P2 2.5 Falling objects

Lesson 4: Falling objects

Textbook section P2 2.5

Aim: How does a parachute work?

Starter (10 min): F=ma becomes weight (N) = mass (kg) x acceleration due to gravity (m/s2)
When objects fall freely, the resultant force acting on them is the Earth’s force provided by Earth’s gravitational field. This will make an object accelerate at about 10 ms2 close to the Earth’s surface.

We call the force of gravity ‘weight’ and the acceleration ‘the acceleration due to gravity’.

F=ma becomes weight = mass x gravitational field strength.

When objects fall through fluids (such as water and air), the fluid exerts frictional forces on the object. > the velocity of the object then > frictional force. Eventually the two forces (weight & friction) will balance, resultant force = zero and the object moves at a steady velocity. This is called the ‘terminal velocity’. Possibly show the ball bearing in air, water, and honey demonstration.

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/forces/forcemassrev4.shtml see BBC Bytesize for more on this topic.

Main activity (30 min): Egg parachute competition.

Students have to think about what makes a good parachute (one that creates the most drag force/air resistance so > deceleration, <> time taken to hit the ground). They have 15 min to build an egg parachute which will then be tested outside (materials plastic bags, string, tape, 1 egg per team).
Rules
1. No cushioning for the egg
2. If the egg breaks you are disqualified
3. The team whose parachute took the longest to hit the ground without the egg breaking are the winners. The winning team will receive a prize.

Plenary (15 min):

Students are given a velocity time graph for a falling object, they have to describe what is happening at each stage using the key words and terms. They should also calculate the size of the forces using the equation w = m x g.
Accelerates, Weight, Gravity, Drag force, Increase, Decrease, Terminal velocity, greater than, equal to

AQA Specification – Physics 2.13.2
· The faster a body moves through a fluid the greater the frictional force that acts on it.
· A body falling through a fluid will initially accelerate due to the force of gravity. Eventually the resultant force on the body will be zero and it will fall at its terminal velocity.
· Calculate the weight of a body using: weight (newton, N) = mass (kilogram, kg) x gravitational field strength (newton/kilogram, N/kg)Draw and interpret velocity-time graphs for bodies that reach terminal velocity, including a consideration of the forces acting on the body.

P2 Unit 2.4 On the road

Lesson 3: On the road

Textbook section P2 2.4

Aim: What factors affect the stopping distance of vehicle on the road?

Starter 10 min: 10 key questions that provide a recap of the previous two lessons. These should be attempted without books. Mr C will provide the answers and students should self assess their work.

Main activity 30 min:
When a vehicle is travelling at a steady speed the resultant force acting on it is zero, so driving forces are equal & opposite to frictional forces.
The braking force needed to stop that vehicle depends on the velocity and mass of the vehicle. > velocity and/or > mass = > braking force.
The total stopping distance of a vehicle is the distance it travels during the driver’s reaction time (the thinking distance) plus the distance it travels under the braking force (the braking distance).
Stopping distance = thinking distance + braking distance. With the same driver and car the thinking & braking distance will increase when the initial velocity increases.
The thinking distance will increase if the driver is tired or under the influence.
The braking distance can be increased by poorly maintained roads, bad weather conditions & the condition of the car i.e. worn brakes or tyres.

Students have to...
1. Use the textbook to draw a diagram or series of diagrams that summarise the above, it should be simple and concise and include as many key points as possible. To help them begin a diagram of Mr Average in his average car is shown on the board.
What is a stopping distance? Explained by Mr Average.
What factors affect braking force?
What factors affect the thinking distance and braking distance?
2. Complete summary questions 1 & 2 on pg 203 of the text book. Mr C will provide answers at the end of the lesson.
3. If there is time the lesson can be finished with some reaction tests using software on the computer.

http://www.gcse.com/fm/stopping_distances.htm a good summary of the material covered in this lesson.

Plenary (10 min)...
Students can try some practice exam questions. Mr C will give the answers and students should mark and correct their own work.

AQA Specification – Physics 2.13.2
· When a vehicle travels at a steady speed the frictional forces balance the driving forces.
· The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.
· The stopping distance of a vehicle depends on the distance the vehicle travels during the drivers reaction time and the distance it travels under the braking force.
· A driver’s reaction time can be affected by tiredness, drugs and alcohol.
· A vehicle’s braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.