Exercises: Balanced and Unbalanced Forces and Newton's 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.

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.

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.

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.

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?

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?

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 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.