CSET Practice Test Subtest II Science

GRAVITY

There is a legend which says that while lying against 
a tree in an orchard, Isaac Newton was hit on the head 
by a falling apple. What made the apple fall to the 
ground? Was it a force from the earth on the apple? 
From this event, it is supposed, Newton theorized 
about the force of gravity. He explained the motions 
of the apple, and the earth and other planets around 
the sun as mutual attractions to each other. This 
attraction was called the force of gravity. The more 
mass or volume each object has, the greater the 
force of attraction. As the objects increase their 
distance apart, the attraction gets less or 
decreases.

Without gravity, we would never be able to keep 
our feet on the ground, and at the same time, not 
be able to move off the ground. Without gravity, 
the moon would move in a straight line away from 
the earth; gravitational force of the earth however 
pulls or attracts the moon to orbit around it. 
Gravity also keeps planets on orbit around the 
sun in the solar system. 

Imagine this, when you are biking uphill, the earth's 
gravity is pulling you back down the hill, so that 
makes it hard to bike uphill. On the other hand, 
when you finally reach the top and you are biking 
downhill, the earth's gravity is pulling you down 
the hill making your bike speed faster downhill. In 
fact, anything going downhill gains speed as the 
earth's gravitational force is pulling it down.

Friction

Friction is a force that objects have which makes them 
resist motion or movement across or against one 
another. If one object with a flat surface is placed 
on top of another, the top object can be lifted with 
out any act of resisting except that of gravity. If 
one object is pushed or pulled over another, there is 
an act of resistance that is caused by friction. If 
one object moves over a stationary object, the 
stationary object will slow down the moving object. 
If a force tries to move an object at rest on a 
surface, the surface resists this motion with an 
opposing force of friction. The size or amount of 
the frictional force depends on the kind of surfaces 
which are rubbing together. 

Friction can be used for many important things 
such as helping the wheels of a train stay on the 
rails of the track. It also allows a mechanical 
device that carries things from one place to 
another also known as a conveyer; it helps the 
belt of a conveyor stay on pulleys without slipping. 
We need friction to walk; if we did not have any 
friction, everything would be slipping and sliding 
away. That is why we can't walk on ice very well; 
the ice is so smooth and flat that it produces less 
friction than a sidewalk, making it hard to walk on 
ice. 

Friction occurs any time when two objects or 
surfaces are rubbed together. The more the 
pressure on the objects when they are rubbed 
together, the more friction. If you are trying to 
slow down, friction works with you, but when you 
try to speed up, friction works against you. 

Different methods can be used to reduce the 
amount of friction between the surfaces of the 
objects in contact. One way to reduce friction 
is to apply a lubricant to the surfaces, another 
is to use casters, rollers, or ball bearings between 
the surfaces, and another is to smooth the 
surfaces of the objects in contact.