Let us look at the following two examples. We will start with a familiar example:
Seat Belts
We wear seat belts in cars. This is to protect us when the car is involved in an accident. If a car is travelling at \(\text{120}\)\(\text{km·h$^{-1}$}\), the passengers in the car is also travelling at \(\text{120}\)\(\text{km·h$^{-1}$}\). When the car suddenly stops a force is exerted on the car (making it slow down), but not on the passengers. The passengers will carry on moving forward at \(\text{120}\)\(\text{km·h$^{-1}$}\) according to Newton’s first law. If they are wearing seat belts, the seat belts will stop them by exerting a force on them and so prevent them from getting hurt.
Rockets
A spaceship is launched into space. The force of the exploding gases pushes the rocket through the air into space. Once it is in space, the engines are switched off and it will keep on moving at a constant velocity. If the astronauts want to change the direction of the spaceship they need to fire an engine. This will then apply a force on the rocket and it will change its direction.
Example: Newton’s First Law in Action
Question
Why do passengers get thrown to the side when the car they are driving in goes around a corner?
Step 1: What happens before the car turns
Before the car starts turning both the passengers and the car are travelling at the same velocity. (picture A)
Step 2: What happens while the car turns
The driver turns the wheels of the car, which then exert a force on the car and the car turns. This force acts on the car but not the passengers, hence (by Newton’s first law) the passengers continue moving with the same original velocity. (picture B)
Step 3: Why passengers get thrown to the side?
If the passengers are wearing seat belts they will exert a force on the passengers until the passengers’ velocity is the same as that of the car (picture C). Without a seat belt the passenger may hit the side of the car.
Discussion:
Helium is less dense than the air we breathe. Discuss why a helium balloon in a car driving around a corner appears to violate Newton’s first law and moves towards the inside of the turn not the outside like a passenger.
Answer: As the car goes around the corner all the air keeps moving forward (it acts in the same way that a passenger acts). This causes the air pressure on the one side of the car to increase (this will be on the opposite side to the direction the car is turning). This slight increase in the air pressure pushes the helium balloon to the other side of the car. Because of this it appears that the helium balloon does not obey Newton’s first law.
Optional Video: Newton’s Law Of Inertia
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