1. Which statement below is true?
a. If a car is traveling eastward, its acceleration is eastward.
b. If a car's speed is decreasing, its acceleration must be negative.
c. If a particle is moving, it must have a non-zero acceleration.
d. A moving particle with constant acceleration can never stop and stay stopped.
2. A projectile is launched straight upwards at +45 m/s. Three seconds later, its velocity is approximately:
a. +75 m/s
b. +30 m/s
c. +15 m/s
d. 0 m/s
3. A ball is thrown straight up. At the top of its path its acceleration has a value (magnitude) of __________.
a. 0 m/s/s.
b. about 5 m/s/s.
c. about 10 m/s/s.
d. about 20 m/s/s.
e. about 50 m/s/s.
4. An object is shot vertically upward into the air with a positive initial velocity. Which of the following correctly describes
the velocity and acceleration of the object at its maximum elevation?
Velocity Acceleration
A) Positive Positive
B) Zero Zero
C) Negative Negative
D) Zero Negative
E) Positive Negative
Thank You for answering
Hey, I already took 8:01, freshman physics.
I am here to help if you get stuck, not to do the problems for you. These are very basic questions
1. c is not correct. a particle moving at constant velocity has no acceleration.
However how could a particle with constant acceleration stay stopped?
2. 45 - 10*3 = 15 which is c
3. The acceleration of g = -9.81 m/s^2 is CONSTANT, even if the speed is zero !
4. It is stopped, but the acceleration is STILL -9.81 m/s^2
from the time it is launched until it hits the ground or something.
1. a
1 d
1. The correct statement is b. If a car's speed is decreasing, its acceleration must be negative.
To understand why, we need to remember the fundamental relationship between velocity, speed, and acceleration. Acceleration is the rate at which velocity changes over time, and velocity is the rate at which displacement changes over time. Speed, on the other hand, is a scalar quantity that only tells us the magnitude of velocity.
When a car's speed is decreasing, it means that its velocity is changing in the opposite direction. Since velocity is a vector quantity (having both magnitude and direction), a change in direction means a change in the sign of velocity. Therefore, if the car is originally moving in the positive direction (e.g., eastward), its velocity will become negative (e.g., westward) as it slows down. Since acceleration is the rate of change of velocity, a negative change in velocity means a negative acceleration.
2. The correct answer is b. +30 m/s.
To find the velocity of the projectile three seconds later, we need to consider the acceleration due to gravity acting in the downward direction. When the projectile reaches its highest point, its vertical velocity will be zero, and it will start coming back down.
Using the equation of motion for constant acceleration (v = u + at), where u is the initial velocity, a is the acceleration, t is the time, and v is the final velocity, we can calculate the final velocity three seconds after launching:
v = u + at
v = 45 m/s + (-9.8 m/s^2) * 3 s
v ≈ 45 m/s - 29.4 m/s
v ≈ 15.6 m/s
The approximate final velocity of the projectile after three seconds is +15.6 m/s. Since we are considering the upward direction as positive, the answer is +30 m/s.
3. The correct answer is a. 0 m/s/s.
When a ball is thrown straight up, at the top of its path, its velocity momentarily becomes zero before it starts falling back down. At this point, the ball reaches its maximum height and the velocity changes direction but has zero magnitude.
Acceleration is the rate at which velocity changes over time. At the top of the ball's motion, its velocity is momentarily constant (zero) because it has come to a stop. Since velocity is not changing, the acceleration is zero.
4. The correct answer is D) Zero Negative.
When an object is shot vertically upward, it experiences acceleration due to gravity acting in the downward direction. As it moves upward, the acceleration acts to slow it down until it eventually comes to a stop at its maximum elevation.
At its maximum elevation, the object momentarily stops moving upwards and starts to fall back down. Its velocity at this point is zero since it has zero speed and is changing direction. Since the velocity changes from positive (upward) to negative (downward), the acceleration will be negative.
Therefore, the velocity at the maximum elevation is zero and the acceleration is negative. This is represented by the answer D) Zero Negative.
Sorry,
Answers:
1. c
2. d
3. a
4. b