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Types of Forces Reading
Each of these basketball players is trying to push the ball. One player is trying to push it into the basket, and the other player is trying to push it away from the basket. If both players push the ball at the same time, where will it go? It depends on which player pushes the ball with greater force. Forces like this come into play in every sport, whether it’s kicking a soccer ball, throwing a baseball, or spiking a volleyball. Forces are involved not only in sports such as these but in every motion in our daily lives. In this reading, you’ll see how forces affect the motion of everything from basketballs to planets.
Defining Force
Forceis defined as a push or a pull acting on an object. Any time the motion of an object changes, a force has been applied. Force can cause a stationary object to start moving or a moving object to accelerate. The moving object may change its speed, its direction, or both. How much an object’s motion changes when a force is applied depends on the strength of the force and the object’s mass.
Force as a Vector
Force is a vector because it has both size and direction. The Figure below shows some examples. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force.
Forces can vary in both strength and direction.
SI Unit of Force
The SI unit of force is the newton (N). One newton is the amount of force that causes a mass of 1 kilogram to accelerate at 1 m/s2. Thus, the newton can also be expressed as kg·m/s2.
Types of Forces
Applied Force (Fapp)
An applied force is a force that is applied to an object by a person or another object. If Jack Sparrow is pushing a boat across the sand, then there is an applied force acting upon the object. The applied force is the force exerted on the boat by the person.
Normal Force (Fnorm)
The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a bird is resting on a rock, then the rock is exerting an upward force upon the bird in order to support the weight of the bird. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.
Friction Force (Ffrict)
Frictionis a force that opposes motion between two surfaces that are touching. For example, if a book slides across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together.
Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye appear rough or bumpy when viewed under a microscope. Look at the metal surfaces in the Figure below. The metal foil is so smooth that it is shiny. However, when highly magnified, the surface of metal appears to be very bumpy. All those mountains and valleys catch and grab the mountains and valleys of any other surface that contacts the metal. This creates friction.
The surface of metal looks very smooth unless you look at it under a high-powered microscope.
Air Resistance Force (Fair)
Air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.
Tension Force (Ftens)
The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.
Gravity Force (Fgrav)
Long, long ago, when the universe was still young, an incredible force caused dust and gas particles to pull together to form the objects in our solar system (see Figure below). From the smallest moon to our enormous sun, this force created not only our solar system, but all the solar systems in all the galaxies of the universe. The force is gravity.
Gravity helped to form our solar system and all the other solar systems in the universe.
Gravityhas traditionally been defined as a force of attraction between two masses. According to this conception of gravity, anything that has mass, no matter how small, exerts gravity on other matter. The effect of gravity is that objects exert a pull on other objects. Unlike friction, which acts only between objects that are touching, gravity also acts between objects that are not touching. In fact, gravity can act over very long distances.
Electric Force (Felec)
An electric force is an attractive or repulsive force between two charged objects. Electric forces are attractive when two objects have opposite charges and repulsive when two objects have like charges. Electrons and protons in an atom have opposite charges, so atoms are in part held together by the electric force between protons and electrons.
Magnetic Force (Fmag)
Magnetic force is the force that is created by two magnets. A single magnet cannot create a magnetic force. Magnetic forces are created when a magnet, a magnetic material, or a current carrying wire is placed at an external magnetic field.
Electric forces can be produced by either stationary or moving electric charges, whereas magnetic forces can be produces only by moving charges.
Magnetic force on a moving charge is always normal to the direction of the movement and the magnetic field whereas the force by an electric field on a moving charge is always parallel to the electric field and does not depend on the direction of movement.
Review Questions
- What is force? What does force have to do with the motion of an object?
- What forces are acting on you right now? Explain what those forces are doing to you.
- In the picture of the polar bear, why is the bear sliding on ice? Use friction in your response.
- The bear will not slide forever. Which forces could cause the bear to stop, and why?
- Why is running with a parachute a better workout than running without one? Use what you know about air resistance and force in your explanation.
- Why is the picture of the light bulbs an example of tension force?
- What other force is acting on the light bulbs? How do you know?
- What are two differences between electric force and magnetic force?