Puerto Rico CRECE Science — Grade 8

Which fundamental force keeps planets in stable orbits around the Sun rather than flying off into deep space?

• AMagnetic force
• BElectromagnetic force
• CGravitational force
• DAtmospheric pressure

When two equal forces act on an object in opposite directions, what is the net force acting on the object?

• ADouble the strength of one force
• BZero net force
• CQuadruple the strength of one force
• DNegative velocity

Which of Newton's laws of motion states that an object at rest will remain at rest unless acted upon by an unbalanced external force?

• AFirst Law of Motion (Law of Inertia)
• BSecond Law of Motion (F = ma)
• CThird Law of Motion (Action-Reaction)
• DLaw of Universal Gravitation

A runner completes a 400-meter race around a track in 50 seconds. What is the runner's average speed?

• A4 m/s
• B8 m/s
• C20 m/s
• D200 m/s

According to Newton's Second Law of Motion (F = ma), if you double the net force applied to a toy car while keeping its mass constant, what happens to its acceleration?

• AThe acceleration drops to one-half of its original value.
• BThe acceleration doubles.
• CThe acceleration quadruples.
• DThe acceleration remains completely unchanged.

A swimmer pushes backward against the pool wall with their feet. According to Newton's Third Law, what propels the swimmer forward?

• AThe water absorbing the force without reacting
• BThe wall pushing forward against the swimmer's feet with equal and opposite force
• CFriction between water molecules pulling the swimmer's body forward
• DGravity pulling the swimmer toward the bottom of the pool

According to Newton's Law of Universal Gravitation, what happens to the gravitational force between two planets if the distance between their centers is doubled?

• AThe gravitational force increases by a factor of 4.
• BThe gravitational force is reduced to one-half (1/2) of its original strength.
• CThe gravitational force is reduced to one-fourth (1/4) of its original strength.
• DThe gravitational force remains unchanged.

A 10 kg wooden box is pulled across a flat floor with a horizontal forward force of 50 N. If frictional force opposing motion is 20 N, what is the box's acceleration?

• A7.0 m/s²
• B3.0 m/s²
• C5.0 m/s²
• D2.0 m/s²

A car travels 60 kilometers north in 1 hour. Which quantity describes this motion as a velocity rather than a speed?

• A60 km/h north
• B60 km/h
• C60 kilometers
• D1 hour

A bus pulling away from a stop goes from 0 m/s to 12 m/s in 6 seconds. Which physical quantity describes this change in motion?

• AAcceleration
• BMass
• CDisplacement
• DInertia

Which statement correctly distinguishes the mass of an object from its weight?

• AMass is the amount of matter in the object, while weight is the gravitational force actin…
• BMass is measured in newtons, while weight is measured in kilograms
• CMass changes when the object is moved to the Moon, while weight stays the same everywhere
• DMass and weight are two names for exactly the same physical quantity

In the International System of Units (SI), which unit is used to measure force?

• ANewton (N)
• BJoule (J)
• CWatt (W)
• DKilogram (kg)

A student lifts a backpack onto a shelf. Which SI unit should be used to report the work done on the backpack?

• AJoule (J)
• BNewton (N)
• CWatt (W)
• DMeter (m)

Two elevators lift identical loads through the same height, but one finishes the job in half the time. Which quantity is greater for the faster elevator?

• AIts power, measured in watts
• BThe work it does, measured in joules
• CThe force it applies, measured in newtons
• DThe mass of the load, measured in kilograms

A book rests motionless on a table. What can be concluded about the forces acting on the book?

• AThe forces are balanced, so the net force on the book is zero
• BNo forces at all are acting on the book
• CThe forces are unbalanced, but the book is too heavy to move
• DGravity has stopped acting on the book because it is supported

A wooden crate slides across a concrete floor and gradually slows to a stop. Which force is chiefly responsible for slowing the crate?

• AFriction between the crate and the floor
• BThe magnetic force of the concrete
• CThe upward support force of the floor
• DAir pressure pushing down from above

Which of the following devices is a simple machine?

• AAn inclined plane (ramp)
• BA bicycle
• CA hand-cranked drill
• DA wheelbarrow with a wheel and a lever handle

A mango falls from a tree branch to the ground. How is the force of gravity best described in this situation?

