Net Force and Balanced Forces
At the 2022 Sydney to Hobart Yacht Race, Wild Oats XI experienced 35-knot winds pushing it forward while 120 kN of water drag pushed back — balanced forces kept it from flying apart.
Printable Worksheets
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Q1 · Two people push a box from opposite sides with equal force. Does it move?
Q2 · A skydiver falls at constant speed — they're not accelerating. Does that mean no forces act on them?
● Know
- Net force = sum of all forces on an object (considering direction)
- Balanced forces → no change in motion
- Unbalanced forces → change in motion
● Understand
- How to add forces in the same direction and subtract forces in opposite directions
- Why an object can be moving at constant speed with balanced forces
- How a free-body diagram represents all forces on one object
● Can do
- Calculate net force for parallel forces (qualitative)
- Draw a labelled free-body diagram for a given situation
- Predict motion from a given set of forces
When forces act in directions, you them to find the net force. When forces act in the direction, you them. If net force = zero, the forces are .
Ten people pushing on a door from both sides cancel each other out — net force = zero. But if 11 people push from one side and 10 from the other, the door moves. Net force is what's LEFT over after you add all forces up with direction.
Net force rules:
- Forces in the SAME direction: add their magnitudes.
30 N east + 20 N east = 50 N east - Forces in OPPOSITE directions: subtract the smaller from the larger.
30 N east + 20 N west = 10 N east - Equal and opposite: net force = 0 N (balanced).
30 N east + 30 N west = 0 N
| Situation | Forces | Net force | What happens |
|---|---|---|---|
| Tug of war — equal teams | 500 N left, 500 N right | 0 N | Rope stays still |
| Tug of war — one side stronger | 600 N left, 500 N right | 100 N left | Rope moves left |
| Two kids pushing a trolley forward | 30 N + 40 N forward | 70 N forward | Trolley accelerates forward |
A 50 N force acts north and a 30 N force acts south on a box. The net force is to the . The box will northward because the forces are .
Balanced forces (net force = 0):
- The object either stays still (at rest) OR keeps moving at constant speed in the same direction
- There is NO change in motion — this is called equilibrium
- Examples:
- Book on table: gravity (down) = normal force (up) → net force = 0, book stays still
- Plane at cruise altitude: thrust (forward) = drag (backward) AND lift (up) = weight (down) → constant speed and height
- Skydiver at terminal velocity: gravity (down) = air resistance (up) → constant speed (not zero — still falling fast!)
Unbalanced forces (net force ≠ 0):
- The object changes its motion — starts moving, speeds up, slows down, or changes direction
- Examples:
- Car accelerating: engine force > drag → net force forward → car speeds up
- Rocket launching: thrust > gravity + drag → net force upward → rocket accelerates upward
- Braking car: braking force > engine force → net force backward → car slows down
A free-body diagram (FBD) shows all forces acting on a single object as labelled arrows. It lets scientists and engineers visualise all forces at once.
How to draw an FBD:
- Draw the object as a simple box or dot (in the centre of your space)
- Draw arrows starting FROM the centre of the object, pointing outward in each force's direction
- Make arrow lengths proportional to force magnitudes (longer = bigger force)
- Label each arrow with the force name and magnitude (e.g. "Weight 490 N")
Australian context — surfboard FBD:
In this FBD: weight (down) = buoyancy (up) → balanced vertically. Wave push (forward) > drag (backward) → net force forward → surfboard accelerates toward the beach.
A boat is floating still on calm water. Two forces act vertically: weight (600 N down) and buoyancy (600 N up). A small motor then switches on, pushing the boat forward with 100 N. Water drag is 60 N backward. Predict: (a) Is the boat balanced vertically? (b) What is the net horizontal force and which direction does the boat move?
How close was your prediction?
Well done — you can handle forces in two directions independently.
Key idea: treat horizontal and vertical forces separately. Each pair must be checked independently.
