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Class 9 Science — Force and Newton's three laws

Add forces to find the net force, use F = ma and F = mg, say what Newton's three laws claim, and treat two connected objects as one system.

Name: ________________________

Before you start

Two everyday situations, and the forces acting in each.

12
  1. 1. A box being pushed along a floor — your push one way, friction the other
  2. 2. A ball floating on water — gravity down, buoyancy up

In each case two forces oppose each other. If they match, nothing changes; if they do not, the leftover is the net force.

  1. 1.Boxes of 3 kg and 5 kg are joined by a string and pulled along a frictionless floor by a force of 16 N. What is the acceleration of the pair, in m/s²? Answer with a number.

  2. 2.Which pair of forces is a Newton's-third-law pair?

    • a) The foot pushing the ground backwards and the ground pushing the foot forwards
    • b) The weight of a book and the table's upward push on that same book
    • c) The forward pull of two people pushing one stalled car
    • d) The push of a hand on a box and the friction on that same box

  3. 3.What is the gravitational force the Earth exerts on a 5 kg bag, in N? Answer with a number.

  4. 4.A 1500 kg car accelerates at 2 m/s². What net force is acting on it, in N? Answer with a number.

  5. 5.A block has 14 N acting on it to the right and 6 N to the left. What is the magnitude of the net force, in N? Answer with a number.

  6. 6.Match each of Newton's laws to what it claims.

    • Newton's first law
    • Newton's second law
    • Newton's third law
    • The acceleration is the net force divided by the mass
    • A force on one object is met by an equal and opposite force on the other
    • An object keeps its rest or its constant velocity until a net force acts

  7. 7.Two boxes joined by a string are pulled to the right by one force. Tap the place where the force acting is internal to the system, and so drops out when the two boxes are treated as one object.

    Write the number of the part.

    1234

  8. 8.Sort each situation by whether the forces acting are balanced.

    Groups: Balanced forces · Unbalanced forces

    • A stalled car set moving by two people pushing
    • A tug of war in which both teams pull equally hard
    • A book lying still on a table
    • A rope moving towards the team that pulls harder

Answer key — Class 9 Science — Force and Newton's three laws

  1. 1. 2
  2. 2. a) The foot pushing the ground backwards and the ground pushing the foot forwards
  3. 3. 49
  4. 4. 3000
  5. 5. 8
  6. 6. Newton's first law → An object keeps its rest or its constant velocity until a net force acts; Newton's second law → The acceleration is the net force divided by the mass; Newton's third law → A force on one object is met by an equal and opposite force on the other
  7. 7. 2 — The string joining the two boxes
  8. 8. A tug of war in which both teams pull equally hard → Balanced forces; A book lying still on a table → Balanced forces; A rope moving towards the team that pulls harder → Unbalanced forces; A stalled car set moving by two people pushing → Unbalanced forces