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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