Jack is sat in a wooden box. His friend tries to push Jack across the ground in the box. But it is very hard work. The box hardly moves. Why is it so hard to make the box move?

Do you know how forces affect speed and going faster? What about forces slowing things down and stopping them? We stop cars and bikes by using friction. When you throw a ball up into the air, what happens? Why does it not just keep going up for ever? Because of gravity.