Joe's parents are farmers and they have a huge farm with cows, chickens, and ducks. Joe loves the farm and all the things he gets to play around and play on. One day, Joe's father told him not to get near a tractor that was sitting in the field. His father was worried that Joe would climb on it and hurt himself. Joe went out to the field and was feeding the horses and cows. When he was done, he saw the tractor his father told him not to get near. He knew that climbing on the tractor wouldn't hurt anything, so he did. He climbed on to the seat and sat there. Then, he pretended he was his father and pretended that he was driving the tractor. Joe's father saw him playing on the tractor and called for him. Joe heard his father calling for him and got off the tractor really fast. When he did that, he fell off and hurt his arm. Joe was in pain and his father came running to check on him and picked him up and sat him on a bench and asked him why he did that. Joe looked at his father and said, "I wanted to be like you." Joe's father gave him a hug and asked him if he wanted to ride with him on the tractor. Joe did and after he got a bandage on his arm, he and his father rode in the field on the tractor.

What had he just finished doing when he saw the tractor

How can you describe your location? You might use a familiar system. You might say, 'I live at 1234 Main Street, Springfield, Ohio.' You could also say, 'I live right behind the Elementary School.' This method uses the school as a point of reference. Another example is, I am at the corner of Maple Street and Main Street. Both streets may be a good reference for anyone living in your town. Scientists must be able to pinpoint a feature they are studying. Scientists use a special system to describe locations. They use latitude and longitude as a reference. Lines of latitude and longitude form a grid. You may have used a grid system while doing graphing. This grid is centered on a reference point. Zero latitude is the equator. Lines of latitude run east to west. They divide the Earth from North to South. Lines of longitude run from north to south. They divide the Earth from East to West. Zero longitude runs through Greenwich, England. You may have heard the term, Greenwich Mean Time, or GMT. The system of latitude and longitude works well for objects that do not move.

What line, running north to south, passes through Greenwich, England and is also at the equator?

What if two children push a swing at the same time? Would the swing go higher? Yes, together their force would be more. It is common for forces to be combined. Most objects on Earth have at least two forces acting on them. Do you know what one of them is? Of course, that force is gravity. How many forces do you have on you right now? Gravity pulls you down toward the center of Earth. Your legs exert a downward force. They hold you up against the pull of gravity. Consider the example in Figure 1.3. A book is resting on a table. Gravity pulls the book downward. It has a force of 20 newtons. At the same time, the table pushes the book upward. Its force is also 20 newtons. The table opposes the pull of gravity.
How many forces does an object have?