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GCSE Questions: Forces

Q21. The diagram shows the forces acting on a model air-powered rocket just after it has been launched vertically upwards.

(a)

(i) How does the velocity of the rocket change as the rocket moves upwards?

Give a reason for your answer.

The velocity decreases to zero as it moves upwards. It does this because there is a resultant force acting in opposite direction to motion.

The counter force is composed of the weight of the rocket and air resistance. The weight will remain constant but the air resistance will decrease as the velocity of the rocket decreases.

[2 marks]

(ii) The velocity of the rocket is not the same as the speed of the rocket.

What is the difference between the velocity of an object and the speed of an object?

Velocity includes a direction component as it is a vector quantity - whereas speed is a scalar quantity.

[1 mark]

(b) The speed of the rocket just after being launched is 12 m/s. The mass of the rocket is 0.05 kg.

(i) Calculate the kinetic energy of the rocket just after being launched.

Ek = ½mv2

Ek= 0.5 x 0.05 x 122

Ek= 0.025 x 144

Ek=3.6 J

[3 marks]

(ii) As the rocket moves upwards, it gains gravitational potential energy.

State the maximum gravitational potential energy gained by the rocket. (Ignore the effect of air resistance).

3.6 J

[1 mark]

(iii) Calculate the maximum height the rocket will reach. Ignore the effect of air resistance.

Gravitational field strength = 10 N/kg.

ΔEG = mgΔh

3.6 = 0.05 x 10 x h

h = 3.6/0.5

h = 7.2 m

[2 marks]

(iv) The sketch graphs below show four velocity–time graphs.

Taking air resistance into account, which sketch graph, A, B, C or D, shows how the velocity of the rocket changes as it falls from the maximum height it reached until it just hits the ground?

At the top of its path it will be stationary so graphs A and D are obviously wrong!

As it gets faster air resistance will decrease its acceleration so the answer is B

[1 mark]

(c) The rocket can be launched at different angles to the horizontal. The horizontal distance the rocket travels is called the range.

The diagram below shows the paths taken by the rocket when launched at different angles - air resistance has been ignored.

 

What pattern links the angle at which the rocket is launched and the range of the rocket?

The maximum range is achieved when the angle of launch is 45o. The range increses as the angle changes from zero to 45o and decreases as it exceeds 45o. (max 2 marks)

 

[2 marks]

[12 Marks TOTAL]