Review - Kinematics

Consider a deer that runs from point A to point B. The distance the deer runs can be greater than the magnitude of its displacement, but the magnitude of the displacement can never be greater than the distance it runs.

• True

• False

Answer explanation

The statement is true because the distance a deer runs (the total path length) can exceed the straight-line distance between points A and B (displacement), but not the other way around.

Suppose that an object travels from one point in space to another. Make a comparison between the magnitude of the displacement and the distance traveled by this object.

• The displacement can be either greater than, smaller than, or equal to the distance traveled.

• The displacement is either greater than or equal to the distance traveled.

• The displacement is always equal to the distance traveled.

• The displacement is either less than or equal to the distance traveled.

Answer explanation

Displacement is the shortest distance between two points, while distance traveled accounts for the actual path taken. Therefore, displacement is always less than or equal to the distance traveled.

Consider a car that travels between points A and B. The car's average speed can be greater than the magnitude of its average velocity, but the magnitude of its average velocity can never be greater than its average speed.

• True

• False

Answer explanation

The average speed is the total distance divided by total time, while average velocity is the total displacement divided by total time. Since displacement is less than or equal to distance, average speed can exceed average velocity, making the statement true.

Which of the following quantities has units of a displacement? (There could be more than one correct )

• 40 km southwest

• -120 m/s

• 9.8 m/s^2

• 186,000 mi East

• 32 ft/s^2 vertically downward

Answer explanation

Displacement is a vector quantity that includes direction. '40 km southwest' and '186,000 mi East' both specify a distance and direction, making them displacements. The other options represent speed or acceleration, not displacement.

If the velocity of an object is zero at some point, then its acceleration must also be zero at that point.

• True

• False

Answer explanation

The statement is false because an object can have zero velocity while still experiencing acceleration, such as at the peak of a thrown object's trajectory. Acceleration is independent of velocity.

If the acceleration of an object is zero, then that object cannot be moving.

• True

• False

Answer explanation

The statement is false because an object can be moving at a constant velocity while having zero acceleration. Zero acceleration means no change in velocity, not that the object is at rest.

If the velocity of an object is zero, then that object cannot be accelerating.

• True

• False

Answer explanation

The statement is false because an object can have zero velocity and still be accelerating. For example, an object at rest can start to move, indicating acceleration even when its initial velocity is zero.

Tags

NGSS.HS-PS2-1

Which of the following situations is impossible?

• An object has velocity directed east and acceleration directed west.

• An object has zero velocity but non-zero acceleration.

• An object has constant non-zero velocity and changing acceleration.

• An object has constant non-zero acceleration and changing velocity.

• An object has velocity directed east and acceleration directed east.

Answer explanation

An object with constant non-zero velocity cannot have changing acceleration, as this would imply a change in speed or direction, contradicting the definition of constant velocity.

Tags

NGSS.HS-PS2-1

NGSS.HS-PS2-2

Suppose that a car traveling to the west begins to slow down as it approaches a traffic light. Which of the following statements about its acceleration is correct?

• The acceleration is toward the east.

• The acceleration is toward the west.

• The acceleration is zero.

• Since the car is slowing down, its acceleration must be negative.

Answer explanation

As the car slows down while moving west, its acceleration is in the opposite direction of its velocity, which is east. Therefore, the correct statement is that the acceleration is toward the east.

Under what condition is average velocity equal to the average of the object's initial and final velocity?

• The acceleration must be constantly decreasing.

• The acceleration must be constantly increasing.

• This can occur only when the velocity is zero.

• The acceleration is constant.

• This can only occur if there is no acceleration.

Answer explanation

Average velocity equals the average of initial and final velocity when acceleration is constant. This is because constant acceleration ensures a linear relationship between velocity and time, allowing for this equality.