Gravitational Acceleration & Force

When you drop your phone (accidentally, of course!), it accelerates toward the ground at approximately -10 m/s² on Earth. This means every second it falls, it gains 10 m/s of speed. But drop that same phone on the moon, and it would accelerate much slower, at only -1.625 m/s².

This difference happens because the force of gravity varies depending on where you are in the universe. We calculate this force using the equation Fg = mg, where Fg is the weight (measured in Newtons), m is mass (measured in kilograms), and g is the gravitational acceleration.

An important distinction in physics: mass and weight are different things! Your mass stays the same whether you're on Earth or the Moon, but your weight changes because the gravitational acceleration is different.

Physics Fun Fact: If you weigh 600 Newtons on Earth (about 135 pounds), you'd only weigh about 98 Newtons on the Moon. You could jump six times higher there!

Forces and Free Body Diagrams

Forces are pushes or pulls that can make objects move when unbalanced. They come in two main categories: contact forces and field forces.

Contact forces require physical touching to work. These include applied force (when you push something), tension force (pulling with a rope), friction force (resisting motion), normal force (surfaces pushing back), and spring force. Think about trying to move a heavy box—you need to touch it!

Field forces work without contact. Gravity is the most familiar—it pulls you down even when you're jumping and not touching the ground. Magnetic and electrical forces also work through fields.

The normal force is what a surface exerts on an object sitting on it. It's called "normal" because it acts perpendicular to the surface. When you stand on the floor, it pushes back against you with exactly the force needed to keep you from falling through.

Think about it: When you're sitting in your chair right now, the normal force is pushing up on you with exactly the same strength that gravity is pulling down on you. That's why you don't sink through the chair!

Tension and Free Body Diagrams

Tension force is what a rope, string, or cord applies when it pulls on an object. What makes tension special is that it can only pull, never push—try pushing something with a rope and you'll see it just goes slack!

Free Body Diagrams (FBDs) are simple but powerful visual tools that show all the forces acting on an object. In these diagrams, we represent the object as a dot or box, with arrows showing the direction and relative strength of each force.

Different situations create different force patterns. A box sitting on a flat surface has just two forces: gravity pulling down and normal force pushing up. A free-falling object experiences only gravity. When someone pushes a box across a rough floor, we see normal force, gravity, friction, and the pushing force all at once.

Pro Tip: When drawing Free Body Diagrams, start by identifying if the object is moving or stationary, then figure out what forces could be causing that state. The diagram should tell the complete force "story" of the object.

Newton's Laws

Newton's First Law (Law of Inertia) states that objects keep doing what they're doing unless acted upon by an unbalanced force. If something's moving, it keeps moving; if it's still, it stays still. This resistance to change is called inertia—like when you slide across the floor in socks and can't easily stop.

Forces can be balanced (equal in opposite directions, causing no motion) or unbalanced (causing motion). Imagine two people pushing equally on opposite sides of a door versus one person pushing harder than the other.

Newton's Second Law gives us the formula ΣF = ma, which can be rearranged to a = ΣF/m. This means an object's acceleration equals the net force applied divided by its mass. The heavier something is, the more force needed to accelerate it. The Σ (sigma) symbol means "sum" or "net"—we add forces in the same direction and subtract opposing ones.

Real World Application: This is why your car accelerates faster when it's just you versus when it's full of friends and luggage! The same engine force has to move more mass, resulting in less acceleration.

Newton's Third Law & Practice Problems

Newton's Third Law states that for every force, there is an equal and opposite force acting on the other object. These paired forces are called "third law pairs." When you push against a wall, the wall pushes back on you with equal strength—which is why your hand feels pressure!

This law applies everywhere: The Earth pulls on the Moon with the same force that the Moon pulls on Earth. Even though the effects look different (due to their mass difference), the forces are identical.

Let's see these concepts in practice problems:

Problem 1: A car accelerates at 2m/s² for 5 seconds from rest. To find distance traveled:

• First find final velocity: v = a × t = 2m/s² × 5s = 10m/s
• Then calculate distance: d = average velocity × time = 10m/s × 5s ÷ 2 = 25m

Problem 2: A ball thrown upward at 15m/s reaches its highest point when:

• At highest point, final velocity = 0 m/s
• Using t = Δv ÷ a = 15m/s ÷ 9.8m/s² = 1.53 seconds

Study Tip: For problems like these, always identify what you know, what you need to find, and which equation connects them. Start with the simplest formula first!