Download Day 01- Drawing FBDs Solutions see p2

Be sure to sketch a free­body diagram for each question.
2) The Soviet Union’s G rocket has a mass of about 3.8 × 106kg and it first stage engines exert a thrust of about 5.0 × 107N.
a )What is the force of gravity on the rocket at the Earth’s surface?
b) Calculate the net force at lift­off.
c) Calculate the initial acceleration of the rocket.
d) What should happen to the acceleration if the force exerted by the engine remains constant as it burns?
e) Why is the first stage jettisoned after the fuel is consumed?
Solution:
a)
b)
c)
d) As the fuel is consumed, the mass of the rocket decreases. This decreased mass means a decrease in the downward force of gravity. If the force exerted by the engine remains the same, then the net force up will increase because gravity "cancels" less of the force from the engines. But since the mass of the shuttle , the acceleration of the rocket must increase.
decreases, in order to balance e)Again, this will decrease the mass of the rocket and increase its acceleration. If the rocket pushes the stage one engine down as it jettisons it, then the stage one engine will push back up on the rocket (Newton's third Law) adding another upwards force to the rocket.
3) A drag racing car starting from a standstill can reach a speed of 320 km/h in 6.50s by exerting an average horizontal force of 1.52×104N on the pavement. If the force of friction on the car is 5.2×103N, what is the mass of the car?
Solution:
We will need to use Newton's Second Law,
acceleration and the net force.
, to solve for the mass, so we need to find the 4) A 0.25kg model rocket accelerates from 15m/s [up] to 40m/s [up] in 0.60s. Calculate the force the escaping gases exert on the rocket.
We will use Newton's second law to find the net force and then we will calculate the force of the gas. First we need to find the acceleration and force of gravity.
5) A disc of mass 0.5kg is acted on by three 2N forces as shown in the free­body diagram below. What is the net force on the disc? Justify your answer. [Hint: No calculations required].
2N
120o
2N
2N
Solution: The net force on the disc is 0N. The three 2N forces are spaced evenly around the disc, so they cancel each other out. 6) Draw a free­body diagram which shows all the forces for each of the following situations:
a)
An egg is free­falling from a nest in a tree. Neglect air resistance. b)
A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. c)
A skydiver is descending with a constant velocity. Consider air resistance. d)
A skier is sliding down a hill of snow at an angle of 22°. Neglect frictional forces.
22o
e)
A football is moving upwards towards its peak after having been kicked by the punter. f)
A car is coasting and slowing down.