart than an empty one at the same acceleration. The relationship shows that inertia (resistance to acceleration) increases with mass.

A 20 N force is applied to a 4 kg object. What is the acceleration?

Formula: a = F/m Step 1: Identify given values F = 20 N m = 4 kg Step 2: Calculate acceleration a = F/m = 20 N ÷ 4 kg = 5 m/s² Answer: The acceleration is 5 m/s²

State Newton's Third Law and provide a real-world example.

Newton's Third Law: For every action, there is an equal and opposite reaction. Forces always occur in pairs - when object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. Example: When you walk, your foot pushes backward on the ground (action), and the ground pushes forward on your foot (reaction), propelling you forward.

A 1200 kg car travels at 25 m/s. Calculate its momentum.

Formula: p = mv Step 1: Identify given values m = 1200 kg v = 25 m/s Step 2: Calculate momentum p = mv = 1200 kg × 25 m/s = 30,000 kg⋅m/s Answer: The momentum is 30,000 kg⋅m/s

Define momentum and explain why it's important in collisions.

Momentum (p) = mass × velocity (p = mv). It's a vector quantity measured in kg⋅m/s. Momentum is important in collisions because it's conserved - the total momentum before collision equals the total momentum after collision (in absence of external forces). This principle helps predict outcomes of crashes and explains why heavy, fast-moving objects are harder to stop.

Two balls collide elastically. Ball A (2 kg) moves at 6 m/s, Ball B (3 kg) is at rest. Find Ball A's velocity after collision.

For elastic collision: m₁v₁ + m₂v₂ = m₁v₁' + m₂v₂' Step 1: Apply conservation of momentum (2)(6) + (3)(0) = (2)v₁' + (3)v₂' 12 = 2v₁' + 3v₂' ... (1) Step 2: For elastic collision, kinetic energy is conserved ½(2)(6)² = ½(2)(v₁')² + ½(3)(v₂')² 36 = v₁'² + 1.5v₂'² ... (2) Solving equations: v₁' = 0.6 m/s, v₂' = 3.6 m/s

A 1000 kg car moving at 20 m/s collides with a 1500 kg stationary truck. They stick together. Find their common velocity.

Formula for inelastic collision: m₁v₁ + m₂v₂ = (m₁ + m₂)v' Step 1: Identify given values m₁ = 1000 kg, v₁ = 20 m/s m₂ = 1500 kg, v₂ = 0 m/s Step 2: Apply conservation of momentum (1000)(20) + (1500)(0) = (1000 + 1500)v' 20,000 = 2500v' v' = 8 m/s Answer: Their common velocity is 8 m/s

Define work in physics and state its formula with units.

Work is the energy transferred when a force causes displacement of an object. Formula: W = F × d × cos(θ), where F is force, d is displacement, and θ is the angle between force and displacement. Unit: Joule (J) = N⋅m. Work is done only when there's displacement in the direction of the force. No work is done if force is perpendicular to displacement.

A force of 50 N pushes a box 4 meters horizontally. Calculate the work done.

Formula: W = F × d (when force and displacement are in same direction) Step 1: Identify given values F = 50 N d = 4 m θ = 0° (force and displacement in same direction) Step 2: Calculate work W = F × d = 50 N × 4 m = 200 J Answer: Work done is 200 J

Define power and explain its relationship to work and time.

Power is the rate of doing work or transferring energy. Formula: P = W/t = F⋅v (where v is velocity). Unit: Watt (W) = J/s. Power tells us how quickly energy is converted or work is done. A more powerful engine can do the same work in less time or more work in the same time. Example: A 100W bulb uses 100J of energy per second.

A 0.5 kg ball moves at 10 m/s. Calculate its kinetic energy.

Formula: KE = ½mv² Step 1: Identify given values m = 0.5 kg v = 10 m/s Step 2: Calculate kinetic energy KE = ½ × 0.5 kg × (10 m/s)² KE = 0.25 × 100 = 25 J Answer: The kinetic energy is 25 J

A 2 kg book is placed on a shelf 3 meters high. Calculate its gravitational potential energy.

Formula: PE = mgh Step 1: Identify given values m = 2 kg g = 9.8 m/s² (gravitational acceleration) h = 3 m Step 2: Calculate potential energy PE = mgh = 2 kg × 9.8 m/s² × 3 m = 58.8 J Answer: The gravitational potential energy is 58.8 J

A 10 kg box slides on a surface with coefficient of friction μ = 0.3. Calculate the friction force.

Formula: f = μN = μmg (for horizontal surface) Step 1: Identify given values m = 10 kg μ = 0.3 g = 9.8 m/s² Step 2: Calculate normal force N = mg = 10 kg × 9.8 m/s² = 98 N Step 3: Calculate friction force f = μN = 0.3 × 98 N = 29.4 N Answer: The friction force is 29.4 N

State the impulse-momentum theorem and explain its significance.

Impulse-Momentum Theorem: Impulse = Change in momentum (J = Δp = FΔt). Impulse is the product of average force and time interval. This theorem explains why airbags and crumple zones work - they increase collision time, reducing the average force experienced. A longer collision time means smaller force for the same momentum change, reducing injury.

A 0.2 kg ball moving at 15 m/s is caught by a player in 0.5 seconds. Calculate the impulse.

Formula: J = Δp = m(v_f - v_i) Step 1: Identify given values m = 0.2 kg v_i = 15 m/s (initial velocity) v_f = 0 m/s (final velocity, ball stopped) Step 2: Calculate impulse J = m(v_f - v_i) = 0.2 kg × (0 - 15) m/s J = 0.2 × (-15) = -3 N⋅s Answer: The impulse is -3 N⋅s (negative indicates opposite direction)

Explain the difference between weight and mass, and how weight changes with location.

Mass is the amount of matter in an object (scalar, measured in kg), while weight is the gravitational force acting on that mass (vector, measured in N). Formula: W = mg. Mass remains constant everywhere, but weight varies with gravitational field strength. Example: A person with 70 kg mass weighs 686 N on Earth (g = 9.8 m/s²) but only 114 N on the Moon (g = 1.62 m/s²).

What is normal force and when does it act on objects?

Normal force is the contact force exerted by a surface perpendicular to the surface when an object is in contact with it. It prevents objects from passing through surfaces. On a horizontal surface, normal force equals the object's weight (N = mg). On inclined planes, N = mg cos(θ). Example: When you stand on the floor, the floor pushes up on you with a normal force equal to your weight.

State the law of conservation of momentum and explain when it applies.

Law of Conservation of Momentum: The total momentum of a system remains constant when no external forces act on it. Mathematical form: Σp_initial = Σp_final. This applies in collisions, explosions, and recoil situations when external forces are negligible. Example: In space, when an astronaut throws a tool, both astronaut and tool move in opposite directions with momenta that sum to zero.