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Types of force

边玩边学

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课程笔记

Contact vs Non-Contact Forces

  • Contact forces act when two objects are physically touching.
  • Non-contact forces act between objects that are not touching.
  • Examples of contact forces: air resistance, friction, normal reaction, tension, thrust, upthrust.
  • Examples of non-contact forces: gravitational, magnetic, weight.

Contact Forces

  • Air resistance is a contact force that opposes motion through air.
  • Friction is a contact force that opposes motion when two surfaces rub together.
  • Normal reaction is a contact force acting perpendicular to a surface, opposing weight.
  • Tension is a contact force in a rope or cable being pulled.
  • Thrust is a contact force produced by an engine or rocket, pushing forwards.
  • Upthrust is an upward contact force that allows objects to float in fluids.

Non-Contact Forces

  • Gravitational force is an attractive non-contact force between any two masses.
  • Magnetic force is a non-contact force from magnetic fields; can attract or repel.
  • Weight is a non-contact force due to gravity acting on an object's mass.

Balanced and Unbalanced Forces

  • Balanced forces are equal in size and opposite in direction, resulting in no change in motion.
  • If forces are balanced, an object's speed remains constant (or stays at rest).
  • If the forward force (thrust) is greater than the backward force (friction + air resistance), the object accelerates.

Particle arrangement in solids, liquids, and gases – relevant to understanding how contact forces (e.g., friction) arise from particle interactions.

Particle arrangementSolidLiquidGas

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练习题

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  1. 1.Which of the following is a non-contact force?

    Easy
    • AGravitational force
    • BFriction
    • CAir resistance
    • DTension
  2. 2.Which force allows objects to float in a fluid?

    Easy
    • AUpthrust
    • BWeight
    • CThrust
    • DNormal reaction
  3. 3.Friction is a type of:

    Easy
    • Acontact force
    • Bnon-contact force
    • Cmagnetic force
    • Dgravitational force
  4. 4.Which of the following statements is true?

    Easy
    • AWeight is a non-contact force.
    • BWeight is a contact force.
    • CWeight is a type of friction.
    • DWeight is a magnetic force.
  5. 5.Air resistance acts:

    Easy
    • Aagainst the direction of motion
    • Bin the direction of motion
    • Cperpendicular to the direction of motion
    • Donly when an object is stationary
  6. 6.Which force is produced by an engine or rocket?

    Easy
    • AThrust
    • BTension
    • CUpthrust
    • DNormal reaction
  7. 7.A skydiver falling through the air experiences two main forces. Which are they?

    Easy
    • AWeight and air resistance
    • BWeight and upthrust
    • CThrust and friction
    • DTension and normal reaction
  8. 8.If two forces on an object are balanced, what happens to the object's speed?

    Easy
    • AIt remains constant
    • BIt increases
    • CIt decreases
    • DIt becomes zero

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