Forces and Newton’s Laws

A force is a push or a pull, and forces are what change the way objects move. Sir Isaac Newton summed up how forces and motion work in three laws that show up constantly on the science test. This lesson walks through all three in plain language.

Newton’s First Law: Inertia

The first law says an object at rest stays at rest, and an object in motion stays in motion at the same speed and direction, unless a nonzero net external force acts on it. This tendency to keep doing what it is doing is called inertia. A book resting on a table has balanced gravity and support forces, so its net force is zero. A rolling ball keeps rolling until friction slows it. When a question asks why a moving object keeps going or a still object stays still, the answer is inertia.

Newton’s Second Law: Net Force and Acceleration

The second law connects force, mass, and acceleration: \[ F_{\text{net}} = m \times a. \] For constant mass in an inertial reference frame, net external force equals mass times acceleration. This tells you that a bigger net force produces more acceleration for the same mass, and a heavier object needs more force to accelerate the same amount. If a \(2\) kg object accelerates at \(3\) \(\text{m/s}^2\), the net force is \[ F_{\text{net}} = 2 \times 3 = 6 \text{ newtons}. \] You can rearrange it to find any missing value: acceleration is net force divided by mass.

Newton’s Third Law: Action and Reaction

The third law says that for every action there is an equal and opposite reaction. When you push on something, it pushes back on you with the same force in the opposite direction. A swimmer pushes water backward, and the water pushes the swimmer forward. A rocket pushes gas down, and the gas pushes the rocket up. Look for these force pairs when a question describes two objects interacting.

Watch: A Short Video Lesson

Step by Step Science walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:

A Routine for Force Questions

1. First law: objects keep doing what they are doing (inertia) unless a nonzero net external force acts.
2. Second law: \(F_{\text{net}} = m \times a\). Rearrange it to find any missing quantity.
3. Third law: interaction forces come in equal and opposite pairs acting on different objects.
4. Identify the force pair when two objects push or pull on each other.

Practice

1. What is inertia?
2. Write Newton’s second law as a formula.
3. A \(4\) kg object accelerates at \(2\) \(\text{m/s}^2\). What net force is acting on it?
4. A swimmer pushes water backward. Which law explains why they move forward?
5. Which law explains why a seatbelt is important when a car stops suddenly?
6. If the same net force is applied to a heavy and a light object, which accelerates more?

Answers

1. The tendency of an object to keep doing what it is doing (rest or motion).
2. \(F_{\text{net}} = m \times a\).
3. \(F_{\text{net}} = 4 \times 2 = 8 \text{ newtons}\).
4. The third law (action and reaction).
5. The first law (inertia keeps your body moving forward).
6. The lighter object.

Keep studying

High School Physics includes 12 full-length practice tests: two in the book and ten online.

Forces build on motion and lead into work, energy, and power. The High School Physics learning hub lists all 26 chapter lessons.