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Chapter 2. Motion. Motion and Position . Motion occurs when an object changes its position relative to a reference point. A reference point is needed to determine the position of an object. You don't always need to see something move to know that motion has taken place. . Distance.
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Chapter 2 Motion
Motion and Position • Motion occurs when an object changes its position relative to a reference point. • A reference point is needed to determine the position of an object. • You don't always need to see something move to know that motion has taken place.
Distance • An important part of describing the motion of an object is to describe how far it has moved, which is distance. • The SI unit of length or distance is the meter (m). Longer distances are measured in kilometers (km). Shorter distances are measured in centimeters (cm).
Displacement • Displacement is the distance and direction of an object's change in position from the starting point. • Sometimes you may want to know not only your distance but also your direction from a reference point, such as from the starting point.
Distance and Displacement • In the following picture, are distance and displacement different? • Which is greater?
Displacement Problem • Complete #12 on page 834
Speed • Speed is the distance an object travels per unit of time • Calculating Speed • The SI unit for distance is the meter and the SI unit of time is the second (s), so units of speed are measured in meters per second (m/s). • Sometimes it is more convenient to express speed in other units, such as kilometers per hour (km/h).
Calculating Speed • Complete #’s 13, 14, 21, and 22 on page 834
Changing Speed • If you are traveling at a constant speed, you can measure your speed over any distance interval. • Usually speed is not constant. • Average Speed • Average speed describes speed of motion when speed is changing. • Average speed is the total distance traveled divided by the total time of travel. • Average speed is calculated the same as constant speed
Calculating Average Speed • Complete #15 on page 834
Instantaneous Speed • Instantaneous speed is the speed at a given point in time. • If an object is moving with constant speed, the instantaneous speed doesn't change. • When something is speeding up or slowing down, its instantaneous speed is changing.
Example of Instantaneous Speed • When driving, your speed changes. All of the following are examples of instantaneous speed (speed at a given time.)
Graphing Motion • The motion of an object over a period of time can be shown on a distance-time graph. • Time is plotted along the horizontal axis of the graph and the distance traveled is plotted along the vertical axis of the graph.
Graphing Practice • Complete the Distance Time Graph Worksheets
Velocity • Velocity includes the speed of an object and the direction of its motion. • Speed describes only how fast something is moving. • To determine direction you need to know the velocity. • Because velocity depends on direction as well as speed, the velocity of an object can change even if the speed of the object remains constant.
Calculating Velocity • If a cyclist in the Tour de France traveled southwest a distance of 12,250 meters in one hour, what would the velocity of the cyclist be in meters per second? • Velocity is calculated using the speed formula. You must include direction when calculating velocity.
Acceleration • Acceleration is the rate of change of velocity. When the velocity of an object changes, the object is accelerating • A change in velocity can be either a change in how fast something is moving, or a change in the direction it is moving. • Acceleration occurs when an object changes its speed, it's direction, or both. • When you think of acceleration, you probably think of something speeding up. However, an object that is slowing down also is accelerating.
Calculating Acceleration • To calculate the acceleration of an object, the change in velocity is divided by the length of time interval over which the change occurred. • Acceleration is measure in m/s2 • To calculate the change in velocity, subtract the initial velocity—the velocity at the beginning of the time interval—from the final velocity—the velocity at the end of the time interval. a = vf – vi t
Example Problem • A car starts from rest and reaches a velocity of 12.1 m/s over a time of 5.21 s. Determine the acceleration of the car. 12.1 m/s – 0 m/s 5.21 s a=vf – vi t 2.32 m/s2
Acceleration Problems • Complete #’s 16-18 and 23-24 on page 834
Force • A force is a push or pull. • A force can cause the motion of an object to change. • Force is measure in Newtons. It is derived from mass (kg) and acceleration (m/s2).
Balanced Forces • Force does not always change velocity. • When two or more forces act on an object at the same time, the forces combine to form the net force. • Forces on an object that are equal in size and opposite in direction are called balanced forces. • Object doesn’t move
Unbalanced Forces • When forces acting on an object are not equal in size an unbalanced force is created. • This causes objects to move
Inertia and Mass • Inertia is the tendency of an object to resist any change in its motion. • If an object is moving, it will have uniform motion. It will keep moving at the same speed and in the same direction unless an unbalanced force acts on it. • If an object is at rest, it tends to remain at rest. Its velocity is zero unless a force makes it move. • The inertia of an object is related to its mass. The greater the mass of an object is, the greater its inertia.
A golf ball will remain at rest until a force (hitting the ball with a club) is applied. The tennis ball will remain in constant motion unless a force (gravity and air friction) is applied.
Which one of the objects would be harder to stop if they were moving at the same speed? Why?
Newton's Laws of Motion • The British scientist Sir Isaac Newton (1642–1727) was able to state rules that describe the effects of forces on the motion of objects. • These rules are known as Newton's law's of motion.
Law of Inertia • Newton's first law of motion states that an object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it. • If an object is at rest, it stays at rest unless an unbalanced net force acts on it.