Newton's third law: force pairs

Answer Key - Grade 11 Science Worksheet

Circle the best answer, or write your answer on the line.

1. 1.

   A 1600 kg truck tows a 400 kg trailer. The road pushes the truck 5000 N forward; the trailer meets 400 N of resistance. How hard does the coupling pull the trailer, Ari?

   1320 N
2. 2.

   A 2000 kg truck tows a 1000 kg trailer. The road pushes the truck 7500 N forward; the trailer meets 600 N of resistance. How hard does the coupling pull the trailer, Bo?

   2900 N
3. 3.

   A 900 kg truck tows a 300 kg trailer. The road pushes the truck 3000 N forward; the trailer meets 300 N of resistance. How hard does the coupling pull the trailer, Esme?

   975 N
4. 4.

   Block A rides on block B without slipping, and both accelerate at 3 m/s² on a smooth table. Using the diagrams, which force is the third-law partner of block B's 6 N friction on block A, Jun?

   • The 6 N net force on block A, pointing forward
   • The hand's 18 N push on block B, pointing forward
   • Block A's weight pulling it down onto block B
   • Block A's 6 N friction on block B, pointing backward

5. Read this first

   Use this

   Two free-body diagrams side by side (example data). truck (1000 kg): push from the road 3000 N forward; pull from the coupling 1200 N backward. trailer (500 kg): pull from the coupling 1200 N forward; road resistance 300 N backward. The truck tows the trailer along a level road.

6. 5.

   Which force is the third-law partner of the coupling's 1,200 N pull forward on the trailer, Kira?

   • The 300 N road resistance backward on the trailer
   • The trailer's 1,200 N pull backward on the truck
   • The 900 N net force on the trailer, pointing forward
   • The road's 3,000 N push forward on the truck
7. 6.

   Treat the truck and trailer together as one system. The truck and trailer accelerate at 1.8 m/s². Taking the truck and trailer as one system, which forces set its acceleration, Luz?

   • The road's push and the resistance; the coupling cancels inside
   • The four forces together, since the coupling pulls on each vehicle
   • The road's 3,000 N push alone, since the resistance is small
   • The 1,200 N coupling pull, since it joins the two vehicles together
8. 7.

   A horse pulls a cart, and the cart pulls back on the horse equally. Why does the horse-and-cart system speed up, Milo?

   • The cart's pull is a delayed reaction, so the horse gets ahead
   • The two pulls cancel each other, so the system moves at constant speed
   • The ground pushes the hooves forward more than friction drags back
   • The horse pulls harder on the cart than the cart pulls on the horse
9. 8.

   A 3 kg block sits on a 5 kg block on a table. What is the partner of the table's 78.4 N push up on the bottom block, Pia?

   • The 78.4 N weight of both blocks pulling down
   • The bottom block's 78.4 N push down on the table
   • The top block's 29.4 N push down on the bottom block
   • Earth's 49 N pull down on the bottom block
10. 9.

    A 2 kg cart at 3 m/s hits a 4 kg cart at rest and they stick, moving off at 1 m/s. How do the carts' changes in momentum compare, Tala?

    • Equal in size and opposite in direction, 4 kg·m/s each
    • The 2 kg cart loses 6 kg·m/s and the 4 kg cart gains 4
    • Both are zero, since the momentum of the system is unchanged
    • The heavier cart gains more, since it is the heavier cart
11. 10.

    A cart carries a fan blowing forward at a sail fixed on the same cart. Every bit of that air hits the sail. Does the cart speed up, Zee?

    • No: the fan and sail forces are inside the cart and cancel
    • No: the sail is too small for the air to push it
    • Yes: the air pushes the sail forward, so the cart is pushed ahead
    • Yes: the fan pushes air back, which pushes the cart forward