• AAn attractive force exerted by Earth that pulls the mango toward Earth's center
• BA push exerted by the air that drives the mango downward
• CA magnetic attraction between the mango's iron content and the soil
• DA force that only acts on objects once they begin to move

A worker pushes a cart with a steady horizontal force of 45 N and moves it 8 m in the direction of the force. How much work is done on the cart?

• A360 J
• B53 J
• C5.6 J
• D360 N

A student does 600 J of work carrying boxes up a staircase in 20 seconds. What power did the student develop?

• A30 W
• B12,000 W
• C620 W
• D30 J

A student holds a heavy box perfectly still at chest height for two minutes and becomes tired. In the language of physics, how much work is done on the box during those two minutes?

• AZero, because the box has no displacement
• BA large amount, because the student feels tired
• CA large amount, because the force is applied for a long time
• DIt cannot be determined without knowing the box's temperature

Workers use a long ramp instead of lifting barrels straight up into a truck. What is the main advantage of the ramp?

• AIt reduces the force needed, although the barrel must be moved through a longer distance
• BIt reduces the total work needed to raise the barrel to the truck bed
• CIt removes the force of gravity acting on the barrel
• DIt increases both the force needed and the distance travelled

A student uses a crowbar to lift a heavy rock. Which statement best explains how this simple machine helps?

• AThe lever multiplies the student's input force, so a smaller effort raises a heavier load
• BThe lever eliminates the rock's weight so no force is needed at all
• CThe lever increases the total energy available to the student
• DThe lever converts the student's push into a magnetic force

Which everyday object is best classified as a compound machine?

• AA pair of scissors, which joins two levers to two wedge-shaped blades
• BA single fixed pulley on a flagpole
• CA doorstop shaped like a wedge
• DA metal screw used to join two boards

An astronaut and her equipment have a total mass of 90 kg on Earth. After she lands on the Moon, where gravity is about one-sixth as strong, what happens to her mass and her weight?

• AHer mass stays 90 kg, but her weight becomes about one-sixth of its value on Earth
• BBoth her mass and her weight become about one-sixth of their values on Earth
• CHer mass becomes about one-sixth of 90 kg, but her weight stays the same
• DBoth her mass and her weight remain exactly the same as on Earth

A cyclist speeds up steadily from 4 m/s to 16 m/s over 4 seconds. What is the cyclist's acceleration?

• A3 m/s²
• B4 m/s²
• C5 m/s²
• D12 m/s²

A net force of 24 N gives a toy wagon an acceleration of 3 m/s². Using F = ma, what is the mass of the wagon?

• A8 kg
• B72 kg
• C27 kg
• D21 kg

A model rocket pushes hot gases downward out of its nozzle and rises upward. Which of Newton's laws explains this and what is the action-reaction pair?

• AThe third law: the rocket pushes the gases down and the gases push the rocket up with an…
• BThe first law: the rocket stays at rest until the fuel is used up
• CThe second law: the rocket's mass increases as the gases leave the nozzle
• DThe third law: gravity pulls the rocket down and air resistance pushes it up equally

Two spacecraft drift far from any star. If the distance between them stays the same but the mass of one spacecraft is doubled, how does the gravitational force between them change?

• AIt doubles, because gravitational attraction increases with the masses of the interacting…
• BIt is cut in half, because heavier objects attract less strongly
• CIt stays the same, because only distance affects gravitational force
• DIt becomes zero, because gravity acts only near planets

Sirius is a star far more luminous than the Sun, yet sunlight reaching Earth is overwhelmingly brighter than the light from Sirius. What best explains this observation?

• AThe Sun is vastly closer to Earth, and the light reaching us weakens rapidly as distance…
• BSirius emits no visible light, only invisible radiation
• CEarth's atmosphere blocks all starlight except sunlight
• DThe Sun is the most luminous star in the entire galaxy

Which statement correctly describes what holds the components of the solar system in their observed motions?

• AThe Sun's gravitational attraction keeps planets, moons, meteoroids, and other bodies in…
• BMagnetic forces from the Sun's poles hold each planet in place
• CEach planet is pushed along its path by the solar wind
• DThe planets remain in orbit because there is no force at all acting on them in space

A hoist raises a 250 N crate straight up through 6 m in 10 seconds. What work is done on the crate and what power does the hoist develop?

