Download Chapter 10 TEST - Study Guide

Chapter 10 – Test 1 ~ Grade 8 Physical Science
STUDY GUIDE
1. A force is a push or a pull.
2. Forces are described not only by how strong they are, but also by the direction in which they
act.
3. The strength of a force is measured using the SI unit the newton, named for Isaac Newton, an
English mathematician.
Forces can be shown using arrows. The length of the arrow represents the size of the force, and the
direction of the arrow shows the direction of the force.
The overall force on an object, called the net force, is found by combining all of the forces acting on
the object.
The size of the net force determines whether the object’s motion changes. The direction of the net
force determines the direction of the object’s motion. When two forces act in the same direction, they
add together. When forces act in opposite directions, they are combined by subtracting the smaller
force from the larger force. The direction of the resulting force is the direction of the larger original
force.
 When there is a net force acting on an object, the forces are said to be unbalanced.

Unbalanced forces can cause an object to speed up, slow down, or change direction.

Unbalanced forces acting on an object will change the object’s motion.

Equal forces acting in opposite directions are called balanced forces

.Balanced forces acting on an object will not change the object’s motion.

When you add equal forces in opposite direction, the net force is zero.
The force that two surfaces exert on each other when they rub against each other is called
Friction. It acts in a direction opposite to the direction of the moving object. Friction will eventually
cause an object to come to a stop.
The strength of the friction force depends on two factors: how hard the surfaces push together and
the types of surfaces involved.
Rough surfaces produce greater friction than smooth surfaces. Friction also increases if the surfaces
push hard against each other.
 Static friction acts on objects that aren’t moving.

Sliding friction occurs when solid surfaces slide over each other.

Rolling friction occurs when an object rolls over a surface.

Fluid friction results when an object moves through a fluid—a liquid or a gas.
The force needed to overcome rolling friction or fluid friction is usually less than that needed to
overcome sliding friction.
Gravity is a force that pulls objects toward each other.
 The law of universal gravitation states that the force of gravity acts between all objects in the
universe.
 Any two objects in the universe, without exception, attract each other.

Two factors affect the gravitational attraction between objects: mass and distance.
Weight = Mass x Gravity
Gravity on Earth is 9.8 N/kg
Mass is a measure of the amount of matter in an object.
The more mass an object has, the greater its gravitational force. The farther apart two objects are, the
less the gravitational force between them.
Weight is the force of gravity on a person or object at the surface of a planet. Weight is a measure of
the force of gravity on an object, and mass is a measure of the amount of matter in that object.
When the only force acting on an object is gravity, the object is in free fall. In free fall, the force of
gravity is an unbalanced force that causes an object to accelerate.
Near Earth’s surface, acceleration due to gravity is 9.8 m/s2
.
Objects falling through air experience a type of fluid friction called air resistance



Air resistance is not the same for all objects.
The greater the surface area of an object, the greater the air resistance.
Air resistance also increases with velocity.
So, as the velocity of a falling object increases, air resistance increases until it is equal to the force of
gravity. When forces are balanced, the velocity stops increasing.

The greatest velocity that can be obtained by an object in free fall is called terminal
velocity