Forces, Motion, and Energy

What is Force? A force is a push or pull that causes an object to move faster or slower, stop, change direction, or change size or shape. Without force, nothing would ever move.

Magnetic Force Magnetism is the force of pushing or pulling between magnetic poles. It acts at a distance and cannot be seen. Materials that create this force are said to be magnetic and are called magnets.

Magnetic Force Continued… When LIKE poles of magnets are near each other, a repulsive force exists, and the magnets move away from each other if the force is great enough. When OPPOSITE poles of magnets are near each other, an attractive force exists and the magnets move toward each other if the force is great enough. The closer the objects, the greater the magnetic force. The magnetic force is greatest at the poles of magnets.

1. 2. 3. Let’s look at what we already know about forces. What would happen with these pairs of magnets? Will they attract or repel each other? Write and draw what you think will happen, and then we will see if you are correct. 1. 2. 3.

Gravitational Force Gravity is a force that pulls all objects toward each other.

The force of gravity causes two objects of different weights to fall to the ground at the same time. The force of gravity is a pull that attracts objects to each other. This attraction is not noticeable unless one of the objects is very large (like a planet, moon, or the Sun). The force of gravity between Earth and anything on it is extremely noticeable because the mass of Earth is so large. The pull of Earth’s gravity makes any object fall to the ground. As The Moon goes around Earth, its gravity pulls on Earth causing water in the oceans to move toward the Moon. Gravity Continued…

Earth’s gravity also pulls on the Moon Earth’s gravity also pulls on the Moon. This force of gravity keeps the Moon moving around Earth. Similarly, the pull of the Sun’s gravity keeps Earth moving around the Sun.

Which will hit the ground first? They travel at the same speed! The size of the mass does not change the rate of speed that objects fall. The paper was slowed by air getting trapped under as it fell. Shape of object may affect the speed in which it falls!

Frictional Forces Friction is the force that is applied by a surface as an object moves across it. It is always present when two surfaces rub together. Friction is affected by the type of surface (bumpy, smooth) and by how hard they are pressed together

Friction Continued… Friction occurs in liquids and gases as well as between solids. Without friction, it would be very hard to slow or stop the motion of objects. Friction can be reduced by using lubricants (like motor oil, wax, or grease), by making surfaces smoother, or by using rollers.

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The effect of friction can be changed in the following ways:

Surface Texture of the The rougher the surface, the greater the friction. Smooth surfaces reduce friction. So, carpets have more friction than tile floors. Soles of shoes have rough textures to increase friction between the shoes and the floor so that it is possible to walk without slipping.

Amount of Surface Area The greater the surface area, the greater the friction. If more surface of an object touches another object, the friction will be greater. If tires of a car or truck are larger, more surface area of the tire will touch the road making friction greater. Trucks have larger tires to make it easier for them to stop or slow down.

Lubrication Lubricants- a substance put on a surface to reduce friction Without lubrication, moving parts of machines would slow down or stop very quickly. We use oil in a car to reduce friction on the motor.

Motion The motion of an object can be described by its position, direction of movement, and speed. How do we know this flag is in motion?

Speed One way to describe motion is speed. Speed is a measure of the distance an object moves in a certain amount of time. Speed is a measure of how fast an object is moving.

Position The position of an object is its location relative to another object (the reference point). For example, we can use the words “above”, “below”, “beside”, “behind”, “ahead of” plus the distance from the other object. The distance (length) is the change in position of an object over time when compared with a reference point. Direction of motion can be determined by reading a compass. Students should give directions using the terms “north”, “south”, “east”, or “west.” shown by the needle of a compass lined up with the direction of the moving object. They should also describe the direction of motion by using the terms “right”, or “left” relative to another object, or “up”, or “down” relative to Earth.

Point of Reference Point of reference is your point of view when you observe a motion

Starting Position Speed, ramp height, length, and surface type can affect motion. The higher the position, the greater the speed and distance will be in the object’s motion.

Balanced Forces Balanced forces are forces equal in size and opposite in direction. Neither object moves because it is equal in both directions.

Unbalanced Forces Unbalanced forces occur when one force is greater than its opposite force. They cause the object’s motion to speed up, slow down, or stop. The object moves in the direction applied by the greater force. Unbalanced forces cause a non-moving object to start moving. They affect the rate and direction of motion in objects. Examples: Tug-of-war A truck pulling a car

Rate of motion Rate of motion is the speed of the object or how fast or slow the object is moving. Unbalanced forces can cause the speed or the rate of motion to change by increasing, decreasing, or stopping the motion.

The speed of an object is the measure of the distance (change in position) an object moves in a given amount of time. Speed is distance divided by time, so you can take the value on the Y axis and divide it by the value on the X axis.

Slower Speed Faster Speed The line on the graph is a flatter line, illustrating a slower speed Faster Speed The line on the graph is a steeper line, illustrating a faster speed Speed Stopped The line illustrates a constant speed from 0-5 seconds. From 5 to 10 sec, the line is flat with no increase in position. This graph illustrates an object that is stopped between 5 and 10 seconds.

A change in force or mass affects the motion of an object as follows:

Force As the force increases, the speed of an object increases. As the force applied to an object is decreased, the object will move slower than the object that was given a greater push or pull. If there is no friction (for example, in outer space), an object that is already moving does not need a force to keep it moving.

Mass As the mass increases, the speed of an object decreases if the force remains the same. The speed decreases as the object’s mass increases. It is much harder to change the speed of a heavy object than a light object. An object with a small mass is easier to stop or cause a change in motion than an object with a large mass.

Changing Forms of Energy Energy is most noticeable as it transforms from one type to another. What are some examples of transforming electrical energy? A light bulb A hair dryer

Changing forms of Energy An example of transforming chemical energy is a car engine. Chemical potential energy in gasoline is transformed into kinetic energy of the car as it moves!!

KE and PE In many situations, there is a conversion between potential and kinetic energy. The total amount of potential and kinetic energy in a system is called the mechanical energy Mechanical energy = PE + KE

Mechanical Energy Mechanical energy is due to the position and motion of the object. What happens to the mechanical energy of an apple as it falls from a tree?

The total amount of energy stays the SAME!! Mechanical Energy The mechanical energy does not change because the loss in potential energy is simply transferred into kinetic energy. The energy in the system remains constant!! The total amount of energy stays the SAME!!

The Law of Conservation of Energy The Law of Conservation of Energy states that energy cannot be created or destroyed. Energy is transformed… not destroyed

Energy in Your Body Even the energy converted in your body follows the law of conservation of energy. Chemical potential energy is transferred to kinetic energy that allows your body to move!!