You've decided to learn the sport of skateboarding. You go to a skate park where skaters are doing all sorts of jumps, flips, slides, and other tricks. One of the most basic jumps, the ollie, is shown below.

[[☃ image 4]]

How does a skater control the motion of a skateboard? Let’s find out.

##Forces are interactions

To start moving forward, a skater can push backward on the ground with one foot as shown below.

[[☃ image 6]]

If the ground beneath the skater's foot wasn’t there, moving his foot backward would not make him move forward.

Pushes require two objects to push against each other. This is called an interaction. The same is true for pulling: two objects interact by pulling on each other. An object can’t change its motion by pushing or pulling on itself.

A push or pull from an interaction between two objects is called a force. The strength of a force is measured in units of newtons $(\text{N}).$ For example, the skater’s push against the ground may have a strength of $30,\text{N}.$

Forces are modeled with arrows. The arrow points in the direction the force acts. The longer the arrow, the stronger the force. An arrow modeling a force can be labeled with a description of the force, the strength of the force, or both, like in the image below.

[[☃ image 7]]

[[☃ graded-group-set 1]]

##Newton’s third law

When two objects interact, they push against each other in opposite directions. For example, when a skater’s foot pushes backward against the ground, the ground pushes forward on the skater’s foot. This makes the skater accelerate forward.

What if the skater pushes back on the ground harder? Then they speed up quicker. When they push the ground harder, the ground pushes back on them harder too. In fact, the ground pushes on the skater with exactly the same strength as the skater pushes on the ground.

These characteristics of interactions are always true and are summarized by Newton’s third law: When two objects interact, the forces they exert on each other are always the same strength, and they always act in opposite directions.

Said another way, when object A exerts a force on object B, object B exerts a force of equal strength on object A. The forces objects A and B exert on each other act in in opposite directions.

“Object A” and “object B” can be replaced with the names of whatever objects are interacting in a given situation. It doesn’t matter which is A and which is B.

[[☃ explanation 2]]

[[☃ graded-group-set 1]]

##Types of forces

A contact force pair results from two objects touching each other. Two types of contact forces are described below.

  • A normal force pair is produced when two surfaces press against each other. Each normal force acts perpendicular $(\perp)$ to the surface it’s applied to. (In math, normal means perpendicular.) For example, a normal force pair is produced when the girl stands on the skateboard.

[[☃ image 29]]

  • A friction force pair resists two surfaces sliding against each other. Each friction force acts parallel $(\parallel)$ to the surface it’s applied to, opposite the direction it's sliding (or attempting to slide) relative to the other surface. For example, a friction force pair is produced when a skateboard skids across the ground to slow down.

[[☃ image 27]]

A non-contact force pair involves two objects that can interact without touching.

  • A gravitational force pair pulls any two objects with mass toward each other. The pull is very weak unless one or both of the masses are very large, like the Earth. In our everyday experience, the only gravitational forces strong enough to notice are those of the Earth interacting with other objects. For example, when a skater performs a jump, he’s not touching Earth’s surface. But the Earth still pulls him back towards the ground.

[[☃ image 41]]

  • The electric and magnetic forces are also non-contact forces.

Net force

When more than one force acts on an object at the same time, it’s helpful to think about their combined effect—the net force. The net force is the strength and direction of a force that could replace all the individual forces acting on an object and have the same effect.

When determining the net force on an object, we only consider forces that act on that object. So, even though every force is part of a pair, we only model the one force from the pair that acts on the object we're analyzing.

[[☃ graded-group-set 1]]

When forces act in opposite directions, their effects cancel. So, subtracting them gives the strength of the net force. The direction of the net force is the direction of whichever force was stronger, because not all of that force is canceled out.

[[☃ graded-group-set 2]]

What happens if two forces in opposite directions are the same strength? Suppose the skater in the problem above stands at rest on their skateboard. The Earth still pulls down on them with a gravitational force of $550,\text{N}.$ But now the skateboard pushes them up with an equal strength force: $550,\text{N}.$ So, the net force is:

$F_\text{net}=550,\text{N}-550\text{N}$

$F_\text{net}=0$

The strength of the net force is zero. The combined effect of the two forces is the same as if no force is acting at all.

Whenever the forces on an object completely cancel out, giving a net force of zero, we say the forces are balanced. If the net force is not zero, the forces are unbalanced.

Sometimes, forces aren’t in the same or opposite directions. They may point in different dimensions. Look again at the boy pushing the girl on a skateboard to the right. The girl also experiences an upward force from the board and a downward force from Earth. We can model these forces with arrows:

[[☃ image 4]]

We can’t just add or subtract the strengths of these three forces. Instead, we have to look separately at the horizontal (side-to-side) forces and the vertical (up-and-down) forces.

The vertical forces are in opposite directions, so we subtract their strengths:

$F_\text{net, vertical} = F_\text{normal}-F_\text{gravity}$

$F_\text{net, vertical} = 475,\text{N}-475,\text{N}$

$F_\text{net, vertical}=0$

The vertical forces are balanced.

The girl experiences only one horizontal force — the push. Since there’s nothing to combine it with, the strength of the push equals the strength of the net force:

$F_\text{net, horizontal}=F_\text{push}$

$F_\text{net, horizontal}=25,\text{N}$

The net horizontal force is $25,\text{N}$ to the right.

##The ollie

This article began with a skater doing an ollie. Here’s the same video, in slow motion. Can you identify the forces involved throughout the ollie?

Once you've given it a try, check your answer below!

[[☃ image 2]]

[[☃ explanation 1]]

##Key points

  • A force is a push or pull. Its strength is measured in newtons $(\text{N}).$

  • A force can be modeled with an arrow. The arrow’s length represents the strength of the force.

  • All forces are produced by two objects interacting. Each of the two objects puts a force on the other.

  • Newton’s third law says if objects A and B interact, A exerts a force on B, and B exerts an equal-strength force on A, but in the opposite direction.

  • Contact forces require contact between objects. Examples are normal and friction forces.

  • Non-contact forces can act without contact. These forces include gravitational, electric, and magnetic forces.

  • Net force is the combined effect of all the forces acting on an object. To find the strength of the net force, add forces that act in the same direction and subtract forces that act in opposite directions.