Acceleration

Imagine a track meet. The runners all line up at the starting line. At this point, their velocity is 0—they aren’t moving. Then, the starting gun goes off, and the runners push off. They begin to increase their speed.

We say that they accelerate. To most people, acceleration means simply “speeding up.” In science, though, the word has a different meaning. It is the rate at which velocity changes. Remember that velocity involves the direction in which an object moves as well as its speed. So accelerating the object may involve changing its speed or changing its direction (or both).

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Balanced and Unbalanced Forces

In a tug-of-war, two teams pull on a rope in opposite directions. The team that uses the most force pulls the other team across a line. This is an example of how motion is affected by unbalanced force. The force of the pull from one team is greater than the force of the pull from the other team. Unbalanced forces acting on an object will change the object’s motion. If the two tug of- war teams are evenly matched, however, the situation is different. The teams both pull as hard as they can, but the one force is exactly balanced by the other force. When balanced forces act on an object, they will not change that object’s motion. Inertia The unit of measurement for force called the Newton is named in honor of the English scientist and mathematician Isaac Newton. In the late 1600s, Newton discovered three basic laws, or principles, that describe how forces affect objects.

Scientists still rely on these laws of motion when figuring out how to get a spacecraft to the Moon. Newton’s first law of motion deals both with objects that are at rest (that is, not moving at all) and with objects that are moving. It says that an object at rest will remain at rest unless it is acted upon by a force strong enough to make it move. The first law also says that an object in motion will move at a constant speed in a straight line unless acted upon by a force strong enough to make it change its speed or direction.

The first law is sometimes called the law of inertia. Inertia is the tendency of an object to resist change in its motion. For example, the passengers in a moving car keep moving forward when the car stops suddenly. The passengers have inertia. The only way to stop inertia is to exert an opposite force. That is what seatbelts do.

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Weight and Mass

People sometimes think the words weight and mass mean the same thing. But for scientists, they mean different things. Weight is the force of gravity on a person or object at the surface of a planet. When you stand on a scale, the scale measures the force with which Earth pulls on you. Mass is something different. It is a measure of the amount of matter in an object. Far out in space, far from the pull of Earth’s gravity, your weight might go down to just about zero, but you would still have the same mass.

The gravitational pull of an object depends on the amount of mass it has. The greater the mass, the stronger the pull. When you fall off your skateboard, you pull Earth to you at the same time Earth pulls you toward its center. But your mass is tiny compared to that of Earth. So the pull you exert on Earth is much, much weaker than the pull of Earth’s gravity on you. Friction is a force that can affect the motion of an object. Friction occurs when two surfaces rub together. Think of the wheels of a skateboard on pavement. It may seem that the wheels and the pavement are both smooth. But actually both have bumps and ridges. Friction is created when the bumps and ridges of the two surfaces come into contact with each other. If a moving object meets continuous friction, sooner or later it will be brought to a stop. Without friction, the object would keep moving at a constant speed forever. With friction, the only way the object can keep moving is if it gets a push (or a pull) from some other force. For the skateboard, you supply the push. How strong the force of friction will depend on a couple of factors. One is the type of surfaces involved. For example, the rougher the surfaces, the greater the friction. Another factor is how hard the surfaces push together. There is more friction if you rub your hands together with some force than if you rub your hands together lightly.

Mass and Payload

Imagine an empty cardboard box. It has very little mass. It is very easy to push. Suppose you fill it with rocks. Now the mass is much greater, and you have to use a lot more force to push it. This fact is explained by Isaac Newton’s second law of motion. This science principle says that the amount of force needed to move an object—that is, change its speed or direction—depends on the size of the object’s mass. The greater the mass, the greater the amount of force required. The law also says that for a given mass, a greater force will produce a greater change in speed or direction. The change in speed or in direction will occur in the same direction as the force. The cardboard box will move in the direction you push.

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