Ready for launch

Fully fueled the shuttle weighs 2 million kilograms and can carry 30 tons of payload. It consists of the Orbiter, two solid fuel rocket boosters and an external fuel tank.To lift the massive shuttle from the pad to maximum orbits of about 600 km above the Earth, the shuttle uses the force generated from the two solid rocket boosters(SRB's) and the three main engines of the Orbiter

 
The solid boosters provide thrust for approximately two minutes. Once ignited, these giant sky-rockets can not be extinguished and must be allowed to burn. This is the most dangerous part of the mission for if something went wrong there is little that can be done. During this time the SRB's, together with the Orbiter's main engines propel the shuttle to speeds of 4,800 km/h. After the solid rockets are jettisoned, the main engines provide thrust which accelerate the Shuttle from a speed of 4,800 km/h to over 27,000 km/h in just six minutes .

 

Why are the first two minutes of flight the most dangerous?

Unlike the SRB's the Orbiter's main engines can be turned off. However, alone the Orbiter's engines are not powerful enough to lift the Shuttle off the launch pad. When viewing video footage of a Space Shuttle launch you will notice that the Orbiter's engines are always turned on first and only when they are fully operational are the SRB's ignited. Why?

The shuttle uses the Solid Rocket Boosters (SRB's) to accelerate to 4,800 km/h in just two minutes. Calculate the acceleration of the Shuttle during the first two minutes.

Calculate the acceleration of the Shuttle, in km/h, as it accelerates from a speed of 4,800 km/h to 27,000 km/h in just 6 minutes.

What is the combined force acting on the Shuttle at lift off of both the Orbiter engines and the SRB's in kilograms?

What is the net force, in kilograms, driving the Shuttle skyward? Consider the weight of the Shuttle acting downwards.

Why are the Orbiter's Main Engines not capable of lifting the Shuttle off the launch pad, but after 2 minutes into the flight they can propel the Orbiter and its external fuel tank to speeds of 27,000 km/h in just 6 minutes?
The forces involved during launch.