Physics Chapter 3

When you jump your hang time is the time your feet are off the ground. Does hang time depend on your vertical component of velocity when you jump your horizontal component of velocity or both? defend your answer

Only the vertical component; once your feet leave the ground, the only acceleration (ignoring air resistance) is in the vertical direction. Your vertical liftoff velocity divided by g will be the time you move upward; twice that will be your total time in the air.

Vector Quantity

Requires both magnitude and direction

Projectile is launched straight up at 141 m/s how fast is it moving. Suppose it is launched at work at 45° above the horizontal plane how fast is it moving at the top of its curved trajectory?

0 m/s at top of trajectory when launched straight up; horizontal velocity of angle-launch = 141 m/s x cos(45) = 100 m/s --- this is the only part of velocity in play at the top of a curved trajectory, so the speed at the top of the curved trajectory is 100 m/s

A shiny new sports car stats in the parking lot of a car dealership above is a cargo plane flying horizontally at 15 m/s at the exact moment the plane is 125 m directly above the car a heavy create accidentally falls from its cargo doors relative to the car where will the crate hit

125 m = ½(9.8)(t2) t = 5 s; 50m/s x 5 s = 250 m from the car

Neglecting air resistance, if you throw a ball straight up with a speed of 20 m/s how fast will be moving when you catch it?

20 m/s. For projectiles, the speed at points of equal elevation will be the same.

The boy on the tower in the figure below throws a ball a distance of 6 m as shown if the tower is 20 m tall what speed in meters per second is the ball thrown.

20m = ½(9.8m/s2)(t2) t = 2s; 60m/2s = 30 m/s

Scalar Quantity

Completely described by motion

Harry and Angela look from their balcony to a swimming pool below that is 15 m from the bottom of the building they estimate the balcony is 45 m high and wonder how fast they would have to jump horizontally to succeed in reaching the pool what is your answer

First, solve for time with d = 1/2at2. 45 m = ½(9.8m/s2)(t2) t = 3 s. Next, horizontal velocity = horizontal distance/time. 15m/3s = 5 m/s

What are the horizontal and vertical components of a 10 unit factor that is oriented 37° above the horizontal

Horizontal component = 10 units x cos(37deg) = 8 units; we see that this is a 3-4-5 triangle (multiplied by two), so the vertical component is 3 x 2 = 6 units (if you didn't remember that, you would solve for vertical with 10 units x sin(37deg) = 6 units)

Calculate the magnitude of the horizontal and vertical components of a vector that is 100 units long and disoriented at 45°

Horizontal component = 100 units x cos(45deg) = 70.7 units; vertical component will also be 70.7 units since vertical and horizontal are equal in a 45-45-90 triangle

Harry accidentally falls out of a helicopter that is traveling at 100 m/s he plunges into the swimming pool two seconds later assuming no air resistance what was the horizontal distance between here in the swimming pool when he fell from the helicopter?

Horizontal distance = horizontal velocity x time; distance = 100 m/s x 2 s = 200 m

How does the downward component of the motion of a projectile compare with the motion of free fall?

The downward motion of a projectile is the same as that of an object in free fall. Both are only under the influence of gravity.

At the instant a ball is thrown horizontally over a level range, a ball held at the side of the first is released and drops to the ground if air resistance is neglected which ball strikes the ground first?

They hit the ground at the same time. Once again, the vertical component of motion is ONLY affected by gravity and the height from which the object is dropped. If the height is the same, the objects will fall at the same time.

The launch in velocity of a projectile is 20 m/s at 53° above the horizontal what is the vertical component of its velocity at alunch? it's horizontal component of velocity? neglecting air friction which of these components remains constant throughout the flight path? which of these components determines the projectiles time in the air

Vertical component = 20 m/s x sin(53deg) = 16 m/s; horizontal component = 20 m/s x cos(53deg) = 12 m/s (another 3-4-5 triangle!); horizontal velocity remains constant; vertical velocity determines time in the air

In the absence of air resistance, why does the horizontal component of velocity for a projectile remain constant while the vertical component changes?

Without air resistance nothing causes horizontal velocity to change, vertical motion is effected by the force of gravity