8th Grade Science - Topics 4, 5, and 6 Common Assessment Review

A girl pushes on the classroom door to open it. Which two terms describe the net force acting on the door?

• Contact and balanced

• Contact and unbalanced

• Noncontact and balanced

• Noncontact and unbalanced

Answer explanation

The girl applies a force directly to the door, making it a contact force. Since she is pushing it open, the forces are not equal, resulting in an unbalanced force. Thus, the correct terms are contact and unbalanced.

Tags

NGSS.MS-PS2-2

A dog is pulling on its leash while being walked down the sidewalk. What reference point would indicate the dog is not moving?

• A nearby building

• The leash

• The sidewalk

• A tree along the sidewalk

Answer explanation

The leash is the reference point that indicates the dog is not moving because it is attached to the dog. If the leash remains taut, it shows the dog is stationary relative to the person holding it, despite any pulling.

Two dogs pull on a rope. One dog pulls with a force of 5 Newtons to the left, and the other dog pulls with a force of 3 Newtons to the right. What is the result?

• The rope remains in place without moving

• The rope moves to the left.

• The rope moves to the right.

• The rope has a balanced force applied to it.

Answer explanation

The leftward force of 5 Newtons is greater than the rightward force of 3 Newtons. Therefore, the net force is 2 Newtons to the left, causing the rope to move to the left.

Tags

NGSS.MS-PS2-2

A bus driver drove from Philadelphia to Washington, D.C. He drove the first 100 kilometers in 2 hours, the next 55 kilometers in 1 hour, and the final 75 kilometers in 2 hours. What is the average speed of the bus throughout the trip?

• 46km/hr

• 50km/hr

• 77km/hr

• 81km/hr

Answer explanation

Total distance = 100 + 55 + 75 = 230 km. Total time = 2 + 1 + 2 = 5 hours. Average speed = total distance / total time = 230 km / 5 hr = 46 km/hr. Thus, the average speed of the bus is 46 km/hr.

Which statement about acceleration is always true?

• Acceleration is always one dimensional.

• Objects can accelerate only by speeding up.

• Either a change in speed or a change in direction causes acceleration.

• Both speed and direction must always change for an object to accelerate.

Answer explanation

Acceleration occurs when there is a change in speed, direction, or both. Thus, the statement 'Either a change in speed or a change in direction causes acceleration' is always true.

A soccer player kicks a ball. Which of the following describes the reaction force to this kick?

• Friction between the ball and the player's foot.

• Friction between the ball and the ground.

• Force applied to the ground by the player's foot.

• Force applied to the player's foot by the ball.

Answer explanation

The reaction force to the kick is the force applied to the player's foot by the ball, as per Newton's third law. When the player kicks the ball, the ball exerts an equal and opposite force back on the player's foot.

Tags

NGSS.MS-PS2-1

Which term describes resistance to change in motion?

• Acceleration

• Intertia

• Net force

• Velocity

Answer explanation

Inertia is the term that describes an object's resistance to changes in its state of motion. It explains why an object at rest stays at rest and an object in motion continues in motion unless acted upon by a net force.

The acceleration of a baseball after it is hit by a bat depends on the mass of the ball and the net force on the ball. This example best illustrates which of Newton's Laws?

• Newton's first law of motion

• Newton's second law of motion

• Newton's third law of motion

• Newton's law of universal gravitation

Answer explanation

This example illustrates Newton's second law of motion, which states that acceleration depends on the net force acting on an object and its mass, as seen when a bat hits a baseball.

Tags

NGSS.MS-PS2-2

When is there static friction between your chair and the floor?

• When the chair sits still.

• When the chair falls to the floor.

• When you lift the chair.

• When you slide your hair across the floor.

Answer explanation

Static friction occurs when the chair sits still, preventing it from moving. This force acts between the chair and the floor, keeping the chair in place until a sufficient external force is applied.

The invisible area around a charge in which the electric force is experienced by other charged objects is the:

• Electric charge

• Electric force

• Electric field

• Electric current

Answer explanation

The electric field is the region around a charged object where it exerts a force on other charges. It describes how the electric force is experienced by other charged objects in its vicinity.

Tags

NGSS.MS-PS2-5