Friction and Gravitational Forces

The bumpy pattern on the soles of shoes is called the tread. Melanie and Keisha are at the store looking at the treads on different basketball shoes. They want new shoes so that they do not slide on the basketball court. Which of these situations produces the most friction between the shoes and the floor?

• a girl with a less mass wearing shoes with more tread

• a girl with a less mass wearing shoes with less tread

• a girl with greater mass wearing shoes with more tread

• a girl with greater mass wearing shoes with less tread

Answer explanation

The correct answer is C. The most friction occurs when a girl with greater mass wears shoes with more tread. The two factors that affect the amount of friction between two surfaces in contact are the types of surfaces involved and how hard the surfaces are pushed together. The floor will always be the same, so the amount of tread and material of the shoe will determine the amount of friction. The girl with greater mass will push the sole and floor together harder, and a lot of tread on the sole will produce more friction.

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NGSS.MS-PS2-2

An astronaut in space might feel weightless, but there still is a gravitational attraction between the astronaut and Earth. Which two factors affect this gravitational attraction?

• the masses of Earth and the astronaut

• the weight of Earth and the astronaut

• the rotation of the Earth

• the distance between Earth and the astronaut

Answer explanation

The correct answers are A and D. Gravitational attraction exists between any two objects. The amount of gravitational attraction depends on the mass of the objects and the distance between them. The rotation of Earth does not change its mass or its distance from the astronaut, so it does not influence the amount of gravitational attraction. Weight is the effect of gravity on mass and does not determine gravitational attraction.

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David delivers meals to elderly people once a week. He uses a cart to move the meals. The cart has four smooth wheels. Which type of friction acts between the cart wheels and the sidewalk?

• sliding friction

• rolling friction

• fluid friction

• static friction

Answer explanation

The correct answer is B. Rolling friction occurs between the cart’s wheels and the sidewalk. The different types of friction are based on different types of contact between surfaces. Sliding friction occurs when one object slides across another. While all friction increases the force needed to move an object forward, rolling friction will be less than sliding friction for the same area of contact between two materials. Fluid friction occurs when a solid object moves through a liquid or a gas.

Miguel learned about the difference between mass and weight. He wanted to investigate what he would weigh at different locations in our solar system. He used this table to organize data he found out about Earth, the moon, Neptune, and Mercury. He needs to fill out the rest of the table. How would Miguel's mass on Neptune compare to his mass on Earth?

• It is less.

• It is greater.

• It is the same.

• It varies compared to the distance between the planets.

Answer explanation

The correct answer is C. Mass is the amount of matter in an object, so mass does not change from location to location. A mass of 60 kg on Earth would be a mass of 60 kg anywhere in the universe. The masses are the same.

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Pam’s family is clearing some land, and there is a large boulder sitting on top of the ground. Even with the entire family pushing on it, they cannot move it. Which type of friction exists between the boulder and the ground?

• fluid

• rolling

• sliding

• static

Answer explanation

The correct answer is D. The rock is on dry land, so A is not correct. The rock does not move in either of those fashions, So B and C are not correct.

Do you believe that you are pulled towards your science book, a bunch of grapes in your lunch, and your school building because of universal gravitation? Which two statements about universal gravitation are true?

• Gravity acts between you and each of these objects because all have mass.

• Gravity does not act between objects that are far apart, like you and the sun.

• Gravity only pulls things in a downward direction, like toward Earth's surface.

• Gravity acts between any two objects in the universe.

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Pascal knows that a falling body accelerates at a constant rate, and he also knows that the same is true of a ball rolling down a ramp, as in the figure. The source of the force that accelerates the ball rolling down the ramp is The ______________ attraction of Earth accelerates the ball.

Answer explanation

The gravitational attraction of Earth accelerates the ball. Gravity acts downward.

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NGSS.MS-PS2-4

Pascal knows that a falling body accelerates at a constant rate, and he also knows that the same is true of a ball rolling down a ramp, as in the figure. The source of the force that accelerates the ball rolling down the ramp is The direction of the force is _____________.

Answer explanation

The gravitational attraction of Earth accelerates the ball. Gravity acts downward.

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Pascal knows that a falling body accelerates at a constant rate, and he also knows that the same is true of a ball rolling down a ramp, as in the figure. The force that moves the ball is not the only force acting on it. The ramp also exerts an upward force on the ramp, which counters much of the force of _______________.

Answer explanation

The ramp also exerts an upward force on the ramp, which counters much of the force of gravity. The force of the ramp is perpendicular to the ramp and to the direction of motion of the ball, so it does no work.

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NGSS.MS-PS2-4

Pascal knows that a falling body accelerates at a constant rate, and he also knows that the same is true of a ball rolling down a ramp, as in the figure. The force of the ramp is perpendicular to the ramp and to the direction of motion of the ball, so it does no _________.

Answer explanation

The ramp also exerts an upward force on the ramp, which counters much of the force of gravity. The force of the ramp is perpendicular to the ramp and to the direction of motion of the ball, so it does no work.

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NGSS.MS-PS2-2

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