Newton's first law: inertia

You place a coin on a piece of card covering the top of a glass and flick the card away suddenly in a horizontal direction, as shown in the figure.

The coin falls into the glass instead of flying off with the card. Explain this trick using Newton's first law.

In an ice hockey match, a player hits the puck and it glides almost without slowing down to the far end of the rink. What does Newton's first law say about the motion of the puck while no one touches it? Why does it eventually stop on a real rink?

Complete the sentence with the missing words: "If the resultant force acting on an object is zero, an object at rest $\dots$ at rest, and a moving object continues to move in a $\dots$ $\dots$ motion at a $\dots$ velocity". Then give an example of each case.

A ball rolls across the playground floor and eventually stops even though no one touches it. A friend says: "This proves that Newton's first law does not hold true in real life." Are they right? Explain which force is involved and what would happen on a perfectly smooth, frictionless surface.

A spacecraft travels through deep space, far from planets and stars, with its engines turned off.

1. a) What does the spacecraft do: does it stop, slow down gradually, or continue at a constant velocity? Justify your answer using the first law.
2. b) Is it necessary to burn fuel to maintain velocity? What about to change it? Explain the difference.

Aitana is sitting on the sofa at home. Is she at rest or in motion? Answer by choosing two different frames of reference:

1. a) Relative to the walls of her house.
2. b) Relative to the Sun.

What do you conclude about simply stating that "she is still"?

Marc is on a train travelling at a constant velocity along a straight track and throws a ball vertically upwards, as shown in the figure.

1. a) Does the ball land back in his hand, in front of him, or behind him? Justify your answer.
2. b) Would the answer change if the train were braking at that exact moment? Explain.

On a plane mid-flight, to you your suitcase is stationary in the overhead locker; however, to someone who could see you from the ground, the suitcase is moving at around $900\ \text{km/h}$.

1. a) Who is correct?
2. b) Explain what a frame of reference is and why rest and motion are always described relative to one.

You are standing on a bus without holding onto anything, and suddenly the driver brakes hard. In which direction does your body lean, forwards or backwards? Explain using Newton's first law.

Why is it compulsory to wear a seatbelt even in the back seats of a car? Explain what would happen to your body during sudden, hard braking if you were not wearing one, relating it to inertia.

A skateboarder goes down the street and their board hits a small stone: the board stops dead and they fly forwards.

1. a) Explain using the first law why they fly forwards.
2. b) Relate this accident to the function of a seatbelt in a car.
3. c) If, instead of hitting a stone, the skateboard slowed down gradually upon entering grass, would they fly off with as much force? Justify your answer.