A large push against the tennis ball will make it move a lot!What would happen if one person stood in front of the swing, and the other person stood behind the stopped swing, and they both pushed with the same force? Nothing would happen! They are two forces of the same size going in opposite directions. They will cancel out each other.The size of a force also affects acceleration. A small push against a swing will make it move a little. A big push will make the swing move a lot.
CAUSE AND EFFECT What happens to an object when force is used on it?
Force is measured in newtons (N). This word honors the work of Isaac Newton, who did early research involving force. Newton’s Second Law helps us understand how forces make objects accelerate.9
Mass and ForceHow much force does it take to move objects? It depends on its mass. It takes more force to accelerate a large mass than it does to accelerate a small mass to the same speed.Think of the difference between a fallen tree and a kitten. It would take very little energy to move a kitten. However, you might not want to move a tree without some help!All matter has inertia—the property that keeps an object at rest or moving in a straight line. A ball that is rolling in one direction on a ?at, straight surface, tends to keep going in the same direction. Inertia also has to do with matter that is not moving. The fallen tree is sitting in one place, so it will take a force to get it moving.It will take a lot of force to move this load of lumber.10 The more mass there is in an object, the more inertia it has. It’s harder to change its motion. If the tree is rolling in one direction, it will be very dif?cult to get it to change direction!
Sometimes we need a lot of force to move objects, so we use tools and machines. You could tie ropes to the tree and pull it, but it would still be
dif?cult. You would
probably use a machine, because you would need a greater force than your own muscles can give.
CAUSE AND EFFECT What would happen if you tried to get an elephant to move by pushing it? Why?
This elephant has a lot of inertia!11
Gravity and WeightWe use force to move our bodies. We also Gravity at the use electric forces to help us run machines, equator is slightly and there are electric charges in nature.weaker than at the North Pole because Gravity is the force that keeps us on the Earth bulges at the ground by pulling us toward Earth. What equator and you are farther from the happens when you throw a ball up in the center of Earth.air? It comes down!Gravity is an example of gravitation—the force that causes all masses to attract one another. Gravitation keeps the moon orbiting around Earth. The moon and Earth are pulled toward each other. Gravitation enables Earth to orbit the sun. Gravitation also keeps all of the other planets pulled toward the sun, as they move in their orbits.The force of gravity pulls the planets and the sun toward one another.12Your weight on the moon would be different from your weight on Earth. The larger and closer a mass is to another mass, the more it affects the object. We are affected by the gravity of Earth because Earth is very large and very close to us.
You can measure the force of gravity by measuring weight on a scale. Weight is the gravitational force acting on an object.
Remember that mass and weight are different. Your mass on the moon would be the same—you have the same
amount of matter inside you wherever you are. Your weight would change because the gravitational force on the moon is different from that of Earth.
MAIN IDEA AND DETAILS How does gravity control the relationship between the planets and the sun?
13
move it
