Newton's Laws and Balanced Forces Review

During a demonstration of Newton’s laws of motion, a student used the setup shown in Figure 1. The student flicked the index card with a fingertip, and the coin fell straight down into a plastic cup as shown in Figure 2. Which of these best explains why the coin fell straight down into the cup instead of remaining on the index card?

• The coin was at rest until the card was removed, so it tended to remain in the same location. Once the card was gone, the unbalanced force of gravity caused the coin to fall.
• Moving the card applied an action force on the coin. Since the card was gone, gravity applied a reaction force on the coin.
• The card had less mass than the coin, so a smaller force of gravity acted on the card. The larger force of gravity on the coin made it fall.
• The acceleration of the coin falling into the cup was equal and opposite to the acceleration of the card.

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The masses of four vehicles and the net forces acting on them as they enter a highway are recorded in the table below. Which vehicle has the greatest acceleration as it enters the highway?

• Sedan
• Coupe
• SUV
• Truck

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NGSS.MS-PS2-2

A student lets a toy car roll four times down a ramp. Each time the student covers the surface of the ramp with a different material. The student measures and records the time it takes the car to roll down the ramp. Which of these would be the best conclusion based on the data in the table?

• Different surfaces affect how fast a toy car accelerates.

• Different toy cars travel at different speeds.

• Gravity has little effect on the speed of toy cars on different surfaces.

• Air resistance is the greatest factor in limiting the acceleration of different toy cars.

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Sometimes tectonic plates do not move easily past one another, and the plates become stuck. Forces build up, and when the plates finally move, tension is released, as shown below. The sudden movement of the plates is caused by -

• the mass of the plates
• the weight of the plates
• unbalance forces
• gravitational force

According to Newton’s Law of Inertia, which of the following objects will change its motion?

• A bike rider on a smooth and level road, pedaling at a constant rate.
• A train pulling on another train, each with the same amount of force.
• A chair sitting on the floor with balanced forces acting in opposite directions.
• A ball rolling across the floor with friction acting in the opposite direction.

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Safety belts protect people in cars in the event of an accident because, according to Newton’s Laws of Motion, when an impact causes the car to suddenly change its motion, -

• the people in the car will continue to move in the same direction and at the same speed as before the impact.
• the speed of the car always increases, giving it greater force since the mass will remain the same.
• the speed of the people always increases, since now they have, not one, but two forces acting on them.
• the car now has a greater overall mass - its own mass plus the mass of the object by which it has been struck.

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A ball thrown into the air starts out with an initial upward velocity, but eventually slows and stops before falling back to the ground. Which of the following contributes the most to the change in motion of the ball?

• The initial force with which the ball is thrown
• The downward force of gravity
• The resistance against the atmosphere
• The increase of inertia

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According to Newton’s Law of Inertia, an object that is moving will change its motion only if what kinds of forces act on it?

• Constant forces
• Unbalanced forces
• Unbalanced forcesStrong forces
• Gravitational forces

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NGSS.MS-PS2-2

A bowling ball is rolled with a certain speed and strikes a bowling pin with a force indicated by the arrow in this illustration. When the bowling ball strikes the bowling pin, the force applied by the pin to the ball -

• depends on the mass of the bowling pin.
• is equal to the force applied by the ball.
• must be zero, since the pin is not moving.
• is equal to the force of gravity on both.

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An object is moving to the right at a constant velocity. What will happen if a force of 20N starts acting on it in the opposite direction?

• The object’s velocity will increase.
• The object’s velocity will decrease.
• The object will come to a stop.
• The object will move at the same velocity.

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