# Force

### Intro

Whenever you open a door, pick up a pencil, or push something away from you, you exert a force. The fundamental unit of force is the Newton (written as N), named after the great Issac Newton. As you saw on the Newtons Laws page, force applied in a straight line is:

** F=ma**

or, **Force = mass x acceleration**

If you had a toy car, and were to push it in a straight line, the force would be how hard you pushed it, the mass is how much the car weighs and acceleration is how fast you can make the car go.

### Gravity

Guess what? Gravity is also a force and it acts upon us every day, only we don't usually notice. Check out the video for the Giant Drop, a theme park ride at Dreamworld on the Gold Coast of Australia.

So how does this relate to roller coasters? Well it's like a vertical roller coaster and it helps to demonstrate how gravity exerts a force on people (and everything else) just like acceleration.

** Force = mass x acceleration**

**Force of gravity = mass x acceleration due to gravity**

On earth, the acceleration due to gravity is 9.8 meters per second per second downwards.

### 'g-forces'

When on a roller coaster, the rider experiences 'g-forces'. These are the same forces that astronauts experience during a rocket launch, only smaller. It is these g-forces which make the ride exciting, and what make you feel sick. They're calculated by:

**g-force = g + acceleration / 9.8**

9.8 ms^{2 }is on g on earth, and 2 'g' s is 19.6 ms^{2 }(2 x 9.8).

Here's a video which explains what g-forces feel like and what they do to you:

Stuff to think about:

1. What force would be required to accelerate a 2000kg roller coaster car to 10k/m per hour? (Hint: re-arrange the equation)

2. How many 'g's would you experience if you were to accelerate downwards at 25ms^{2 }?

### Centripetal force

Ok, so that's up and down sorted, now for the forces experienced when you go around bends- your roller coaster would be pretty boring without a few bends and loops in it.

Have you ever gotten something like a yo-yo and swung it around in circles? If you have, you'll have seen how it goes in a circle, and doesn't just fall down at the top (unless you slow down). Why does it do this? The answer is centripetal force.

**F _{c} = mv^{2}/r**

**centripetal force = mass x velocity squared / radius**

So, the centripetal force on a yo-yo being swung around in circles is equal to its mass times the square of how fast you swing it, divided by the radius of the circle that it travels in (length of the string).

This also allows you to use not only vertical (up and down) g-forces in your roller coaster, but vertical (sideways) as well. For example, This Wild Mouse roller coaster, has a track that doesn't tilt, but can be as every bit as thrilling as a conventional roller coaster, because of the lateral g-forces felt by the riders. Think about why they feel these forces as you watch the video (Hint: think about centripetal force) and discuss this with someone.

Question:

If you were to swing a bucket of water on a string like the guy in the video, the mass of the bucket was 1kg, the length of the string was 0.75 m and the velocity that you swung it at was 50 m/s, what would the centripetal force be in Newton's?