Forces and Motion in the Real World

Learning Objectives

Define friction and identify the different types (static, sliding, rolling, fluid).

Define gravity as a force of attraction between objects with mass.

Explain the difference between mass and weight.

Describe air resistance as a form of fluid friction.

Forces All Around Us

In our study of Newton's Laws, we talked about forces as simple pushes or pulls. In the real world, there are specific types of forces that are acting on us and on everything around us all the time. Let's look at a few of the most important ones.

Friction: The Force of Resistance

Friction is a force that opposes the motion of objects that are in contact with each other. It always acts in the direction opposite to the direction of motion (or intended motion). Friction is what makes it hard to push a heavy box across the floor, and it's also what allows you to walk without your feet slipping.

There are four main types of friction:

1.Static Friction: The friction that acts on objects that are not moving. You have to overcome static friction to get an object to start moving.

2.Sliding Friction: The friction that acts on an object when it slides over a surface.

3.Rolling Friction: The friction that acts on an object when it rolls across a surface. Rolling friction is much weaker than sliding friction, which is why it's easier to move a heavy object on a cart with wheels.

4.Fluid Friction: The friction that acts on objects moving through a fluid (a liquid or a gas). Air resistance is a form of fluid friction. It's the force that pushes against a car as it drives or against a falling parachute.

Gravity: The Universal Attraction

Gravity is a force of attraction that exists between any two objects that have mass. You don't notice the gravitational pull between you and your desk because your masses are so small. But the Earth has a huge mass, so it exerts a strong gravitational pull on everything near it. This is the force that holds you to the ground and makes objects fall.

Mass vs. Weight

These two terms are often confused, but they are not the same thing.

Mass: The amount of matter, or 'stuff', in an object. An object's mass is the same everywhere in the universe. It is measured in kilograms (kg).

Weight: A measure of the force of gravity on an object. An object's weight can change depending on where it is. It is measured in Newtons (N).

Weight = mass × acceleration due to gravity (g)

An astronaut has the same mass on Earth as they do on the Moon. However, because the Moon has much less gravity than Earth, the astronaut's weight on the Moon is only about 1/6th of their weight on Earth. This is why they can jump so high on the lunar surface.

Air Resistance and Terminal Velocity

When an object falls through the air, gravity pulls it down, but air resistance (a type of fluid friction) pushes it up.

As the object falls faster, the force of air resistance increases.

Eventually, the upward force of air resistance becomes equal to the downward force of gravity.

At this point, the forces are balanced, and the object stops accelerating. It continues to fall at a constant maximum speed called terminal velocity. A sky diver uses a parachute to greatly increase air resistance, which lowers their terminal velocity to a safe speed for landing.

Key Terms

**Friction: The force that opposes the motion of one surface as it moves over another.
**Gravity: The force of attraction between any two objects with mass.
**Mass: The amount of matter in an object. It is a measure of an object's inertia.
**Weight: A measure of the force of gravity acting on an object's mass.
**Air Resistance: A type of fluid friction that acts on objects as they travel through the air.
**Terminal Velocity: The constant speed that a freely falling object eventually reaches when the resistance of the medium (such as air) through which it is falling prevents further acceleration.