Forces

Unbalanced 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. The net force equals the size of the larger force minus the size of the smaller force.

Rolling Friction

Rolling friction is the friction force that acts on rolling objects. For a given set of materials, the force of rolling friction is about 100 to 1000 times less than force of static or sliding friction. Example-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.

Sliding Friction

Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface. Because sliding friction is less than static friction, less force is needed to keep an object moving than to start it moving. Example-With the help of a friend, you push on a pot with enough force to overcome the static friction. Once the pot is moving, static friction no longer acts on it.

Balanced Forces

Sometimes, the net force acting on an object is zero. Balanced forces are forces that combine to product a net force of zero. When the forces on an object are balanced, the net force is zero and there is no change in the object's motion.

Static Friction

Static friction is the friction force that acts on objects that are not moving. Static friction always acts in the direction opposite to that of the applied force. Example-You experience static friction every time you take a step. As you push off with each step, static friction between the ground and your shoe keeps your shoe from sliding.

Fluid Friction

The force of fluid friction opposes the motion of an object through a fluid. Fluid friction increases as the speed of the object moving through the fluid increases. Example-Water and a mixture of gases such as air are known as fluide. You feel fluid friction when stirring thick cake batter. The motion of the spoon through the batter is slowed by fluid friction.

Projectile Motioin

When you throw a ball forward, you'll notice that it actually follows a curved path. This curved path is an example of projectile motion, 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 the ball to follow a curved path.

Combining Forces

You can combine force arrows to show the results of how forces combine. That is, 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.

Representing Force

You can use an arrow to represent the direction and strength of a force. The direction of the arrow represent the direction of the force. The length of the arrow represents the strength, or magnitude, of the force. Weight always acts downward.

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.

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. Note that "in contact" includes solid objects that are directly touching one another as well as object moving through a liquid or a gas. There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.

Terminal Velocity

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 become equal to the downward force of gravity. At this point, the forces acting on the object are balanced. Acceleration is zero and the object continues falling at a constant velocity. Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

Falling Objects

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

Air Resistance

Fluid friction acting on an object moving through the air is known as air resistance. At higher speeds, air resistance can become a significant force. For this reason, bicyclist and speed skaters often wear slick racing suits to reduce air resistance.

Units of Force

Force is measured in newtons, abbreviated as N. One newton is the force that causes a 1-kilogram mass to accelerate at the rate of 1 meter per second each second (1 m/s^2). In fact, 1 newton is equal to 1 kilogram-meter per second squared ( 1 N = 1 kg*m/s^2). The newton is named after Sir Isaac Newton, the English scientist who explained how force, mass, and acceleration are related.

Measuring Force

Forces are often easy to measure. In fact, if you've ever shopped at a grocery store, you may have measured forces using a spring scale. The stretch of the spring in the scale depends on the amount of weight (a type of force) acting on it. As more fruit is placed on the scale, the spring is stretched farther and the scale reading increases.

Gravity

Gravity is a force that acts between any two masses. Gravity is an attractive force, that is, it pulls objects together. Earth's gravitational force exerts a force of attraction on every other object that is near Earth. Note that the force of gravity does not requrie objects to be in contact for it to act on them. Unlike friction, gravity can act over large distances. Earth's gravity acts downward toward the center of Earth.

Key Words

-Air Resistance -Fluid Friction -Force -Friction -Gravity -Net Force -Newton -Projectile Motion -Rolling Friction -Sliding Friction -Terminal Velocity

Objectives

-Describe examples of force and identify appropraite SI units 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 product projectile motion