AS/A-level Physics - Kinetic and Gravitational Potential Energy

• Kinetic energy (KE or Ek) is the energy of an object due to its speed.
• kinetic energy is measured in joules (J)
• Kinetic energy increases when a object begins to accelerate or in other words the Gravitational Potential Energy (GPE) decreases.
• Kinetic energy can be derived from suvat equations, word done and GPE!

• If resistive forces, such as friction and air resistance, are ignored, Ek and Ep are related as follows:

• For example, if an object of mass m is released above the ground at height h, it will gain speed, v, as it falls.
• Due to the conservation of energy, and assuming air resistance is negligible, after falling a height of Δh:

Easy question:📝

Champagne bottles are often opened by ‘firing’ the cork out of the bottle. The world record for the horizontal distance travelled by a fired cork is 53 m. The high pressure inside the bottle produces an average force of 150 N on the cork as it leaves the bottle. This force acts on the cork over a distance of 2.5 × 10–2 m. The work done on the cork is equal to 4J.

Calculate the maximum speed at which the cork could leave the bottle.

Mass of cork = 7.5 x 10^-3 kg

Answer:  32.7 ms^-1

Gravitational Potential Energy (GPE)

• Gravitational potential energy is height energy.
• GPE is the energy stored in an object when you raise it to a height against the force of gravity
• Think of GPE as a way of storing kinetic energy. You have to lift something to increase its GPE, and that energy is only released when the object falls.
• The equation:

GPE (J) = Weight (N) x Change in Height (M)

• When something falls, its GPE is converted into kinetic energy. So the further it goes the faster it goes.
• Kinetic energy gained = potential energy lost 

Easy question:📝

A car is at the top of a hill. The height of the hill is 11cm. If the combined mass of the car and the passengers is 117.5 kg, what is the GPE at this point? (3 sig figs)

Answer: 127 J

These types of questions always appear is AS/A-level Physics exam! Make sure you know when to use the right equation! 👨‍🏫

Hard question: 📝

A tennis ball with a mass of 400 g is thrown upwards at a speed of 5 ms-1.

1. What is the ball’s Ek as it is released?
2. What is the ball’s maximum gain of Ep?
3. What is the ball’s maximum height?

This is the end of the topic! 

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