• A1,500 J of work and 150 W of power
• B1,500 J of work and 15,000 W of power
• C150 J of work and 1,500 W of power
• D2,500 J of work and 250 W of power

A 5 kg cart is pushed forward with a force of 30 N while friction opposes the motion with a force of 10 N. What is the cart's acceleration?

• A4 m/s²
• B6 m/s²
• C8 m/s²
• D2 m/s²

What energy transformation occurs as a cart rolls down a steep hill from the top peak?

• AKinetic energy transforms into potential energy
• BGravitational potential energy transforms into kinetic energy
• CChemical energy transforms into nuclear energy
• DThermal energy transforms into electrical energy

Which mechanism of heat transfer is responsible for heating a metal spoon when it sits inside a hot bowl of soup?

• AConvection
• BRadiation
• CConduction
• DInsulation

If a hot copper block at 90°C is placed in contact with a cool aluminum block at 20°C in an isolated container, what will happen?

• AThermal energy flows from the copper block to the aluminum block until both reach the sam…
• BThermal energy flows from the aluminum block to the copper block, making the copper hotte…
• CNo heat transfer occurs because both objects are solid metals.
• DBoth blocks instantly cool to 0°C.

What fundamental principle explains why energy in a closed system can change forms (e.g. electrical to light and thermal) but total energy remains constant?

• ALaw of Conservation of Energy
• BLaw of Universal Gravitation
• COhm's Law of Resistance
• DNewton's First Law of Inertia

Equal masses of iron, sand, aluminum, and liquid water receive identical amounts of thermal heat energy under full sunlight. Why does liquid water experience the smallest temperature increase?

• AWater has the highest specific heat capacity among the tested substances.
• BWater has zero thermal mass.
• CWater conducts heat faster than metals.
• DWater molecules destroy thermal energy upon heating.

Which statement best defines energy as it is used in physics?

• AThe capacity of a system to do work or to cause change
• BThe total amount of matter contained in an object
• CThe force that an object exerts on its surroundings
• DThe distance an object moves in a fixed time interval

A stone is held motionless at the top of a tall wall. Which type of energy does the stone possess because of its raised position?

• AGravitational potential energy
• BKinetic energy
• CSound energy
• DElectrical energy

Which of the following is a renewable source of energy?

• AWind
• BCoal
• CPetroleum
• DNatural gas

Which material is the best electrical conductor?

• ACopper
• BRubber
• CGlass
• DDry wood

What is electric current?

• AThe flow of electric charge through a conductor
• BThe total charge stored on an insulating surface
• CThe magnetic force between two bar magnets
• DThe energy needed to lift an object against gravity

A bar magnet is brought near a second bar magnet. Which pairing of poles will repel each other?

• ANorth pole facing north pole
• BNorth pole facing south pole
• CSouth pole facing north pole
• DAny two poles, because magnets always repel

Two carts of equal mass roll along a track, but Cart B moves twice as fast as Cart A. How does Cart B's kinetic energy compare with Cart A's?

• ACart B has four times the kinetic energy of Cart A
• BCart B has twice the kinetic energy of Cart A
• CCart B has the same kinetic energy as Cart A
• DCart B has half the kinetic energy of Cart A

A student graphs the kinetic energy of a rolling ball against its mass while keeping its speed constant. What shape should the graph have?

• AA straight line rising from the origin, because kinetic energy is directly proportional t…
• BA curve that bends upward, because kinetic energy depends on the square of the mass
• CA horizontal line, because kinetic energy does not depend on mass
• DA line falling from left to right, because heavier balls store less energy

Two magnets are held with like poles facing each other and then pushed closer together. How does the energy stored in the system change?

• AThe stored potential energy increases, because work must be done against the repulsive fo…
• BThe stored potential energy decreases, because the magnets are closer together
• CThe stored potential energy stays the same, because the magnets never touch
• DNo energy is stored, because magnetic forces do not act at a distance

A crane does 4,000 J of work lifting a load. Ignoring friction and air resistance, how much gravitational potential energy does the load gain?

• A4,000 J
• B0 J, because work and energy are unrelated quantities
• C8,000 J, because work always doubles the stored energy
• DIt cannot be determined without knowing the crane's power rating