Falling and Gravity

Last lesson The equations of motion (SUVAT) Measuring “g”

The equations of motion The equations of motion can be used when an object is accelerating at a steady rate There are four equations relating five quantities u initial velocity, v final velocity, s displacement, a acceleration, t time SUVAT equations

The four equations 1This is a re-arrangement of 2This says displacement = average velocity x time 3With zero acceleration, this becomes displacement = velocity x time 4Useful when you don’t know the time NOT in data book

Beware! All quantities are (except time) vectors. These equations are normally done in one dimension, so a negative result means displacement/velocity/acceleration in the opposite direction.

Today’s lesson Falling and terminal speed Weight and gravity

Falling!

Imagine a dog being thrown out of an aeroplane. Woof! (help!)

Force of gravity means the dog accelerates gravity To start, the dog is falling slowly (it has not had time to speed up). There is really only one force acting on the dog, the force of gravity. The dog falls faster (accelerates) due to this force.

Gravity is still bigger than air resistance gravity As the dog falls faster, another force becomes bigger – air resistance. The force of gravity on the dog of course stays the same The force of gravity is still bigger than the air resistance, so the dog continues to accelerate (get faster) Air resistance

Gravity = air resistance Terminal velocity gravity As the dog falls faster and air resistance increases, eventually the air resistance becomes as big as (equal to) the force of gravity. The dog stops getting faster (accelerating) and falls at constant velocity. This velocity is called the terminal velocity. Air resistance

Terminal Velocity The dog will continue to fall at constant speed (called the terminal speed) until…………. gravity Air resistance air resistance = gravity

Terminal Speed gravity Air resistance

Oooooops!

That definitely was terminal!

Falling without air resistance

gravity This time there is only one force acting in the ball - gravity

Falling without air resistance gravity The ball falls faster….

Falling without air resistance gravity The ball falls faster and faster…….

Falling without air resistance gravity The ball falls faster and faster and faster……. It gets faster by 9.81 m/s every second (9.81 m/s 2 ) This number is called “g”, the acceleration due to gravity.

Falling without air resistance? distance time

Falling without air resistance? distance time

Falling without air resistance? speed time

Falling without air resistance? speed time

Falling without air resistance? speed time Gradient = acceleration = 9.8 m.s -2

Velocity/time graphs Taking upwards are the positive direction velocity/m.s -1 Time/s Ball being thrown into the air, gradient = constant = m.s -2

Falling with air resistance? distance time

Falling with air resistance? distance time

Falling with air resistance? velocity time

Falling with air resistance? velocity time Terminal velocity

Gravity

True or false? If the earth stopped spinning we would float into space. There is no gravity on the moon. NASA have an anti-gravity room where gravity can be turned off and people can float. It is possible to shield the force of gravity by using lead or other materials. Isaac Newton discovered gravity. Nottingham Forest won the Champions league in 1979 and You have no mass in space. There is no gravity in space because you are far from the earth. Weight is measured in kilograms. There is no gravity in space because space is a vacuum. If we could suck all the air out of the classroom, we would float around.

All false! If the earth stopped spinning we would float into space. There is no gravity on the moon. NASA have an anti-gravity room where gravity can be turned off and people can float. It is possible to shield the force of gravity by using lead or other materials. Isaac Newton discovered gravity. Nottingham Forest won the Champions league in 1979 and You have no mass in space. There is no gravity in space because you are far from the earth. Weight is measured in kilograms. There is no gravity in space because space is a vacuum. If we could suck all the air out of the classroom, we would float around. Only this one is true!

Gravity What is gravity?

Gravity Gravity is a force between ALL objects! Gravity

Gravity is a very weak force. The force of gravitational attraction between two people (when 1 metre apart) is only around Newtons!

Gravity The size of the force depends on the mass of the objects. The bigger they are, the bigger the force! Small attractive force Bigger attractive force

Gravity The size of the force also depends on the distance between the objects.

Gravity We only really notice the gravitational attraction to big objects! Hola! ¿Como estas?

Gravity The force of gravity on something is called its weight. Because it is a force it is measured in Newtons. Weight

Gravity On the earth, Mr Fletcher’s weight is around 950 N. 950 N I love physics!

Gravity On the moon, his weight is around 160 N. Why? 160 N

Gravity In deep space, far away from any planets or stars his weight is almost zero. (He is weightless). Why? Cool!

Mass Mass is a measure of the amount of material an object is made of. It is measured in kilograms.

Mass Mr Porter has a mass of around 95 kg. This means he is made of 95 kg of blood, bones, hair and poo! 95kg

Mass On the moon, Mr Fletcher hasn’t changed (he’s still Mr Fletcher!). That means he still is made of 95 kg of blood, bones, hair and poo! 95kg

Gravity In deep space, Mr Fletcher still hasn’t changed (he’s still Mr Fletcher!). That means he still is made of 95 kg of blood, bones, hair and poo! 95kg I feel sick!

Mass and weight Mass is a measure of the amount of material an object is made of. It is measured in kilograms. Weight is the force of gravity on an object. It is measured in Newtons.

Calculating weight The force of gravity on one kilogram is called the gravitational field strength. It s measured in Newtons per kilogram (N/kg)

Calculating weight To calculate the weight of an object you multiply the object’s mass by the gravitational field strength wherever you are. Weight (N) = mass (kg) x gravitational field strength (N/kg)

Example The gravitational field strength on earth is around 9.81 N/kg. What is your weight if your mass is 45 kg? Weight (N) = mass (kg) x gravitational field strength (N/kg) Weight (N) = 45 kg x 9.81 N/kg Weight = 441 N to 3sf

Easy isn’t it?