Page 350 - ABCTE Study Guide_Neat

What Causes Acceleration?

In order to have acceleration, there must be an unbalanced force applied to an object. This statement is
the essential meaning of Newton's first law. If there are no unbalanced forces acting on an object that
doesn't mean the object is at rest. The object could be moving at a constant velocity. Macroscopic forces
that we experience include gravity, friction, electromagnetic forces, and contact forces such as pushing on
a surface.

Think about the following:

A box is sliding across the floor. When you stop pushing the box—applying a force—the box slows to a
stop because

• there is no longer a force acting on the box.
• air resistance slows the box down.
• an object can move only when a force is acting on it.
• the friction from the floor acts on the box against its motion to change its velocity.

Friction is the key here.

Objects do not need to have force acting on them in order to keep moving. They only need the force to
accelerate them (change their velocity). The frictional force of the floor accelerates the box in the direction
opposite its velocity, so it slows to a stop.

How is Pressure Related to Force?

Pressure is force divided by area. A sharp knife cuts food more easily than a dull one because with a
sharp knife the same force is applied to a smaller area. Hence you can exert more pressure with a sharp
knife than a dull one. We also feel pressure from air, water, and other fluids. This pressure increases with
depth. So atmospheric pressure at sea level is greater than on a mountaintop. Similarly when we dive to
the bottom of a swimming pool, we feel a greater water pressure at a greater depth.

Review

• Velocity includes the direction of motion while speed does not.
• A constant velocity is represented by a straight line on a graph, and the slope will be the
value of the velocity or speed. A steeper slope indicates a higher velocity.
• Newton’s first law: When no force acts on an object (or when the forces acting on it
cancel), it moves in a straight line at constant speed.

Electricity and Magnetism

Lesson Objective

Now let’s turn our attention to energy. First, we quickly review some ideas about electricity, and then we’ll
move on to the different types of energy and discuss some important concepts about heat and light.