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Force and Newton Laws - Braemar College

Looks at types of forces and how Newtons Laws explain these occurences
by

DJ Horvat

on 20 July 2014

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Transcript of Force and Newton Laws - Braemar College

Force
Newton's First Law
Newton's Second Law
Newton's Third Law
Contact Forces
Non-Contact Forces
Weight
Air Resistance
Terminal Velocity
The force you apply is very obvious when you physically push or pull something.
This is a very obvious contact force.
Force is measured in the Newton (N)
Other forces such as gravity act on objects without any actual contact at all.
Friction acts between any two surfaces that try to slide over one another. Friction acts in the opposite direction to the movement or attempted movement.
Air resistance and drag are caused by friction between air (or liquid or other gases) and a moving object. Like friction, it acts in a direction opposite to the movement.
Thrust is caused by gases or liquid being pushed out the rear of an engine, jet or rocket.
Weight is caused by gravity. It acts downwards, towards the centre of the planet.
Magnetic force is the force of repulsion of like poles (N/N or S/S) or attraction of unlike poles (N/S).
Sir Isaac Newton
Electrostatic force is the force of repulsion of like charges (+/+ or -/-) or attraction of unlike charges (+/-).
“I can calculate the motion of heavenly bodies, but not the madness of people.”
Why?
Because of this guy...
For that reason we can classify force as either contact on non-contact forces
Sir Isaac Newton was an English scientist who in 1687 was able to describe the motion of everything using three Laws...
These Laws are called...
Newtons's Laws
Place a pen on the desk. Watch what it is doing. Of course, it’s not moving. This effect is called inertia.
Anything at rest will stay that way unless pushed or pulled.
This means that a force is required to get something moving.
Why do you wear seatbelts in a moving car?
Anything that is moving will keep moving at the same speed and in the same direction unless a force changes it.
If a car is traveling at 60 km/h, then so are you. If the car collides with something, it will stop very quickly
Unbelted passengers will keep traveling at 60 km/h until stopped by the windscreen or dashboard, generally hitting it with their head.
Seatbelts and Airbags restrain you and allow you to decelerate with the car. They also spread the stopping force over your chest waist and head.
Instead, the car stops moving and you keep moving just as you were before the accident.
This suggests that something pushed you …
If you answered ‘Because you are thrown forwards in a car accident’, then you’re wrong!
The Force is with you
Put simply this is called...
Acceleration is any change in velocity. This may be a change in speed, such as from 10 to 20 m/s, or a change in direction, such as from north to east.
All acceleration requires a force. Bigger forces produce higher accelerations.
Two people pushing a car will be more effective than just one person pushing it.
Newton’s First Law says that if there is no force, then nothing happens. Newton’s Second Law says:
Something will happen if a force is applied: the object will accelerate and the acceleration will depend on the mass of the object.
Some of these forces may balance each other by canceling each other out. If cancellation is complete then the overall force is zero and there will be no acceleration. You don’t speed up, nor do you slow. You just stay stationary, or keep traveling, as you were before.
Things fall, roll or accelerate downwards because there is a non-contact and invisible force called weight pulling them to Earth.
Weight is the force that pulls objects down to the surface of a planet.
On the Earth’s surface, the acceleration of all objects is 9.8 m/s/s. This means that the speed of a falling object increases by about 9.8 m/s every second it is falling.
An object pushes air out of its way as it falls. The air pushes back with an equal, upwards force called air resistance.
Air resistance increases as speed increases...the faster something is falling, the more the resistance on it.
Eventually air resistance balances weight and the total force on the object is zero.
All objects have a terminal velocity, but its value will depend on the shape and size of the object.
As we know...
A higher air resistance will produce a lower acceleration when falling
When this happens, there can be no more acceleration and the object falls at a constant speed called its terminal velocity.
Weight depends on the mass of the object and the acceleration caused by the planet’s gravity.
Mass is the amount of matter in an object. It never changes unless you snap a bit off or add more to it.
A 2 kg mass, for example, is always 2 kg regardless of where you take it around the universe.
Whenever a weapon is shot, it recoils or moves backwards, just like a hose shooting water.
The explosion of gunpowder in a cannon, for example, will push a cannonball out (the action) and the cannon will recoil because of the force the ball applies back on it (the reaction).
Newton explained the action-reaction phenomena in his Third Law, which states:
For every action force there is an equal and opposite reaction force.
Getting moving
To move forwards, you first need to push backwards. This is most obvious when riding a bike or driving a car.
The driving wheel pushes the road backwards when it turns, sometimes also spraying sand, mud or water backwards. The road will then push the bike or car forwards.
This action-reaction pair depends on the traction or friction between the tyre and the road. On ice, the wheel will simply spin on the spot and you will go nowhere.
Aircraft and ships also use action-reaction to move. Propellers or jet engines push air or water backwards and so the water pushes the aircraft or ship forwards.
Rockets use action-reaction to provide the thrust force needed for launch.
The rocket expels massive quantities of gases out and the gases push the rocket back, usually upwards.
By D. J. Horvat
Inertia depends on the mass of the object. The greater the mass, the greater the inertia.
Momentum
When you hammer a nail into a wooden wall, you will find that a hammer with a greater mass will give a greater drive…
The momentum of an object depends on its mass and speed
Momentum often indicates the difficulty an object has in stopping…
For example...
A truck has more momentum than a car moving at the same speed. If the truck collides with something then it would do more damage.
Change in Momentum
We have seen that Newton’s second law of motion can be written as…
By substituting the acceleration formula you can find the force due to change in momentum.
Considering the change in momentum, you can reduce the force during a collision by lengthening the time of the collision.
Crumple zones and airbags are designed to increase the time of the collision ultimately reducing the force on the occupant.
Put simply...
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