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.