Types of force

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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.

Friction happens because imperfections on the two surfaces at the molecular level push against each other.

Friction happens because imperfections on the two surfaces at the molecular level push against each other.

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.

Phenomena caused by gravitational attraction and the weight force: falling objects and a satellite held in orbit around Earth.

Phenomena caused by gravitational attraction and the weight force: falling objects and a satellite held in orbit around Earth.

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.

When the forces and moments on an object are balanced, the object remains in equilibrium.

When the forces and moments on an object are balanced, the object remains in equilibrium.

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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