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Projectile motion can be described by the horizontal and vertical components of motion. In the previous chapter we studied simple straight-line motion—linear motion. Now we extend these ideas to nonlinear motion—motion along a curved path. Throw a baseball and the path it follows is a combination of constant- velocity horizontal motion and accelerated vertical motion. 5 Projectile Motion
A vector quantity includes both
magnitude and direction, but a scalar
quantity includes only magnitude.
5.1 Vector and Scalar Quantities
5 Projectile Motion A quantity that requires both magnitude and direction for a complete description is a vector quantity. Velocity is a vector quantity, as is acceleration. Other quantities, such as momentum, are also vector quantities.
5.1 Vector and Scalar Quantities
5 Projectile Motion A quantity that is completely described by magnitude is a scalar quantity. Scalars can be added, subtracted, multiplied, and divided like ordinary numbers.
- When 3 kg of sand is added to 1 kg of cement, the resulting mixture has a mass of 4 kg.
- When 5 liters of water are poured from a pail that has 8 liters of water in it, the resulting volume is 3 liters.
- If a scheduled 60-minute trip has a 15-minute delay, the trip takes 75 minutes.
5.1 Vector and Scalar Quantities
5 Projectile Motion How does a scalar quantity differ from a vector quantity?
5.1 Vector and Scalar Quantities
The resultant of two perpendicular vectors
is the diagonal of a rectangle constructed
with the two vectors as sides.
5.2 Velocity Vectors
By using a scale of 1 cm = 20 km/h and drawing a
3 - cm-long vector that points to the right, you
represent a velocity of 60 km/h to the right (east).
5.2 Velocity Vectors
5 Projectile Motion The airplane’s velocity relative to the ground depends on the airplane’s velocity relative to the air and on the wind’s velocity.
5.2 Velocity Vectors
5 Projectile Motion The velocity of something is often the result of combining two or more other velocities.
- If a small airplane is flying north at 80 km/h relative to the surrounding air and a tailwind blows north at a velocity of 20 km/h, the plane travels 100 kilometers in one hour relative to the ground below.
- What if the plane flies into the wind rather than with the wind? The velocity vectors are now in opposite directions. The resulting speed of the airplane is 60 km/h.
5.2 Velocity Vectors
5 Projectile Motion Now consider an 80-km/h airplane flying north caught in a strong crosswind of 60 km/h blowing from west to east. The plane’s speed relative to the ground can be found by adding the two vectors. The result of adding these two vectors, called the resultant, is the diagonal of the rectangle described by the two vectors.
5.2 Velocity Vectors
5 Projectile Motion An 80-km/h airplane flying in a 60-km/h crosswind has a resultant speed of 100 km/h relative to the ground.
5.2 Velocity Vectors
Often we will need to change a single vector into an equivalent set of two component vectors at right angles to each other:
- Any vector can be “resolved” into two component vectors at right angles to each other.
- Two vectors at right angles that add up to a given vector are known as the components of the given vector.
- The process of determining the components of a vector is called resolution.
5.3 Components of Vectors
A ball’s velocity can be resolved into horizontal and vertical components.
5.3 Components of Vectors
5 Projectile Motion Vectors X and Y are the horizontal and vertical components of a vector V.
5.3 Components of Vectors
5 Projectile Motion How do components of a vector affect each other?
5.3 Components of Vectors
5 Projectile Motion
The horizontal component of motion for a
projectile is just like the horizontal motion
of a ball rolling freely along a level surface
without friction.
5.4 Projectile Motion
The vertical component of a projectile’s
velocity is like the motion for a freely
falling object.
5 Projectile Motion A projectile is any object that moves through the air or space, acted on only by gravity (and air resistance, if any). A cannonball shot from a cannon, a stone thrown into the air, a ball rolling off the edge of a table, a spacecraft circling Earth—all of these are examples of projectiles.
5.4 Projectile Motion
Projectiles near the surface of Earth follow a curved path that at first seems rather complicated. These paths are surprisingly simple when we look at the horizontal and vertical components of motion separately.
5.4 Projectile Motion
Projectile motion can be separated into components. a. Roll a ball along a horizontal surface, and its velocity is constant because no component of gravitational force acts horizontally.
5.4 Projectile Motion
5 Projectile Motion Projectile motion can be separated into components. a. Roll a ball along a horizontal surface, and its velocity is constant because no component of gravitational force acts horizontally. b. Drop it, and it accelerates downward and covers a greater vertical distance each second.
5.4 Projectile Motion
5 Projectile Motion Most important, the horizontal component of motion for a projectile is completely independent of the vertical component of motion. Each component is independent of the other. Their combined effects produce the variety of curved paths that projectiles follow.
5.4 Projectile Motion
5 Projectile Motion Describe the components of projectile motion.
5.4 Projectile Motion
5 Projectile Motion
The downward motion of a horizontally
launched projectile is the same as that of
free fall.
5.5 Projectiles Launched Horizontally
The vertical distance a projectile falls
below an imaginary straight-line path
increases continually with time and is
equal to 5 t^2 meters.
5.6 Projectiles Launched at an Angle
No matter the angle at which a projectile is launched, the vertical distance of fall beneath the idealized straight-line path (dashed straight lines) is the same for equal times.
