1. Estimating sizes. I'm sorry that I forgot to put any scaling objects in the picture, but the rock was maybe 1 metre high, so let's say the volume was 1 cubic metre. You need to be able to do things like this.
2. Density is the mass per unit volume. 1 cubic metre IS unit volume. Looking up rock density on the Internet, it's about 3000 kg per cubic metre. (1000 kg is 1 tonne) http://geology.about.com/cs/rock_types/a/aarockspecgrav.htm So this rock is as heavy as a car.
3. Are you confident with the difference between mass and weight? Mass is a measure of the stuff things are made of; weight is the pull of gravity on the object. Mass is in kg; weight is in Newtons. Weight = mass x g, where g is gravitational field strength, taken as 9.81 N/kg in Physics. So the weight of the rock is 29430N.
4. The Principle of Moments: a moment is a turning force. Moment = force x perpendicular distance from the pivot. Say we put a pivot 1 metre from the rock and I stood on the other end. I would just be lifting the rock when the moments were equal. Moment for rock = 29400N x 1 metre. Moment for me = 650N x d metres. So how big is d? d = 29430 x 1 /650 = 45 metres. That's a big lever!
