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

  2. 2.A fielder catching a fast ball draws his hands backwards as he takes it. Why does that help?

    • a) It stretches out the stopping time, so the acceleration and the force are both smaller
    • b) It increases the ball's acceleration, which spreads the force out
    • c) It makes the ball lighter, so less force is needed
    • d) It cancels the ball's velocity with an equal and opposite velocity

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

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

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

  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.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.

  7. 7.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 book lying still on a table
    • A tug of war in which both teams pull equally hard
    • A rope moving towards the team that pulls harder

  8. 8.A box slides across a floor and, once nothing is pushing it, slows down and stops. What does Newton's first law say is going on?

    • a) A moving object naturally runs out of motion and stops
    • b) Friction is a net force on the box, and it is what changes the velocity
    • c) The box stops because the push that started it has been used up
    • d) The box keeps a constant velocity, so no force can be acting

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

  1. 1. 3000
  2. 2. a) It stretches out the stopping time, so the acceleration and the force are both smaller
  3. 3. 49
  4. 4. c) The foot pushing the ground backwards and the ground pushing the foot forwards
  5. 5. 8
  6. 6. 2
  7. 7. 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
  8. 8. b) Friction is a net force on the box, and it is what changes the velocity