Changing Motion Ch. 6 Lesson 2 EQ: How do unbalanced forces change the speed and direction of an object’s motion?

Vocabulary Balanced forces Unbalanced forces Newton

Balanced Forces Balanced forces are forces that cancel each other out when acting together on a single object. Each force is equal in size and opposite in direction. Balanced forces don’t cause a change in motion. When an object is sitting still, all the forces acting on it are balanced.

Unbalanced Forces Forces that are not equal to each other are called unbalanced forces. Unbalanced forces can cause a change in motion. The motion is in the direction of the greater force.

Isaac Newton Over 300 years ago, the English scientist Sir Isaac Newton told how force and motion are related. Today we measure force in metric units called newtons (N). The newton is named after the famous scientists.

Weight and Force Remember that the newton is also the metric unit for weight. Force and weight are related. An object has weight because the force of gravity pulls down on the object. Therefore, weight is a force.

How do forces affect acceleration? Do you swim? If so, you push harder against the water to swim faster. The size of a force affects the object’s acceleration. A greater force gives the object more acceleration. The mass of the object matter, too. If you apply the same force to an object with more mass, the object accelerates slower.

Forces Add Up

Mass Affects Inertia Remember that inertia is the tendency of a moving object to stay in motion. Inertia also keeps objects with no motion at rest. Suppose you apply the same force to two objects with different masses. The object with less mass accelerates faster. It has less inertia. The more massive the object, the more inertia it has.

How does friction affect motion? Friction is a force that works against motion. The amount of friction depends on the surfaces involved. There is little friction between steel blades and ice, but a lot of friction between rubber soles of sneakers and sidewalks.