Category Archives: High School

Items appropriate for use in a high school physics course.

A2L Item 145

Goal: Reasoning with energy

Source: UMPERG-ctqpe64

Two
masses, M > m, travel down the surfaces shown. Both surfaces are
frictionless. Which mass has the largest speed at the bottom?

  1. m
  2. M
  3. Both have the same speed
  4. Cannot be determined

Commentary:

Answer

(3) By energy considerations, both would have the same speed.
Students frequently get confused about the mass, thinking that the
larger mass has the greatest potential energy change and therefore has
the greatest speed.

A2L Item 143

Goal: Reasoning with energy

Source: UMPERG-ctqpe62

Two
identical blocks fall a distance H. One falls directly down, the other
slides down a frictionless incline. Which has the largest speed at the
bottom?

  1. The one falling vertically
  2. The one on the incline
  3. Both have the same speed
  4. Cannot be determined

Commentary:

Answer

(3) The only force doing work is gravity and both block undergo
the same vertical displacement.

A2L Item 136

Goal: Reasoning with dynamics

Source: UMPERG-ctqpe43

Block
m1 sits on block m2 and both sit on the floor of
an elevator at rest. When the elevator starts to move down, the normal
force on the upper block will …

  1. increase.
  2. remain the same.
  3. decrease.
  4. Cannot be determined

Commentary:

Answer

(3) As it starts the elevator must accelerate downward and so
will the upper block. The only forces on the block are gravity and the
normal force. The normal force must diminish so gravity can provide the
downward acceleration.

Students answering #2 may have interpreted the question to mean ‘as the
elevator moves’ and think that the elevator moves with constant velocity.

A2L Item 131

Goal: Reasoning with dynamics

Source: UMPERG-ctqpe30

A
block of mass m, when placed on a rough inclined plane and moved, moves
down the plane with constant speed. If a block of mass 2m were placed
on the same incline and moved, it would …

  1. return to rest.
  2. accelerate until the speed is half.
  3. move with some constant speed.
  4. None of the above.
  5. Cannot be determined

Commentary:

Answer

The block will have the same motion. Both the gravitational force
and the friction force scale with the mass so there is no net force in
either case.

A2L Item 130

Goal: Hone the concept of static friction

Source: UMPERG-ctqpe29

A block of mass m sits at rest on a
rough incline of angle θ. The coefficient of static friction is
μ.

The friction force on the block is

  1. mgcos(θ), down the incline.
  2. mgsin(θ), up the incline.
  3. μmgsin(θ), down the incline.
  4. μmgcos(θ), up the incline.
  5. none of the above.
  6. cannot be determined

Commentary:

Answer

(2) this is all that is needed to hold the block at rest. Some
students will give #4 as the answer having specified the maximum static
friction force.

It helps to classify forces as model forces obtainable from a formula,
and procedure forces. Static friction is an example of a procedure
force, one that cannot be determined without application of the 2nd law.

A2L Item 128

Goal: Hone the concept of static friction

Source: UMPERG-ctqpe27

A mass of 5 kg sits at rest on an incline making an angle of 30° to
the horizontal.

If μs = 0.7 the friction force on the block is

  1. 43.3N, down the incline
  2. 25N, up the incline
  3. 10N, down the incline
  4. 30.3N, up the incline
  5. none of the above
  6. cannot be determined

Commentary:

Answer

(2) this is all that is needed to hold the block at rest. Some
students will give #4 as the answer having calculated the maximum static
friction force.

It helps to classify forces as model forces obtainable from a formula,
and procedure forces. Static friction is an example of a procedure
force, one that cannot be determined without application of the 2nd law.

A2L Item 126

Goal: Problem solving

Source: UMPERG-ctqpe23

If m2 = 2m1 , m3 = 3m1 and
the force that the surface exerts on the bottom block is 120N, the mass
of the large block is

Use g = 10 N/kg.

  1. 2 kg
  2. 4 kg
  3. 6 kg
  4. 8 kg
  5. 12 kg
  6. 20 kg
  7. none of the above

Commentary:

Answer

(3) the most common other answers are #1 and #6, each of which
represents a typical mistake students make. This problem presents a good
opportunity to encourage students to check their answer.

A2L Item 125

Goal: Problem solving with dynamics

Source: UMPERG-ctqpe21

Two blocks rest on a frictionless surface. Both blocks move to the right
with acceleration of 2 m/s2. The force on the big block due
to the small block is

  1. 14N to the right
  2. 10N to the left
  3. 8N to the right
  4. 6N to the left
  5. 4N to the right
  6. 2N to the left
  7. none of the above
  8. cannot be determined

Commentary:

Answer

(7) The force on the small block must cause the specified acceleration.
The 3rd law requires that the force on the big block be equal and
opposite. The magnitude is 4N but it is directed to the left.

A2L Item 123

Goal: Reasoning with electric fields.

Source: UMPERG-A2LEM7

The diagrams show two uniformly charged spheres. The charge on the
right sphere is three times as large as the charge on the left sphere.
Each vector represents the electric field at the center of one sphere
caused by the other sphere. Which choice best represents the
magnitude and direction of the electric field vectors caused by one
sphere at the position of the other sphere?

Commentary:

Answer

(5) The electric field at the sphere with smaller charge is three times
larger than the field at the sphere with larger charge.

A2L Item 122

Goal: Reasoning with Coulomb’s law

Source: UMPERG-A2LEM6

The diagrams show two uniformly charged spheres. The charge on the
right sphere is three times as large as the charge on the left sphere.
The arrows on each charge represent the force on the charge. Which
force diagram best represents the magnitude and direction of the
electric forces on the two spheres?

Commentary:

Answer

(4) By Newton’s third law the forces are equal and opposite.