Physics Praxis

Which of the following physical properties is a measure of the force exerted by a fluid on a given area?

• viscosity

• pressure

• buoyancy

• density

Answer explanation

A fluid exerts pressure on the walls of its container. The pressure equals the force exerted by the fluid divided by the area of contact.

Viscosity is the resistance of a fluid (liquid or gas) to a change in shape or movement of neighbouring portions relative to one another

buoyancy- the ability or tendency to float in water or air or some other fluid.

Jorge uses a constant force F to push a frictionless shopping cart along a sidewalk. The cart experiences an acceleration a. What acceleration will the cart experience if the force remains the same but the mass of the cart doubles?

• F/4

• 2F

• F/2

Answer explanation

According to Newton's second law, the net force F on a cart equals the cart's mass m multiplied by its acceleration a. F_net = ma. Mass and acceleration are inversely proportional. If the force remains constant and the mass doubles, the acceleration must be halved.

Tags

NGSS.MS-PS2-2

Which distance vs. time graph best represents the motion of a child running toward you at constant speed?

• 
  a

• b

• c

• d

Answer explanation

Graph A best represents the motion. The child starts at a distance far from you and moves toward you, as represented by the line starting high on the y-axis and moving toward 0 distance. The child runs at constant speed, so they cover the same distance each second as represented by the straight line with uniform slope.

Tags

NGSS.MS-PS3-1

The gravitational force from the sun extends:

• to infinity.

• only to the edge of the Solar System.

• only to the edge of the Milky Way Galaxy.

• only to the core of the Earth.

Answer explanation

The gravitational force from any object extends to infinity. The strength of the force decreases with distance, but the force never decreases to zero. This means every object in the Universe is pulled by every other object in the Universe.

Tags

NGSS.MS-ESS1-1

NGSS.MS-ESS1-2

During a tug-of-war game, the following students pull on the rope: Billy pulls leftward with 380 N, Susie pulls leftward with 250 N, Omar pulls leftward with 310 N, Julius pulls rightward with 290 N, Sarina pulls rightward with 270 N, and Ricardo pulls rightward with 420 N. What is the magnitude of the net force on the rope?

• 
  1920 N

• 980 N

• 940 N

• 40 N

Answer explanation

The net force on the rope is the vector sum of the forces.

The sum of the leftward forces is 380 N + 250 N + 310 N = 940 N.

The sum of the rightward forces is 290 N + 270 N + 420 N = 980 N.

Subtract the total leftward force from the total rightward force to find the magnitude of the net force: 980 N – 940 N = 40 N.

Tags

NGSS.MS-PS2-2

A block of ice of mass m slides down a frictionless ramp inclined 15° from the horizontal. Which of the following is the correct expression for the magnitude of the acceleration of the box?

• 
  g sin 15°

• mg sin 15°

• mg cos 15°

• 
  g cos 15°

Answer explanation

Two forces act on the ramp: the force of gravity, F_g = mg, and the normal force F_n, which points upward, perpendicular to the ramp. The component of gravity that points parallel to the ramp is mg sin 15°. From Newton's second law, the force along the ramp divided by mass equals the acceleration along the ramp. The acceleration is g sin 15°.

Tags

NGSS.HS-PS2-1

If an object has a mass of 3 kilograms on Earth, which of the following correctly describes its mass in interstellar space where there is no gravity?

• zero kilograms

• exactly 3 kilograms

• more than 3 kilograms

• less than 3 kilograms

Answer explanation

Mass is a measure of the amount of matter in an object. Mass does not depend on the strength of gravity. A mass that is 3 kilograms on Earth will still be 3 kilograms in interstellar space.

weight is different would be less in space

The graph gives the position of x, an object, from t = 0 seconds, to t = 15 seconds. Which statement is true about the motion of the object?

• The object has a constant positive acceleration for the first five seconds, a constant negative velocity for the next five seconds, and a constant negative acceleration for the last five seconds.

• The object has a constant positive velocity for the first five seconds, is stopped for the next five seconds, and has a constant negative velocity for the last five seconds.

• The object has a constant positive acceleration for the first five seconds, is stopped the next five seconds, and has a constant negative acceleration for the last five seconds.

Answer explanation

On a position-vs-time graph, constant, positive velocity is shown by a straight line with a constant, positive slope. Constant negative velocity is shown by a straight line with a constant, negative slope. A straight horizontal line indicates no movement. Acceleration and deceleration are represented with curves. In the graph shown, the object has a constant positive velocity for the first five seconds, is stopped for the next five seconds, and has a constant negative velocity for the last five seconds.

Tags

NGSS.HS-PS2-1

A 1-kilogram bowling ball weighs about 10 Newtons on Earth. Which of the following correctly describes its weight and mass on Mars?

• weight on Mars < 10 Newtons; mass on Mars < 1 kilogram

• weight on Mars < 10 Newtons; mass on Mars = 1 kilogram

• weight on Mars < 10 Newtons; mass on Mars = 1 kilogram

Answer explanation

The weight of the bowling ball on Mars will be less than its Earth weight of 10 Newtons, but its mass on Mars will equal its 1 kilogram mass on Earth. Weight is a measure of how hard one mass pulls on another. The weight of the bowling ball is lower on Mars because Mars is less massive than Earth. Mass is a measure of the amount of matter in an object, so the mass of the ball does not change when the ball is moved to Mars.

Which of the following is a fundamental force in nature?

• Gravitational force

• Frictional force

• Centripetal force

• Tension force

Tags

NGSS.MS-PS2-4