Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
12 Cards in this Set
- Front
- Back
|
What is work done? |
Work done is the energy transferred when a force moves an object through a distance. |
|
|
Which equation shows the relationship between work done, force applied and distance moved?
|
work done (joule, J) = force (newton, N) × distance (metre, m) |
|
|
A force of 120 N is applied to a suitcase to move it 40 m along the floor. What is the work done on the suitcase? |
The work done is 120 × 40 = 4800 J. |
|
|
If the woman pulling the suitcase then moved onto a much harder floor surface, the friction would reduce significantly. For half a second she does not realise that the friction has reduced. Describe what happens during this half a second, and justify your answer using estimates for the forces and masses involved. |
There will be a resultant force on the suitcase, because the friction force has reduced, and she is still pulling with a force of 120 N. The case will accelerate. If the friction dropped to 100 N there would be a resultant force of 20 N to the right. If the mass of the case was 20 kg, it would accelerate at 1 m/s^2. (^2 = power of 2) |
|
|
A force of 200 N pushes a car 30 m. Calculate the amount of work done. If a drawer moves 40 cm and the person opening it transfers 1 J of energy against friction forces, what was the friction force in the drawer system? |
6000 J. F = E/d. F = 1/0.4. Friction force was therefore 2.5 N. |
|
|
A crane lifts a 2 t load 40 m to the top of a building. How much work has been done against the weight of the load?
Note: 1 t (tonne) =1000 kg. Remember that weight = mass ×gravitational field strength (10 N/kg on Earth). |
Weight = 20 000 N, distance = 40 m so work done is 800 000 J. |
|
|
To keep a bus moving on a flat road the engine needs to be pushing the bus all of the time. Describe the data you would need to collect to calculate the work done by the bus engine against friction. |
You would need to know the force the engine is pushing with and the distance the bus moves. |
|
|
Explain why a lubricant can reduce the energy lost through heating in a car engine. |
The lubricant reduces friction so although the parts still move past each other, the force needed is less and so the work done is less. This means that less heat is lost. |
|
|
As the Space Shuttle enters the Earth’s atmosphere it heats up. Draw a diagram to explain the changes in energy as the Space Shuttle goes from high above the Earth’s atmosphere to landing on an airfield. |
The diagram should include the changes from kinetic energy and potential energy to heat and light due to air friction. |
|
|
If you slide down a rope, you can get a friction burn. Explain why your hands get hot in this situation in terms of force, distance and work done. |
The force in this case is the friction to slow you down instead of falling, the distance is the distance moved along the rope and the work done is energy transferred from movement into heat. Your hands get hot because the energy is transferred through friction between your hands and the rope. |
|
|
Two people, one walking and one riding a bike, move along a path at about the same speed. What similarities and differences can you think of between the forces on the cyclist and those on the walker? |
The wind resistance forces would be similar as they are moving at the same speed and are roughly the same shape. The cyclist would find that there is much less energy lost through friction as their leg movements would be much less and a lot of energy is ‘lost’ in the walking process: joints, muscles, etc. moving inside the body. In general the work done will be less for the cyclist as they are moving the same distance but with a smaller force. |
|
|
What are the key points to remember? |
Work done is energy transferred. Work done = force × distance, W = F × d. Energy is transferred when a force acts through a distance. Energy is never created or destroyed. ‘Lost’energy usually ends up as heat. |
