CHAPTER 12:FORCES AND MOTION

SECTION 12–1: FORCES

OBJECTIVES

Describe examples of force and identify appropriate SI units used to measure force.

Explain how the motion of an object is affected when balanced and unbalanced forces act on it.

Compare and contrast the four kinds of friction.

Describe how Earth’s gravity and air resistance affect falling objects.

Describe the path of a projectile and identify the forces that produce projectile motion.

WHAT IS A FORCE?

A force is a push or a pull that acts on an object.

A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

Forces are often easy to measure. Scales and balances are used to measure gravitational forces.

Force is measured in newtons (N). One newton is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second per second. One newton is equal to a kilogram-meter per second squared (kgm/s2).

COMBINING FORCES

You can combine force arrows to show the result of how forces combine. Forces in the same direction add together and forces in opposite directions subtract from one another.

The net force is the overall force acting on an object after all the forces are combined.

When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion. Much like an equally matched game of tug-of-war or arm-wrestling.

COMBINING FORCES

Often, the forces on an object are unbalanced. An unbalanced force is a force that results when the net force acting on an object is not equal to zero.

When an unbalanced force acts on an object, the object accelerates.

Forces acting in opposite directions can also combine to produce an unbalanced force.

FRICTION

All moving objects are subject to friction, a force that opposes the motion of objects that touch as they move past each other.

Friction acts at the surface where objects are in contact.

There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.

STATIC FRICTION

Static friction is the friction force that acts on objects that are not moving. It always acts in the direction opposite to that of the applied force.

Static friction is at work when you take a step or try to push an object that will not move.

SLIDING FRICTION

Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface.

You must be able to overcome static friction before sliding friction can manifest itself.

Less force is needed to keep an object moving than to start it moving, so static friction is a stronger force than sliding friction.

ROLLING FRICTION

When a round object rolls across a flat floor, both the object and the floor are bent slightly out of shape. This change in shape at the point of rolling contact is the cause of rolling friction, the friction force that acts on rolling objects.

Depending upon the materials, the force of rolling friction is about 100 to 1000 times less than the force of static or sliding friction. This is why it’s easier to roll heavy objects rather than push them.

FLUID FRICTION

A fluid is a liquid or gas.

The force of fluid friction opposes the motion of an object through a fluid.

Fluid friction works as a diver jumps into a pool, as you stir cake batter, and as an airplane flies across the sky.

Fluid friction acting on an object moving through the air is known as air resistance. At high speeds, it can become a significant force.

GRAVITY

Gravity is a force that acts between any two masses. It is an attractive force, or a force that pulls objects together.

Earth’s gravity acts downward toward the center of Earth. Fortunately, an upward force usually balances the downward force of gravity.

FALLING OBJECTS

Gravity and air resistance affect the motion of falling objects. Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

As objects fall to the ground, they accelerate and gain speed. With increasing speed comes increasing air resistance. If an object falls for a long time, the upward force of air resistance becomes equal to the downward force of gravity. Acceleration is zero.

Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

PROJECTILE MOTION

Projectile motion is the motion of a falling object (projectile) after it is given an initial forward velocity. Air resistance and gravity are the only forces acting on a projectile.

The combination of an initial forward velocity and the downward vertical force of gravity causes an object to follow a curved path.

Horizontal motion and vertical motion are independent of one another.

VOCABULARY

Force

Newton

Net force

Friction

Static friction

Sliding friction

Rolling friction

Fluid friction

Air resistance

Gravity

Terminal velocity

Projectile motion