1. Newton’s Third Law

2. Imagine a rocket is being launched from the earth.
Hot gases are pushed out from the bottom of the rocket as the rocket is thrust upward.
The force of the gases pushing against the surface of the earth is equal and opposite to the force with which the rocket moves upward.
The motion of the rocket can be explained by Newton's third law, for every action there is an equal and opposite reaction.

3. The earth and the moon are like two rocks in space
The earth and the moon are like two rocks in space. Why does the moon orbit the earth orbit the moon?
The force on each body is the same – but in opposite directions (Newton’s 3rd Law).
The moon is accelerated much more than the earth, because it has so much less mass than the earth (Newton’s 2nd Law).

4. Your empty hand is not hurt because there is little mass, therefore there is little inertia (little resistance to a change in motion).
Carrying a heavy load causes pain because there is significant mass and inertia, therefore your hand is tougher to stop and continues to move.
As a result, your hand gets squashed a bit and it hurts.

5. There is no net force on the horizontal direction of the parcel being dropped from the plane or the ball being dropped from the mast.
As a result, it will continue to move at the same speed of the plane or ship.
The parcel or ball will move the same amount as the plane or ship.

6. Bob’s weight is applying 500N on his bosun
Bob’s weight is applying 500N on his bosun. The rope used can hold 300N, however Bob is using two ropes. As a result, the bosun can support up to 600N.

7. When Bob reties the bosun, he ties one end to the flag pole and it breaks.
This occurs because the force Bob’s weight applies on the rope is 500N, greater than the amount that a single rope can support (300N).

8. We know if an object is not moving or moving at constant speed in a straight line there are still forces acting upon it.
Because there is no change in motion, we can assume that there is NO net force acting on the object.
Thus, the forces acting upon the object counteract each other.

9. If the car is traveling at a constant speed, there is NO net force acting on it.
We still need to step on the gas to maintain the constant speed. This is required to counteract the forces of friction (ground and air).
These forces are in opposite directions so that the net force is zero.

10. If you push with a force of 200N to slide a box across the floor at a constant speed in a straight line, how much friction is exerted by the floor on the box?
200N in the opposite direction, there must be zero net force in order that the box in a straight line at a constant speed.

11. If Mark starts pulling on the rope, they both move towards each other.
Mark’s pull on the rope is transmitted to Sua causing her to accelerate towards Mark.
By Newton’s 3rd Law, the rope pulls back on Mark with equal force causing him to accelerate towards Sua.

12. Each page is like a different body, so when the nail is hit, there is a chain of collisions taking place on the pages all the way down to her head.
As a result, there is little change in motion of the last page on top of her head, so it doesn’t hurt.

13. The stack of books consists of many pages that are at rest
The stack of books consists of many pages that are at rest. They all have inertia (because they all have mass) so they resist accelerating (they resist a change to their rest).