Newton's Laws of Motion Quiz

Newton's Laws of Motion

• Newton's First Law of Motion - An object at rest will remain at rest, and an object in motion will continue in motion at a constant velocity unless acted upon by an external force.
• Inertia - The tendency of an object to resist changes in its state of motion. Inertia increases as the mass of an object increases.
• Newton's Second Law of Motion - Force equals mass times acceleration (F = m * a).
  • Net Force is the sum of forces acting on an object.
  • 1 Newton (N) is equal to 1 kg * m/s².
  • 1 pound (lb) is equal to 4.45 Newtons.
  • Weight is a type of force that is the force of gravity acting on an object. It is calculated as: W = m * g.
• Newton's Third Law of Motion - For every action, there is an equal but opposite reaction.
  • If you push an object with a force of 100 N, the object pushes back on you with a force of 100 N.
  • The forces are equal in magnitude but opposite in direction.
  • The object with less mass will experience a greater acceleration due to the same force.
• Normal Force - A contact force exerted by a surface on an object in contact.
  • Always perpendicular to the surface.
  • Equal in magnitude and opposite in direction to the weight of the object if the object isn't accelerating vertically.
  • Can be affected by applying forces to the object.
• Tension Force - The force transmitted through a rope, cable, or similar when taut.
  • Calculated using: T = m * (a + g).
  • Needed to lift an object and must be greater than its weight.
  • Needed to lower an object and must be less than its weight.
  • Equal to the object's weight if it's stationary or moving vertically at a constant velocity.

Practical Examples of Newton's Laws of Motion

• Rocket Propulsion: A rocket expels gas downward; the gas pushes back on the rocket upward, propelling it forward (Newton's Third Law).
• Boat in the Ocean: If you throw a package to the right from a boat, the recoil force from the throw will push you and the boat to the left.
• Earth and Moon: The Earth and Moon exert equal and opposite gravitational forces on each other; the Earth's greater mass results in smaller acceleration compared to the Moon's.
• Free Body Diagram: A diagram that displays all forces acting on an object.

Tension Force

• When an object is at rest or moving at a constant speed, acceleration is zero and tension remains constant.
• When accelerating upward, tension exceeds weight (more work required).
• When accelerating downward, tension is less than weight (lifting not required).
• Tension equals weight minus the force needed to accelerate downward.

Newton's Third Law

• When an object exerts a force on another, the second object exerts an equal and opposite force on the first.
• This is Newton's Third Law.
• These forces are action-reaction pairs.
• Action-reaction pairs act on different objects (e.g., weight and normal force act on the same object).

Finding Net Force

• Parallel forces in the same direction: add.
• Parallel forces in opposite directions: subtract.
• Perpendicular forces: use Pythagorean Theorem.
• Net force's magnitude: square root of the sum of squares of individual force magnitudes.
• Net force's direction: use arctangent function.

Finding Resultant Force

• Resultant force: vector sum of multiple forces.
• Magnitude: Pythagorean Theorem.
• Direction: using arctangent.
• Angle measured from positive x-axis.

Tension in Ropes

• Tension in a rope connecting two objects equals the pulling force on both.
• Tension is uniform throughout the rope.
• Find net acceleration to calculate tension.
• Tension equals mass times net acceleration.

Forces on a Suspended Block

• Tension forces in ropes supporting a suspended block must balance its weight.
• Tension forces have horizontal and vertical components.
• Horizontal components are equal and opposite (no horizontal motion).
• Vertical components equal the block's weight.

Forces and Equilibrium

• Sum of y-direction forces = T1y + T2y – mg = 0 (mg acts downwards)
• Net force is zero for non-accelerating objects.
• Rearranging: T1y + T2y = mg
• Solve for T1 and T2, needing two equations (two variables).

Finding T2 in terms of T1

• T2x = T2 cos(θ) (θ = 30°)
• T1x = T1 cos(60°)
• T2 = 0.5774T1 (from the ratio of the x-components)

Using the Second Equation

• Substitute T1y and T2y into T1y + T2y = mg.
• T1y = T1 sin(60°), T2y = T2 sin(30°)
• Substitute values:
  • sin(60°) = 0.866
  • sin(30°) = 0.5
  • mg = known value.
• Solve for T1, then use the T2 equation to find T2

Forces on Connected Blocks

• Net force on a system of connected blocks equals total mass multiplied by acceleration.
• Net force on individual blocks varies.
• Action-reaction forces apply between connected blocks.
• Example scenario: Masses and forces on interconnected blocks are determined by analyzing forces on each block individually.

Forces on a Block on Top of Another

• Total mass of two blocks means a weight of 1500 Newtons (also normal force).
• Weight of block A is 500 Newtons. (equal to the normal force).
• Force B exerts on A is 500 N. Force A exerts on B is 500 N.

Friction

• Friction opposes motion.
• Kinetic friction: surfaces sliding; calculated as µ_k * normal force.
• Static friction: surfaces not sliding; less than or equal to µ_s * normal force.
• Normal force on a flat horizontal surface equals mg.
• Static friction increases until maximum value, then motion begins.
• Kinetic friction is constant when in motion.

Example Problems Involving Friction

• Cases with applied force, static friction, kinetic friction and results for acceleration have been illustrated.

Inclines

• Gravitational force on an incline has parallel and perpendicular components.
• Normal force = mg cos(θ).
• Fg (parallel) = mg sin(θ).
• Acceleration on a frictionless incline = g sin(θ).

Acceleration Down an Incline

• Acceleration down a frictionless incline: a = g sin θ (e.g., 5 m/s² with θ = 30°).
• Initial velocity for a stationary object = 0 m/s.
• Use v_f² = v_i² + 2ad to find final velocity (given initial velocity and distance).

Friction on a Horizontal Surface

• Net force on sliding object = kinetic frictional force.
• Kinetic frictional force = µ_k * N (where N = mg).
• Results in deceleration.
• v_f^2 = v_i^2 + 2ad used to find stopping distance (with a being negative as it is deceleration).

Static Friction on an Incline

• Determine if an object will slide with static friction.
• Static frictional force (F_s) ≤ µ_s * N ≤ µ_s * mg * cos θ.
• Object slides if gravity force down the incline (mg sin θ) exceeds the static frictional force.
• Critical angle of sliding: θ = tan⁻¹(µ_s).

Pulley Systems

• Net force in pulley systems = total mass * acceleration.
• Acceleration in vertical pulley (heavier object): a = (M2g - M1g) / (M1 + M2).
• Calculate the tension by finding the net force acting on each block.
• Calculate difference between the weight and the net force.

Pulley Systems with Friction

• Net force in pulley systems with friction = calculated by subtracting frictional force from weight force.
• Example pulley system equation with friction: a = (M1 * g - µ_kM2g) / (M1 + M2)
• Tension force altered by the frictional forces.
• The tension force required to move the lighter block is the net force acting on it, in addition to the frictional force.

Understanding Forces and Direction

• • Net force: upward acceleration.
• • Net force: downward acceleration.
• Net force: sum of all forces.

Note & Important Considerations

• Notes are a condensed summary for conceptual understanding.
• Practice using free body diagrams to better illustrate forces.

Test your knowledge of Newton's three laws of motion, including concepts of inertia, force, and the relationship between mass and acceleration. This quiz will cover fundamental principles that govern the motion of objects in physics.