5.6 Projectiles Launched at an Angle
5 Projectile Motion The dashed straight lines show the ideal trajectories of the stones if there were no gravity. Notice that the vertical distance that the stone falls beneath the idealized straight-line paths is the same for equal times. This vertical distance is independent of what’s happening horizontally.
5.6 Projectiles Launched at an Angle
5 Projectile Motion With no gravity the projectile would follow the straight-line path (dashed line). But because of gravity it falls beneath this line the same vertical distance it would fall if it were released from rest.
5.6 Projectiles Launched at an Angle
5 Projectile Motion With no gravity the projectile would follow the straight-line path (dashed line). But because of gravity it falls beneath this line the same vertical distance it would fall if it were released from rest.
5.6 Projectiles Launched at an Angle
5 Projectile Motion With no gravity the projectile would follow the straight-line path (dashed line). But because of gravity it falls beneath this line the same vertical distance it would fall if it were released from rest.
5.6 Projectiles Launched at an Angle
If there were no gravity the cannonball would follow the straight-line path shown by the dashed line. The vertical distance it falls beneath any point on the dashed line is the same vertical distance it would fall if it were dropped from rest:
5.6 Projectiles Launched at an Angle
Height
For the component vectors of the cannonball’s motion, the horizontal component is always the same and only the vertical component changes. At the top of the path the vertical component shrinks to zero, so the velocity there is the same as the horizontal component of velocity at all other points. Everywhere else the magnitude of velocity is greater, just as the diagonal of a rectangle is greater than either of its sides.
5.6 Projectiles Launched at an Angle
5 Projectile Motion The velocity of a projectile is shown at various points along its path. Notice that the vertical component changes while the horizontal component does not. Air resistance is neglected.
5.6 Projectiles Launched at an Angle
5 Projectile Motion The velocity of a projectile is shown at various points along its path. Notice that the vertical component changes while the horizontal component does not. Air resistance is neglected.
5.6 Projectiles Launched at an Angle
5 Projectile Motion The velocity of a projectile is shown at various points along its path. Notice that the vertical component changes while the horizontal component does not. Air resistance is neglected.
5.6 Projectiles Launched at an Angle
5 Projectile Motion The velocity of a projectile is shown at various points along its path. Notice that the vertical component changes while the horizontal component does not. Air resistance is neglected.
5.6 Projectiles Launched at an Angle
Maximum range is attained when the ball is batted at an angle of nearly 45°.
5.6 Projectiles Launched at an Angle
Speed
Without air resistance, a projectile will reach maximum height in the same time it takes to fall from that height to the ground. The deceleration due to gravity going up is the same as the acceleration due to gravity coming down. The projectile hits the ground with the same speed it had when it was projected upward from the ground.
5.6 Projectiles Launched at an Angle
5 Projectile Motion Without air resistance, the speed lost while the cannonball is going up equals the speed gained while it is coming down. The time to go up equals the time to come down.
5.6 Projectiles Launched at an Angle
5 Projectile Motion In the presence of air resistance, the path of a high-speed projectile falls below the idealized parabola and follows the solid curve.
5.6 Projectiles Launched at an Angle
5 Projectile Motion
5.6 Projectiles Launched at an Angle
5 Projectile Motion
think!
A projectile is launched at an angle into the air. Neglecting air resistance, what is its vertical acceleration? Its horizontal acceleration?
5.6 Projectiles Launched at an Angle
think!
A projectile is launched at an angle into the air. Neglecting air resistance, what is its vertical acceleration? Its horizontal acceleration? Answer: Its vertical acceleration is g because the force of gravity is downward. Its horizontal acceleration is zero because no horizontal force acts on it.
5.6 Projectiles Launched at an Angle
think!
At what point in its path does a projectile have minimum speed?
5.6 Projectiles Launched at an Angle
5 Projectile Motion
think!
At what point in its path does a projectile have minimum speed? Answer: The minimum speed of a projectile occurs at the top of its path. If it is launched vertically, its speed at the top is zero. If it is projected at an angle, the vertical component of velocity is still zero at the top, leaving only the horizontal component.
5.6 Projectiles Launched at an Angle
5 Projectile Motion Describe how far below an imaginary straight-line path a projectile falls.
5.6 Projectiles Launched at an Angle
5 Projectile Motion
- Which of these expresses a vector quantity? a. 10 kg b. 10 kg to the north c. 10 m/s d. 10 m/s to the north
Assessment Questions
5 Projectile Motion
- Which of these expresses a vector quantity? a. 10 kg b. 10 kg to the north c. 10 m/s d. 10 m/s to the north Answer: D
Assessment Questions
- When no air resistance acts on a projectile, its horizontal acceleration is a. g. b. at right angles to g. c. upward, g. d. zero.
Assessment Questions
- When no air resistance acts on a projectile, its horizontal acceleration is a. g. b. at right angles to g. c. upward, g. d. zero. Answer: D
Assessment Questions
5 Projectile Motion
- Without air resistance, the time for a vertically tossed ball to return to where it was thrown is a. 10 m/s for every second in the air. b. the same as the time going upward. c. less than the time going upward. d. more than the time going upward.
Assessment Questions
5 Projectile Motion
- Without air resistance, the time for a vertically tossed ball to return to where it was thrown is a. 10 m/s for every second in the air. b. the same as the time going upward. c. less than the time going upward. d. more than the time going upward. Answer: B
Assessment Questions
