# Which law of Newton applies to the following situations?

1. You pitch forward in a car that comes to an abrupt stop
2. A ball on a pool table hits the side wall then bounces off and goes into the side pocket
3. A rocket lifting off the launch pad fires its thrusters against the ground

t-nez | Certified Educator

1. The first situation is an example of Newton's First Law, An object in motion stays in motion and an object at rest stays at rest unless acted on by an external force.

This is also called the Law of Inertia. A passenger in the car has inertia, which is resistance to change in motion. When the car stops abruptly, the passenger keeps moving forward until acted on by a force, most likely the seatbelt/shoulder strap locking up.

2. This is an example of Newton's Third Law: When one object exerts a force on a second object, the second object also exerts a force on the first object that's equal in magnitude and in the opposite direction. This can be summed up in the statement "for every action there's an equal and opposite reaction."  The ball exerts a force on the side of the table, and the table exerts an equal force on the ball that changes its direction.

3. This is also an example of Newton's Third Law. As the rocket thruster pushes out the hot gas, the gas pushes on the rocket with equal force in the opposite direction. There's additional force from the gas pushing on the launch pad, the launch pad pushing back on the gas and the gas then pushing on the rocket.

Newton's second law also applies in examples 2 and 3. This law says: A force acting on an object produces acceleration equal to the force divided by the mass of the object, or F = ma.  The force of the gas on the rocket accelerates the rocket and the force of the pool table on the ball changes its acceleration by changing its direction. The large difference in the mass of the ball and the mass of the table explains why the table does't accelerate noticeably. The forces are equal in magnitude, so mass x acceleration is equal, so a much larger mass will have a much smaller acceleration.

vnessawong21 | Student

1. This is a classic example of Newton's first law, the law of inertia. The law of inertia tells us objects have the tendency to continue going in the same direction of motion unless a force acts against it. If there is no seat belt, when the car stops you will have the tendency to continue going forward.

2. This is newton's second law which essentially describes the workings of the formula f = ma. A force is exerted by the side wall causing an acceleration in the same direction which sends the ball into the pocket.

3. Newton's third law states for every force there is an opposing force of equal magnitude. The rocket pushes against the ground to attain liftoff, however, the ground will push back up against the rocket.