Forces and motion

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  • LN
    linh.tranLittle Lab Notes

    • 🎯 Try it

    Single choice

    A cart has 7 N left and 2 N right. Resultant?

    A still book has gravity downward and support upward. Still does not mean no forces.

    ⏱️ Practice timer
    05:00

    🔎 Worked example

    📝 Force cards

    Arrows show force direction, not the direction of travel. For the horizontal cards, vertical forces balance.

    Card⇅
    Forces⇅
    Current motion⇅
    A
    8 N right; 3 N left
    Moving right
    B
    6 N right; 6 N left
    Moving right
    C
    2 N right; 7 N left
    Moving right
    D
    4 N up; 4 N down
    Stationary book

    Draw a longer arrow for a larger force. Label each arrow before calculating.

    Forces have size and direction. Equal opposite forces give zero resultant force. A stationary object then stays stationary; a moving object can continue at constant velocity. Unequal forces cause acceleration, which can mean speeding up, slowing down, or changing direction. Example: 8 N right and 3 N left give 5 N right. Motion direction alone does not tell us the resultant force direction.

    ✏️ Practice

  • 1.
    Find each card’s resultant force, including direction.
  • 2.
    Which moving card has constant velocity? Explain.
  • 3.
    Which moving card slows down initially? Explain why it can still be moving right.
  • 4.
    Correct “An object with balanced forces must be stationary.”

    • 💭 Your answer

    Single choice

    Balanced forces on an already moving object imply what?

    ✅ Quick check

    Single choice

    A still book has gravity downward. What keeps it still on a table?

    🔑 Answers

    🔑 Practice answers

  • 1.
    A: 5 N right; B: 0 N; C: 5 N left; D: 0 N.
  • 2.
    B: zero resultant means no acceleration, so its existing velocity continues.
  • 3.
    C: its acceleration is left while its velocity is right, reducing its speed initially. A force does not instantly reverse motion.
  • 4.
    Balanced forces mean constant velocity, which may be zero or nonzero.

    • 💡 Hint and challenge

    💡 Hint: Draw only horizontal forces first; label each arrow before subtracting.

    ⭐ Challenge: Explain how a turning object can accelerate while its speed stays constant.

    🔎 Challenge answer

    Velocity includes direction. A turning object changes velocity even if its speed stays constant; that change is acceleration.

    ➡️ Next lesson

    Distance–time graph practice.

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  • LN
    linh.tranLittle Lab Notes
    • Forces and motion - Docmino