Balanced and Unbalanced Forces and Newton's First Law
You are on the school bus, going a steady 15 m/s. The driver slams the brakes, and your body lurches forward even though nobody pushed you. What is going on? By the end of this page you will be able to explain it.
Forces and Net Force
A force is a push or a pull. We measure it in newtons (N). A force has a size and a direction, so we draw it as an arrow: a longer arrow means a stronger force.
Usually more than one force acts on an object at the same time. What matters for motion is their combined effect, called the net force.
To find the net force along one line:
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Forces in the same direction add.
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Forces in opposite directions subtract (the bigger one wins).
Two students push a crate across a floor from opposite sides. Ethan pushes east with 40 N, and Chloe pushes west with 25 N. Friction is small enough to ignore. (a) Find…
Two students push a crate across a floor from opposite sides. Ethan pushes east with 40 N, and Chloe pushes west with 25 N. Friction is small enough to ignore.
(a) Find the net force, with its direction.
(b) Are the forces balanced or unbalanced?
(c) Predict how the crate's motion will change.
(d) What force would Chloe need to push with for the crate's motion not to change if it started at rest?
Balanced and Unbalanced Forces
When the net force on an object is zero, the forces are balanced. When the net force is not zero, the forces are unbalanced.
| Balanced forces | Unbalanced forces | |
|---|---|---|
| Net force | 0 N | More than 0 N |
| Effect on motion | No change | Speeds up, slows down, or changes direction |
Balanced forces do not mean there are no forces. A book on a table has gravity pulling it down and the table pushing it up. They are equal and opposite, so the book stays put.
Use the interactive below to see how unequal forces create a net force.
Classify each situation as having balanced forces or unbalanced forces acting on the object: (a) A book rests on a table. (b) A soccer ball is kicked and speeds up. (c)…
Classify each situation as having balanced forces or unbalanced forces acting on the object:
(a) A book rests on a table.
(b) A soccer ball is kicked and speeds up.
(c) A car drives at a constant 25 m/s on a straight, flat road.
(d) A skydiver speeds up at the start of a fall.
Newton's First Law and Inertia
Imagine a hockey puck on smooth ice. Give it a push, and it glides a long way. The smoother the ice, the farther it goes. What if there were no friction at all? The puck would never stop.
That idea became Newton's First Law of Motion.
In other words: if the net force is zero, the motion does not change. Only an unbalanced force can change an object's state of motion (its speed or direction).
Inertia is not a force. It is a property of matter. Newton's First Law is sometimes called the law of inertia.
Friction is a force that acts between surfaces and opposes motion. On Earth, almost everything slows down because of friction or air resistance. That is why a car coasting with the engine off slows down. In deep space, with almost no forces, a spacecraft keeps coasting at the same velocity forever.
State Newton's First Law of Motion in your own words. Then define inertia and explain how it relates to the first law.
State Newton's First Law of Motion in your own words. Then define inertia and explain how it relates to the first law.
A hockey puck slides across smooth ice. Which of these best explains why it eventually stops: (1) it runs out of force, (2) friction acts on it, or (3) it wants to rest?…
A hockey puck slides across smooth ice. Which of these best explains why it eventually stops: (1) it runs out of force, (2) friction acts on it, or (3) it wants to rest? Explain your choice using Newton's First Law.
A spacecraft far from any planet or star fires its engine, reaches a speed of 2,000 m/s, and then shuts the engine off. Assume no significant forces act on it afterward.…
A spacecraft far from any planet or star fires its engine, reaches a speed of 2,000 m/s, and then shuts the engine off. Assume no significant forces act on it afterward.
(a) Describe the spacecraft's motion after the engine shuts off.
(b) Explain your answer using Newton's First Law.
(c) Why does a car on Earth, in contrast, slow down when the driver takes a foot off the gas?
Inertia in Everyday Life
Now we can answer the school bus puzzle.
The bus and Olivia move forward together. When the driver brakes, friction between the tires and the road is an unbalanced force that slows the bus. At first, no horizontal force acts directly on Olivia's body. By inertia, her body keeps moving forward at the old speed while the bus slows under her. It feels like she is thrown forward, but really she is just continuing to move.
This is why seat belts matter. In a crash, the car stops suddenly, but your body wants to keep going. The seat belt provides the unbalanced force that slows you down with the car.
Back to the start: you lurch forward on the bus because your body keeps its forward motion (inertia) while the bus slows down.
Olivia is riding in a school bus that is moving forward at a steady speed. The driver suddenly brakes, and Olivia's body lurches forward. Use Newton's First Law and the…
Olivia is riding in a school bus that is moving forward at a steady speed. The driver suddenly brakes, and Olivia's body lurches forward. Use Newton's First Law and the idea of inertia to explain why this happens. Say what force acts on the bus and whether any horizontal force acts directly on Olivia's body at first.
A cart moves at constant velocity along a level track. Liam draws a free-body diagram with these forces: weight 20 N downward, normal force 20 N upward, applied force 6…
A cart moves at constant velocity along a level track. Liam draws a free-body diagram with these forces: weight 20 N downward, normal force 20 N upward, applied force 6 N forward, and friction 6 N backward.
(a) Calculate the net force in the vertical direction and in the horizontal direction.
(b) Does the cart's state of motion change? Justify with Newton's First Law.
(c) Liam claims, "The cart moves at constant velocity because the 6 N forward force keeps it going, so no force means no motion." Identify the error in his reasoning and correct it.
(d) Describe what happens to the cart's motion if the applied force is suddenly increased to 10 N, and give the net force.
A coin rests on an index card placed on top of a glass. Camila flicks the card horizontally and quickly, and the coin drops straight into the glass. (a) Identify which…
A coin rests on an index card placed on top of a glass. Camila flicks the card horizontally and quickly, and the coin drops straight into the glass.
(a) Identify which object has forces acting on it horizontally and which object does not, at the moment right after the flick.
(b) Use inertia and Newton's First Law to explain why the coin falls into the glass instead of moving with the card.
(c) Explain what force makes the coin fall once the card is gone.
(d) Predict what would change if Camila pulled the card slowly, and explain why.