AP Physics 1 Unit 4 Progress Check A

A 5 kg block moves with a constant speed of 10 ms to the right on a smooth surface where frictional forces are considered to be negligible. It passes through a 2.0 m rough section of the surface where friction is not negligible, and the coefficient of kinetic friction between the block and the rough section μk is 0.2. What is the change in the kinetic energy of the block as it passes through the rough section?

20 J of energy is removed from the block.

A block of mass M on an inclined surface is attached to a spring of negligible mass, as shown. The other end of the spring is attached to a wall, and there is negligible friction between the block and the incline. The block is pulled to a position such that the spring is stretched from its equilibrium position. The block is then released from rest. Which of the following systems can be classified as a closed system?

A system consisting of the block, spring, and Earth

A rock of mass M is thrown from the edge of a cliff of height h with an initial velocity v0 at an angle θ with the horizontal, as shown in the figure. Point P is the highest point in the rock's trajectory, and point Q is level with the initial position of the rock. All frictional forces are considered to be negligible. Which of the following could correctly describe the total energy of the rock-Earth system at points P and Q?

Both P and Q: Mgh+12Mv20

A student must determine the effect of friction on the mechanical energy of a small block as it slides up a ramp. The block is launched with an initial speed v0 from point A along a horizontal surface of negligible friction. It then slides up a ramp, where friction is not negligible, that is inclined at angle θ with respect to the horizontal, as shown in the figure. The student measures the maximum vertical height h attained by the block while on the ramp, labeled as point B in the figure. At point B, the block comes to rest. The student performs three trials with the ramp at different angles, launching the block at the same initial speed v0 for each trial. The results from the trials are displayed in the table. Consider the trial with the 45° ramp. Suppose the block is launched up the ramp such that it comes to rest at point B and then travels down the ramp. Which of the following best describes the block's kinetic energy KA when it reaches point A at the bottom of the ramp in comparison to the initial kinetic energy K0 before it travels up the ramp?

KA<K0KA<K0 , because the force of friction removes mechanical energy from the block-ramp-Earth system on its way up the ramp and back down the ramp.

A toy car has an initial acceleration of 2m/s2 across a horizontal surface after it is released from rest. After the car travels for a time t=5 seconds, the speed of the car is 25m/s. Is the system consisting of only the car an open system or a closed system, and why?

Open system, because an external force is applied to the car that causes it to accelerate.

A student performs an experiment in which a ball travels in a perfect circle. The ball is attached to a string and travels in the horizontal, circular path, as shown in Figure 1. At time t0, the ball has a speed ν0. During the time interval of 0s to 2s, the force of tension in the string is recorded and graphed, as shown in Figure 2. Is the system consisting of the ball, string, and student an open system or closed system, and why?

Open system, because the force due to gravity from Earth is an external force that is exerted on the ball-string-student system

A planet orbits a star along an elliptical path from point X to point Y, as shown in the figure. In which of the following systems does the total mechanical energy of the system remain constant?

The closed system containing the planet and the star

Objects X and Y are connected by a string of negligible mass and suspended vertically over a pulley of negligible mass, creating an Atwood's machine, as shown in the figure. The objects are initially at rest, and the mass of object Y is greater than the mass of object X. As object Y falls, how does the kinetic energy of the center of mass of the two-object system change? Justify your selection. All frictional forces are considered to be negligible.

The kinetic energy increases because the gravitational force due to Earth does positive net work on the system.

A small block of mass M=0.10 kg is released from rest at point 1 at a height H=1.8 m above the bottom of a track, as shown in the diagram. It slides down the track and around the inside of a loop of radius R=0.6 m. The speed of the block is 2.5 m/s at point 3. Which of the following claims about the situation is correct?

The mechanical energy of the block-Earth system at point 3 is less than the mechanical energy of the block-Earth system at point 1.

A ball of mass M is attached to a string of negligible mass that has a length R. The ball moves clockwise in a vertical circle, as shown above. Which of the following is true about the ball-string-Earth system as the ball moves from point 1 to point 2?

The potential energy decreases by 2MgR2MgR and the tension in the string increases by more than 2Mg2Mg.

A 5 kg object near Earth's surface is released from rest such that it falls a distance of 10 m. After the object falls 10 m, it has a speed of 12 m/s. Which of the following correctly identifies whether the object-Earth system is open or closed and describes the net external force?

The system is open, and the net external force is nonzero.

The total mechanical energy of a system as a function of time is shown in the graph. Which of the following statements is true regarding the system?

The system should be classified as an open system because mechanical energy can be added and removed from the system.

A student must determine the effect of friction on the mechanical energy of a small block as it slides up a ramp. The block is launched with an initial speed v0 from point A along a horizontal surface of negligible friction. It then slides up a ramp, where friction is not negligible, that is inclined at angle θ with respect to the horizontal, as shown in the figure. The student measures the maximum vertical height h attained by the block while on the ramp, labeled as point B in the figure. At point B, the block comes to rest. The student performs three trials with the ramp at different angles, launching the block at the same initial speed v0 for each trial. The results from the trials are displayed in the table. Consider the trial in which the ramp is at a 20° angle with the horizontal. The surface of the ramp has been replaced with a surface in which frictional forces are considered to be negligible. If the mass of the block is doubled and the initial launch speed is doubled, how could the student predict the new vertical of the block at point B?

Use 12mv2initial=mgyfinal12mvinitial2=mgyfinal to solve for yfyf

A student must determine the effect of friction on the mechanical energy of a small block as it slides up a ramp. The block is launched with an initial speed v0 from point A along a horizontal surface of negligible friction. It then slides up a ramp, where friction is not negligible, that is inclined at angle θ with respect to the horizontal, as shown in the figure. The student measures the maximum vertical height h attained by the block while on the ramp, labeled as point B in the figure. At point B, the block comes to rest. The student performs three trials with the ramp at different angles, launching the block at the same initial speed v0 for each trial. The results from the trials are displayed in the table. How should the student use the data collected and the known quantities from the experiment to determine the total mechanical energy of the block-ramp-Earth system for all trials in the experiment?

Use K=1/2 mv2K=1/2mv2 with the block's initial speed for one trial because the initial speed is the same in all trials.

A block on a rough, horizontal surface is attached to a horizontal spring of negligible mass. The other end of the spring is attached to a wall. The spring is compressed such that the block is located at position X. When the block-spring system is released, the block travels to the right through position Y and continues to travel to the right through position Z. Free body diagrams for the block at positions X, Y, and Z are shown in the figure. At which position does the block have the greatest kinetic energy?

Y

A small object of mass M is shot horizontally from a spring launcher that is attached to a table. All frictional forces are considered to be negligible. The ball strikes the ground a distance D from the base of the table, as shown in the figure. A second object of mass M2 is launched from the same launcher such that the spring is compressed the same distance as in the original scenario. The distance from the base of the table that the object lands is

greater than D but less than 2D