Physics Newton's Laws Introduction

Introduction

• The lecture covers force, motion, and Newton's laws of motion.
• Topics include force, types of forces, and inertia.

Force

• A force is a push or pull on an object.
• The SI unit of force is the Newton (N).
• Force is a vector quantity, having both magnitude and direction.
• The dyne is another unit, but rarely used.

Effects of Force

• Forces change an object's state of motion (e.g., start, stop, change direction, speed).
• Forces can also change an object's shape or size.

Types of Forces

• Balanced Forces: Equal and opposite forces, resulting in no net movement (rest or constant speed).
• Unbalanced Forces: Unequal or non-opposite forces resulting in a net force and acceleration.

Understanding Balanced Forces

• Balanced forces allow for constant-speed motion, not just rest.
• For example, a car at a constant speed has balanced forces (engine vs. friction).

Calculating Net Force

• Net force is the overall force on an object.
• Calculate by adding forces in the same direction and subtracting those in opposite directions.

Galileo's Contribution to Motion

• Aristotle thought objects naturally rested.
• Galileo's experiments showed objects continue motion unless acted upon, implying resistance to change in motion.

Inertia

• Inertia is the tendency of an object to resist changes in motion.
• An object at rest stays at rest, and an object in motion stays in motion with the same speed and direction, unless acted upon by external forces.
• Inertia is a fundamental property of all matter.
• Two types of inertia:
  • Inertia of rest: Objects at rest resist motion.
  • Inertia of motion: Objects in motion resist changes in speed and direction.

Examples of Inertia

• Coin on a card trick: the coin remains still due to inertia of rest, while the card moves.
• Car braking: Passengers lurch forward because their bodies continue moving at the car's previous speed.
• Moving from a stationary position on a bus, you experience inertia of rest, pushing you backward.

Inertia and Its Examples (continued)

• Objects in motion tend to stay in motion with the same velocity.
• Examples include the behaviour of objects during sudden braking or starting of a vehicle.
• Also includes throwing water out of a glass when spinning quickly.

Momentum

• Momentum is the measure of an object's motion. It is the product of mass and velocity.
• Equation: momentum (p) = mass (m) × velocity (v).
• High momentum objects (high mass and/or velocity) exert significant forces on impact.
• Momentum relates to the ability of an object to cause a change in motion in a collision.

Impulse

• Impulse is the product of force and time.
• Equation: impulse = force × time.

Newton's First Law of Motion

• Objects at rest stay at rest and objects in motion stay in motion at a constant velocity unless acted upon by a net force. Explains inertia.

Newton's Second Law of Motion

• Acceleration of an object is directly proportional to net force and inversely proportional to its mass.
• Equation: force (F) = mass (m) × acceleration (a).
• Change in momentum per unit time equals force.
• Force and acceleration are directly proportional, mass and force are directly proportional, time and force are inversely proportional.

Impact of Force and Momentum

• Force depends on time it takes to stop an object's motion.
• Softer surfaces reduce force by increasing impact time.
• This is why landing on cushions/sand reduces injury in a fall.

Momentum Equation and Units

• The equation for Force is: Force (F) = Change in Momentum (Δp) / time (t)
• Force calculated by dividing change in momentum by the time it takes.
• The standard unit for force is the Newton (N). One Newton = acceleration of 1 meter per second squared on a 1-kilogram mass.

Understanding Force, Momentum, and Time

• Applying a force changes an object's momentum.
• Momentum is a measure of how much an object is moving.
• Change of momentum over time equals force.

Examples of Force and Momentum

• Bullets have high impact due to high velocity and momentum.
• Catching a ball: extending the catching time (therefore extending the time for momentum to change) reduces the force on the hand.
• Karate chop: quick motion maximizes the force needed to break an object as little time is needed for momentum to change.

Applying Force and Momentum Principles

• High jumpers prefer soft landings to reduce force.
• Longer impact times result in lower forces.
• Longer force application times can potentially lead to stronger forces in activities such as long jump athletic performance.

Analyzing Velocity-Time Graphs

• Maximum force occurs at greatest change in velocity over time (highest acceleration).
• Velocity-time graphs illustrate forces: constant velocity = no force, changing velocity = force.

Further Applications

• Principles are crucial for understanding car accidents, movement in space, and machine operation.

Forces and Momentum (continued)

• Momentum = mass × velocity
• Force = mass × acceleration
• Stationary objects can experience forces.
• Momentum is zero for stationary objects.
• Force and acceleration have the same direction as momentum and velocity.
• Force is inversely proportional to time for the same change in momentum.
• Momentum is directly proportional to time for the same change in force.

Newton's Third Law of Motion

• Every action has an equal and opposite reaction.
• Forces always occur in pairs; when one body exerts a force on another, the second body simultaneously exerts an equal but opposite force on the first.

Examples of Newton's Third Law of Motion (continued)

• Collision, walking, swimming, firing a gun, rowing a boat, and rocket propulsion are examples of action-reaction pairs.

Key Concepts

• Friction: Opposes motion due to surface roughness, influences movement.
• Inertia: Resistance to change in motion, impacted by mass.
• Velocity: Speed and direction of motion.
• Acceleration: Change in velocity over time.

Understanding Key Concepts (continued)

• Why does a football stop moving?: Friction causes a force in the opposite direction, eventually stopping the motion.
• Why does only the bottom coin move when striking a stack?: Only the coin in contact with the striker experiences the necessary force to overcome its inertia.
• High-velocity bullets are dangerous: Higher velocities mean greater momentum and impact.
• Why is walking on ice difficult?: Ice's smoothness means low friction, preventing sufficient reaction force for movement.

Solving Problems

• Calculating Bullet Force: Use force (F)= mass (m) × acceleration (a) and the equation v^2 - u^2 = 2as for acceleration.
• Mass and Penetration Relationship: A higher velocity bullet penetrates deeper than lower velocity, or higher-mass bullets penetrate more.

Additional Information

• Force: Newtons (N)
• Distance: Meters (m)
• Speed: Meters per second (m/s)
• Acceleration: Meters per second squared (m/s^2)
• Mass: Kilograms (kg)

How to Solve Equations

• Follow the order of operations (PEMDAS/BODMAS).
• Remember key equations (e.g., force = mass × acceleration, v^2 - u^2 = 2as).
• Ensure units are consistent.