For each scenario, find the net force and state what happens to the object (stays still / moves left / moves right / speeds up / slows down).
| # | Forces | Net force | What happens? |
|---|---|---|---|
| 1 | 30 N east, 30 N west | ||
| 2 | 50 N north, 20 N south | ||
| 3 | 100 N forward, 100 N forward | ||
| 4 | 80 N right, 50 N left | ||
| 5 | 200 N up (thrust), 500 N down (weight) |
In your book, draw a free-body diagram for each of these three situations. Label every force with its name and direction. Then answer the question below.
Situation A: A stationary car parked on a flat road (forces: weight down, normal force up)
Situation B: A bike accelerating forward (forces: pedalling force forward, air resistance backward — pedalling force is larger)
Situation C: A skydiver at terminal velocity (forces: gravity down, air resistance up — both equal)
Q1. Two people push a shopping trolley: person A pushes with 30 N east, person B pushes with 20 N west. (a) What is the net force? (b) Which direction does the trolley move? (2 marks)
Q2. Draw a free-body diagram of a book sitting on a table. Label all forces and explain why the book stays still. (3 marks)
Q3. Explain the difference between balanced and unbalanced forces. For each, describe what happens to an object's motion. Give one example of each. (4 marks)
Answers
▾MCQ 1
C — Equal and opposite forces cancel out: 10 N − 10 N = 0 N. Net force = 0, forces are balanced. Note: "−10 N" (D) is not a valid net force magnitude — net force is always zero or a positive value with a direction stated.
MCQ 2
B — Acceleration means the car's speed is increasing, which only happens when there is a net force. The engine force must exceed air drag and road friction to produce a net forward force. Balanced forces (A/C) would mean no change in speed.
MCQ 3
B — Terminal velocity means constant falling speed (not zero!). That constant speed only occurs when forces are balanced — gravity down = air resistance up, net force = 0. If gravity were larger (A), the skydiver would still be accelerating downward.
MCQ 4
C — Arrow length shows the magnitude (size) of the force. Arrow direction shows which way the force acts. A longer arrow means a bigger force, not higher speed or greater mass.
MCQ 5
C — A floating, still boat is in equilibrium. Weight (gravity) pulls it down and buoyancy pushes it up with equal force. Net force = 0, so the boat does not move. Forces are definitely present (A is wrong); they are just balanced.
Short Answer 1
Model answer: (a) 30 N east − 20 N west = net force of 10 N east. (b) The trolley moves east (in the direction of person A's push).
Short Answer 2
Model answer: FBD: draw a rectangle labelled "book", with a downward arrow labelled "Weight (gravity)" and an upward arrow of equal length labelled "Normal force". The book stays still because the two forces are equal in magnitude and opposite in direction, giving a net force of zero. With no net force, there is no change in motion — the book remains stationary (Newton's First Law preview).
Short Answer 3
Model answer: Balanced forces: net force = 0. The object's motion does not change — it stays still if at rest, or continues at the same speed and direction if moving. Example: a book on a table (gravity down = normal force up, net = 0, book stays still). Unbalanced forces: net force ≠ 0. The object's motion changes — it speeds up, slows down, starts moving, or changes direction. Example: a rocket launching (thrust > gravity + drag, net force upward, rocket accelerates upward).
Remember the hook? A plane flies at constant speed and constant altitude with four forces acting on it — yet it's not accelerating or diving. That's exactly what balanced forces look like in real life!
Now explain how balanced forces keep that plane flying level. Then do the same for a skydiver falling at constant speed — why don't they keep accelerating? Use the terms net force, balanced, gravity, and air resistance.
- Net force = total force after adding all forces with direction. Same direction: add. Opposite: subtract.
- Balanced (net = 0): no change in motion. Unbalanced (net ≠ 0): motion changes — object accelerates.
- Free-body diagrams show all forces on one object as arrows — length = magnitude, direction = which way the force acts.