Big Idea 13 - Forces and Changes in Motion

Ben, Brandon, and Nico, are skydiving and jump out of an airplane, beginning to fall toward the Earth. Which of the following best describes the primary force acting on them?

• A push toward the clouds

• A pull toward the center of the Earth

• A push from the airplane’s engines

• A magnetic pull from the North Pole

Answer explanation

The primary force acting on the skydiver is gravity, which pulls objects toward the center of the Earth. This force causes the skydiver to accelerate downward as they fall.

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NGSS.MS-PS2-4

Which of the following is an example of a "pull" being used to change the motion of an object?

• Sailor kicking a soccer ball across a field.

• Liam hitting a tennis ball with a racket.

• Izzy's dog tugging on a leash during a walk.

• Ellie typing on a computer keyboard.

Answer explanation

A dog tugging on a leash during a walk is an example of a 'pull' because the dog exerts a force on the leash, changing its motion. The other options involve pushing or striking, not pulling.

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Kinsley kicks a soccer ball high into the air during recess.

What happens to the force of gravity on the ball as it is kicked high into the air?

• Gravity stops pulling on the ball at its highest point.

• Gravity changes into a push to help the ball fall back down.

• Gravity continues to pull the ball toward the Earth at all times.

• Gravity increases as the ball gets farther away from the ground.

Answer explanation

Gravity always acts on objects with mass, including the ball, pulling it toward the Earth at all times, even at its highest point. It does not stop or change into a push.

Aaron is dragging a heavy crate across the floor. If Aaron wants to make the crate speed up (increase its change in motion), what must he do?

• Apply a smaller pulling force.

• Apply a greater pulling force.

• Keep the pulling force exactly the same.

• Remove all force from the crate.

Answer explanation

To increase the crate's speed, the student must apply a greater pulling force. This greater force overcomes friction and accelerates the crate, aligning with Newton's second law of motion.

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NGSS.MS-PS2-2

Landon and Luca each have identical toy cars at the starting line. Landon pushes car A with a force of 5 Newtons, and Luca pushes car B with a force of 15 Newtons. Which statement is most likely true?

• Car A will travel a greater distance than Car B.

• Car B will have a greater change in motion than Car A.

• Both cars will move at exactly the same speed.

• Car B will stop sooner because it had more force.

Answer explanation

Car B is pushed with a greater force (15 Newtons) compared to Car A (5 Newtons). According to Newton's second law, a greater force results in a greater change in motion, making Car B's change in motion larger than Car A's.

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NGSS.MS-PS2-2

Julie and Ali are testing how force affects motion. They use a rubber band to launch a paper airplane. If they stretch the rubber band farther back before releasing it, what is the expected result?

• The airplane will have a smaller change in motion.

• The airplane will move at the same speed as before.

• The airplane will have a greater change in motion because of the increased force.

• The airplane will not move because the force is too great.

Answer explanation

Stretching the rubber band farther increases the force applied to the paper airplane. According to Newton's second law, greater force results in a greater change in motion, making this the expected outcome.

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NGSS.MS-PS2-2

Anders and Gavin are both sitting on the grass with a bowling ball and a soccer ball. If they kick both balls with the exact same amount of force, which will happen?

• The bowling ball will move faster because it is heavier.

• The soccer ball will move faster because it has less mass.

• Both balls will move the same distance.

• Neither ball will move because the forces are balanced.

Answer explanation

When the same force is applied, the soccer ball, having less mass, will accelerate more according to Newton's second law (F=ma). Thus, it will move faster than the heavier bowling ball.

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Jane and Ava are comparing how hard it is to start moving a large, loaded semi-truck versus a small bicycle. Why does the semi-truck require a much more powerful engine to start moving than the bicycle?

• The truck has more mass, so it requires more force to change its motion.

• The truck has less mass, so it is easier to push.

• The bicycle has more gravity acting on it than the truck.

• The truck’s wheels are larger, which removes the need for force.

Answer explanation

The truck has more mass, so it requires more force to change its motion. This is due to Newton's second law, which states that force equals mass times acceleration, meaning heavier objects need more force to start moving.

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NGSS.MS-PS2-2

Maggie is pushing a shopping cart, and her mom drops a 50-pound bag of dog food into it. What will they notice about the force needed to keep the cart moving at the same speed?

• You will need less force because the cart is heavier.

• You will need more force because the increased mass resists change in motion.

• You will need the same amount of force as before.

• The cart will automatically speed up due to the extra weight.

Answer explanation

You will need more force because the increased mass of the cart, due to the added weight, resists changes in motion according to Newton's second law of motion. More force is required to maintain the same speed.

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NGSS.MS-PS2-2

Owen and Noah push against a large stone wall with all their strength, but the wall does not move. Which of the following explains why the wall remains still?

• There are no forces acting on the wall at all.

• They are not pushing hard enough to create a force.

• The wall is pushing back with an equal and opposite force.

• Gravity is pushing the wall forward at the same time.

Answer explanation

The wall remains still because it exerts an equal and opposite force against the student's push, as described by Newton's third law of motion. This balance of forces prevents any movement.