Balanced forces, equilibrium and inertia

Answer Key - Grade 11 Science Worksheet - Building Confidence

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

1. 1.

   A 4 kg box sits at rest on a table while Ari presses straight down on it with 15 N. What is the table's normal force on the box?

   54.2 N
2. 2.

   A 2.5 kg box sits at rest on a table while Bo presses straight down on it with 6 N. What is the table's normal force on the box?

   30.5 N
3. 3.

   A 6 kg box sits at rest on a table while Esme presses straight down on it with 20 N. What is the table's normal force on the box?

   78.8 N

4. Read this first

   Free-body diagram (example data)

   a crate on the floor of mass 5 kg, with arrows from its center: weight 49 N down; normal force 49 N up; push 20 N right; friction 12 N left.

5. 4.

   Using the free-body diagram, is the crate in equilibrium, Jun?

   • No: the net force is 8 N to the right
   • Yes: friction balances the push, so nothing is left
   • Yes: the weight and the normal force cancel each other
   • No: the net force is 32 N to the right, from 20 + 12
6. 5.

   The cart rolls right at a steady speed. Using the diagram, is it in equilibrium, Kira?

   • No: the push and friction add up to 30 N of net force
   • No: it is moving, so some net force must act to the right
   • Yes, but only if the cart were at rest and not moving
   • Yes: every pair cancels, so its velocity stays the same
7. 6.

   Using the diagram, what is true of the lamp, Luz?

   • The net force is 5.6 N upward, so it is not in equilibrium
   • The net force is 35 N upward, the cable's whole pull
   • It is in equilibrium, since the cable holds the lamp up
   • The net force is 5.6 N downward, so the lamp is falling
8. 7.

   A hockey puck is hit and then glides over very smooth ice at nearly the same speed. Why, Milo?

   • The smooth ice pushes it forward to keep up its speed
   • Almost no force acts on it, so its velocity barely changes
   • Its inertia is a forward force that carries it across the ice
   • The force from the stick keeps pushing it forward along the ice
9. 8.

   Which one is not in equilibrium, Ren?

   • A car cruising at 25 m/s along a straight, level road
   • A heavy book resting on a flat desk in a classroom
   • A skydiver falling through the air at a steady 55 m/s
   • A ball at the top of its flight, momentarily at rest
10. 9.

    A box rests on a ramp. Tala says friction must be zero, since nothing moves. What is wrong with that?

    • Friction is zero for boxes that rest near the top of a ramp
    • The normal force by itself balances the whole weight, so nothing is left over
    • Without friction, the weight's part down the ramp would make it slide
    • Friction acts down the ramp and adds to gravity's pull on the box
11. 10.

    An object has zero net force on it. What must be true, Zee?

    • It must be at rest, since any motion would need an unbalanced force
    • No force acts on it, so nothing pushes or pulls it
    • Its speed must be small, since a fast object needs a force to keep up
    • Its velocity is not changing: it may be at rest or moving steadily