Newton's Law of Inertia

What is a force?

• A touch

• Any push or pull

• Power or energy

• Gravity only

Answer explanation

A force pushes or pulls on an object. One example is gravity. Another is friction. Forces can cause changes in motion, such as stopping, starting, speeding up, slowing down or changing direction.

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What is inertia?

• Objects changing their motion on their own.

• Objects moving in a straight line.

• Objects staying still when they are acted on by a force.

• Objects not changing motion unless acted on by an unbalanced (net) force.

Answer explanation

"Inertia" is the tendency of objects to follow Newton's first law: Objects in motion remain in motion and objects at rest remain at rest unless acted on by an unbalanced force.

A car moves at a constant speed without changing direction. What HAS to be true about the forces on the car?

• A force is pushing the car in the direction it's moving.

• The forces on the car are not balanced.

• The forces on the car are balanced.

• Forces push on the car in all directions.

Answer explanation

The car isn't changing speed or direction, so the forces have to be balanced!

Sara swings a bucket full of water around her head. The water doesn't spill out of the bucket. Why not?

• A force is pushing out on the water.

• Inertia causes the water to move in a straight line.

• A force is pushing out on the bucket.

• The water would spill out of the bucket.

• T

Answer explanation

The force of the string pulls on the bucket, causing it to move in a circle. The water moves in a straight line because of inertia, which pushes it against the bottom of the bucket.

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A ball is sitting still on a plate. How could the ball start moving?

• The ball's inertia disppears.

• All the forces on the ball stop.

• A new, balanced force acts on the ball.

• A new, unbalanced (net) force acts on the ball.

• A

Answer explanation

To cause a change in motion, a force must be unbalanced (greater in one direction than in others.)

A driver stops suddenly at a red light. He is thrown forward against the seatbelt. Why?

• A force pushes him forward when he stops.

• The force of the seatbelt pushes him backwards.

• A force pushes the whole car forward, including the driver.

• The force of the brakes stops the car, but not the driver.

Answer explanation

Because the force of the brakes acts on the car but not the driver, the driver keeps moving forward because of inertia until the seatbelt stops him.

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You are throwing a ball to your friend across the room. Which ball will take more force to throw?

• They are the same size, so they will take the same force.

• The bowling ball has a greater mass, so it will take more force.

• The basketball has a smaller mass, so it will take more force.

• T

Answer explanation

The larger the mass, the larger the force required to cause the same change in motion. It is harder to throw a bowling ball across the room than it is to throw a basketball. 

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A car runs into a wall. What will happen to the people in the car?

• They will be pushed backwards by the force of the wall.

• They will be thrown forward because of the force of the car.

• They will keep moving forward until they hit their seatbelt or the dashboard.

• They will be pushed in all directions equally.

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A car turns left suddenly. The drink in the drink holder falls over onto the person sitting in the right-hand seat. Why?

• The car turns left, but the drink continues going straight, tipping it to the right.

• The car turns left, and the drink is pushed in the opposite direction.

• The car turns left, and the force causes the drink to move. It could have fallen either way.

• The force of the turn pushes both the car and drink to the left, but the drink isn't pushed as much.

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The planets stay in orbit around the sun because of gravity and ______________.

• weight

• Earth

• inertia

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