Work done and energy transfer

Describe — 4 marks

A student pushes a heavy box across a classroom floor. The box moves in the direction of the applied force, and the student notices the box becomes slightly warm after being pushed.

(a) Describe what is meant by work done on an object. [1 mark]
(b) Describe how energy is transferred when the student pushes the box across the floor. [2 marks]
(c) Describe why the box becomes warm during this process and identify the form of energy responsible for this temperature increase. [1 mark]

Show mark scheme

(a) Work done is the energy transferred when a force moves an object in the direction of the force / when a force acts over a distance
(b) The student's muscles transfer chemical energy (1 mark)
(b) This is converted to kinetic energy of the box and heat energy due to friction between the box and floor (1 mark)
(c) Friction between the box and floor converts kinetic energy into thermal/heat energy, causing the temperature to increase

Show — 3 marks

A student pushes a crate across a horizontal floor with a constant force of 45 N. The crate moves a distance of 8 m in the direction of the force. The student wants to calculate the work done on the crate.

(a) Show that the work done on the crate is 360 J. [2 marks]
(b) Explain what happens to the energy transferred to the crate if the student pushes with the same force but the crate only moves 4 m. [1 mark]

Show mark scheme

(a) Uses correct formula: W = F × d (or equivalent)
(a) Correct substitution: W = 45 × 8 = 360 J (or shows clear calculation leading to 360 J)
(b) Recognises that less energy is transferred / work done is reduced / energy transfer is proportional to distance moved (accept: energy transferred would be 180 J or half the original amount)

Evaluate — 5 marks

A student designs an experiment to investigate energy transfer in a pulley system. A 2 kg mass is lifted vertically upwards through a height of 3 m using a rope over a pulley. The student measures that a force of 25 N is required to lift the mass at a constant velocity. The student then calculates the work done by the applied force and compares it to the gravitational potential energy gained by the mass. (Use g = 10 m/s²)

(a) Calculate the work done by the applied force when lifting the mass through 3 m. [1 mark]
(b) Calculate the gravitational potential energy gained by the mass. [1 mark]
(c) The work done by the applied force is greater than the gravitational potential energy gained. Evaluate why this difference exists and what it tells us about the efficiency of the pulley system. [3 marks]

Show mark scheme

(a) Work = Force × Distance = 25 × 3 = 75 J (1 mark)
(b) GPE = mgh = 2 × 10 × 3 = 60 J (1 mark)
(c) The difference (75 - 60 = 15 J) represents energy lost to overcome friction (1 mark)
(c) Friction acts in the pulley system (at the pulley axle/rope contact) opposing the motion (1 mark)
(c) The efficiency of the system is 60/75 = 80% / not all input energy is converted to useful gravitational potential energy / some energy is wasted as heat (1 mark)

Explain — 4 marks

A student pushes a heavy box across a classroom floor. The box moves 5 metres in the direction of the push. The student applies a constant force of 40 N to move the box at a steady speed.

(a) Calculate the work done by the student in pushing the box across the classroom floor. [2 marks]
(b) Explain what happens to the energy transferred to the box as it moves at steady speed across the floor. [2 marks]

Show mark scheme

(a) Correct formula used: W = F × d
(a) Correct calculation and unit: 40 × 5 = 200 J
(b) Energy is transferred from the student to the box / work is done on the box
(b) The energy is dissipated/transferred as heat due to friction between the box and floor

Describe — 4 marks

Show — 3 marks

Evaluate — 5 marks

Explain — 4 marks

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