AP Physics 1: Newton's Laws and Forces (Class Slides)AP Physics 1: Newton's Laws and Forces (Class Slides)

Newton's First Law and Forces

Goal: Know the Law of Inertia, the difference between mass and weight, the common types of forces, and how to draw a free-body diagram.

THE SHORT VERSION

Newton's First Law says motion only changes when an unbalanced force acts. This guide follows the class slides in order: who Newton was, the Law of Inertia, mass versus weight, every type of force on the slides, and how to draw free-body diagrams (FBDs) for six example situations.

1 Isaac Newton (1643-1727)

English physicist, mathematician, astronomer, and philosopher.

Played a major role in developing calculus.

His three Laws of Motion and his Law of Universal Gravitation are the foundation of classical mechanics.

Fun fact from the slides: he also studied alchemy (trying to turn lead and other base metals into gold) and wrote 169 books on it. None were published in his lifetime, because making gold and silver was considered a felony.

2 Newton's 1st Law: The Law of Inertia

Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. In other words: objects at rest stay at rest, and objects in motion stay in motion, unless acted upon by an unbalanced force. Two things to know

Inertia: the tendency of an object to resist changing its motion.

Mass: measured in kg. It is the amount of "stuff" an object has, and it is the same everywhere in the universe.

Mass is inertia. Physicists consider your mass to be your inertia. More mass means more resistance to a change in motion. 3 What Is a Force?

A force is a push or a pull.

A force results in the object being accelerated.

Objects cannot exert forces on themselves.

4 Types of Forces

The slides first list the contact forces (they involve touch), then go through each force one at a time.

Force Symbol Key idea from the slides

Applied force F_A The force usually "applied" to an object, such as you pushing something.

Friction force F_f Always opposes the motion.

Tension force F_T The force in a string, rope, or cable. Usually the same everywhere in the cable. Always a pulling force. Normal force F_N Keeps one object from "crashing" through another. Always perpendicular to the surface. The object must be touching a surface to have one.

Drag force F_D Like friction but with fluids. The opposing force air exerts on an object (air resistance).

Thrust F_Th The force typically used by engines.

Spring force F_S The force a spring exerts to get back to its unstretched position (think trampoline). Also true of elastic objects like a rubber band. Buoyant force F_B The upward force that a fluid exerts on an object.

Gravity

(weight)

F_g Your weight is a force. F_g = mg.

5 Weight Is a Force

F_g = m g, where F_g is weight in newtons (N), m is mass in kg, and g is the acceleration due to gravity. On Earth g = 9.8 m/s².

g is different on different planets.

Slide example: a 30 kg person weighs (30)(9.8) = 294 N.

6 Mass vs. Weight

Mass Weight

Unit kg N

Changes with location? No. The same everywhere. Yes. Varies with gravity.

Same on the moon as on Earth? YES NO

So you have the same mass on the moon as on Earth, but you do not weigh the same.

Mass stays 30 kg everywhere. Weight changes with g.

7 Free-Body Diagrams (FBDs)

Physicists draw FBDs to see all the forces acting on an object.

How to draw an FBD

Draw a dot for each object in the system. Each object gets its own FBD. Pretend the object is alone in empty space (a "free body"). Include all forces acting on the body. Do not include forces the body exerts on other objects.

Draw an arrow away from the dot for each force.

The arrow's direction is the force's direction. Its length is proportional to the force's magnitude. Label each arrow clearly.

8 Slide Examples

Situation Forces on the object

Person sitting in a chair F_g down, F_N up (from the chair).

Car driving at constant speed to the right F_g down, F_N up, F_thrust (engine) forward, F_D (drag / air resistance) backward. Pig hanging from a ceiling F_g down, F_T up (rope).

Polar bear floating in water F_g down, F_B (buoyant) up.

Fish hanging on a spring scale F_g down, F_S (spring) up.

Runner sliding into 2nd base F_g down, F_N up, F_f (friction) opposing the slide.

Person leaning against a wall F_g down, F_N horizontal (perpendicular to the wall), F_f up along the wall. Important: a normal force does not have to point up. It only has to be at 90° to the surface the object touches.

Free-body diagrams for the slide examples. Same dot, different arrows.

9 Words to know

Inertia

Tendency of an object to resist changing its motion.

Mass

Amount of "stuff" in an object, in kg. Same everywhere. Mass is inertia.

Weight (F_g)

Force of gravity on an object, F_g = mg, in N. Varies with g.

Unbalanced force

A net force. It is what changes an object's motion.

Free-body diagram

Drawing of one object as a dot with an arrow for every force acting on it.

Normal force

Contact force perpendicular to the surface, stopping objects from passing

through it.

CHECK YOURSELF Why does a 30 kg student have the same mass but a smaller weight on the moon? What is the weight of a 30 kg student on Earth? Draw an FBD for a book resting on a table, and name the type of each force. COMMON MISTAKES Calling mass and weight the same thing. Drawing a force the object exerts on something else. Forgetting that tension only pulls. Assuming F_N always points up.

AP Physics 1 Slide Notes · original study notes