Use Newton's second law to solve this problem.

When the elevator is not moving, we get 

However, when the elevator is accelerating downward, the man appears to be lighter since the elevator is negating some of the force from gravity. Written as an equation, we have:

Where  is acceleration due to gravity and  is the acceleration of the elevator.

Putting in our values, we get:

This question is testing your understanding of terminal velocity and Newton's second law. Since the skydiver is at terminal velocity, the force of the air is equal to the force of gravity, resulting in zero net force and thus no acceleration. We just need to calculate the force of gravity on the skydiver to find the force of the air:

There are two forces in play in this scenario. The first is gravity, and the second is air resistance. Since they are opposing each other, we can write:

Substituting in Newton's second law, we get:

Rearranging for the force of air resistance, we get:

Plugging in our values from the problem statement:

We can use the equation for conservation of energy to calculate the velocity of the diver as he hits the water:

Cancel out initial kinetic and final potential energies, and plug in our expressions:

Cancel out mass and rearranging for final velocity:

Plug in our values:

We know that the diver then decelerates from this velocity to zero in 0.8 seconds, so we can calculate the acceleration:

Then use Newton's second law to calculate the force on the diver:

The car and the truck experience equal and opposite forces, but since the car has a smaller mass it will experience greater acceleration than the truck according to the equation F = ma.

A greater mass will decrease the acceleration.

Since we are neglecting air resistance, there are two forces in play: gravity and friction. Therefore, we can use Newton's second law to write the following:

Substituting in an expression for the force of gravity and rearranging for the force of friction, we get:

Use Newton's second law.

The box has a constant force acting on it pulling it towards the left. Therefore we can write this as:

, where the negative sign indicates that the force is directed towards the left.

Since the force is constant, this means that is is causing the box to move with a constant acceleration that we can calculate using Newton's second law of motion:

Now that we know the acceleration, we can calculate the final velocity after 5 seconds:

Where  and  since the box is originally at rest.

So we have that

.

Note that the negative sign indicates that the box is moving to the left.

The force applied by the weaker person can be calculated using Newton's second law, which states: 

The net force is equal to the product of the mass of the object and the acceleration of the object. We were given the mass and acceleration of the object, but only the ratio of the applied forces:

Solve for , the applied force from the weaker person:

Newton's second law states that the net force, or the vector sum of all the forces acting on an object, equals the mass times the acceleration. So, it is possible to have forces act on an object without acceleration if the forces are oriented such that they vector sum to zero. An example would be a person sitting in a chair. Gravity and the normal force both act on the person. However, these forces are equal in magnitude and opposite in direction. So the person doesn't accelerate.