Force
Forces
The earth pulls you down
When you dive in the pool or ocean, the water pushes against your skin
When you drive fast, the car seat pushes into your back
Your enemy may push you to the ground, but your friend will pull you back up to your feet
If your walk you dog, and he is strong and fast, like Luka, then you will be pulled along
A truck pulls a trailer
A bulldozer pushes the soil
A digger pushes its bucket. When the bucket is tipped, the earths gravity pulls the soil to the ground
So Forces push or they pull.
A Force is a Push or a Pull
Force
Forces - Specific Learning Objectives
Identify examples of forces and represent them in diagrams.
Add forces to determine the net force on an object.
Distinguish between contact and non-contact forces, giving examples of each.
Describe the different effects of balanced and unbalanced forces on the motion of an object.
Explain that an object, on which the forces are unbalanced, must accelerate in the direction of the net force.
Explain that the acceleration of an object is proportional to the net force acting on it.
Name the units of measurement for force, mass and acceleration of an object which is acted upon by a given constant unbalanced force.
Use the relationship F=ma to calculate the net force, mass or acceleration of an object.
Forces are a push or a pull
Forces cause things to move.
When a Force is applied to a thing it might
Change direction
Change velocity
Change shape
Force can only cause movement if it is unbalanced
A Balanced force is like you leaning on a wall
A Unbalanced force is like the Hulk leaning on a wall and it tips over
Movement will only happen if the Force is Unbalanced
Balanced Forces = like tug of war and there is no movement
Unbalanced = tug of war and one side wins
Force in Newtons
The unit for Force is named after the person that dedicated his life to the study and exploration of physics - Newton
Contact vs Non Contact Forces
Although a force is a push or a pull. The force doesn't need to be touching!
When you push on a toy truck to move it - this is contact - so its a contact force
When you use a magnet to push the truck around you are not touching the truck, this is non-contact
Contact forces are whenever two things are touching. This includes
Friction
Air friction and water friction - Drag
Tension - springs, rope, rubber band, slingshot
Support or Normal force (the force of the ground)
Thrust
Buoyancy (in air or water)
Non-contact forces are forces that don't touch. This includes
Magnets
Electrostatic force - this is what causes your hair to raise up with a balloon
Gravity
Mr Cowley Lecture
Watch the GCSE Physics clip above, it is really good!
Forces, Balanced and Unbalanced
Mr Cowley Lecture
If this force is balanced by an opposing force, then nothing happens.
Like when you push against a wall - your force balanced by the wall, so nothing happens.
Likewise if you squeeze on a unopened can of coke, then nothing happens to it's shape as the force you exert on the can is balanced with the force that the can pushes back onto your hand with.
If you push against an un-latched door, then the door can-not balance your force, thus the forces are unbalanced and the door swings open
Likewise, if you squeeze a opened can of coke, it will crumple. This is because the force you exert on the can is not in balance with the force the can applies to your hand, so it crumples in the direction that the unbalanced force is applied.
You only get a change in motion if the forces are unbalanced.
If they are balanced then their is no change in motion (regardless of if the object is in motion or stationary)
The YouTube clips below look at the consequences of unbalanced forces
Balanced = No Change in Motion (or shape)
Unbalanced = Change in Motion (or shape)
Force Vectors
Forces can be drawn from the center of the object in straight lines, to show the direction they are adding.
Forces in the same direction add together
Forces in opposite direction subtract from each other
The addition and subtraction will give a net force
If the Net force is zero, then you have No change in motion
If the Net force is not zero, then you will have motion in the direction that the force is pushing (or pulling) the object
You draw the arrows away from the center
If the opposing arrows in the same axis are the same size, then the forces are equal in that axis
If the opposing arrows in the same axis are different sizes, the the forces are unbalanced, and the change in motion will be in the direction of the bigger arrow
Mr Cowley Lectures
Mr Cowley Lecture Force Diagrams and Net Force
Force = Mass x Acceleration
If you push the house as hard as you can you will notice that ... It will not move - this is because the force the house exerts on your hand is equal to the force that you exert on it, so the forces are balanced
If you push on a car (that is on level ground, and has the brakes off) - it will move
The more force you can generate the faster it will move
That's assuming that you can generate enough force to overcome friction within the rolling components of the object
The greatest force is used to get the object to move, this is because of inertia. An object will resist change in its movement. It it at rest then it should remain at rest. Once the object is moving, it should remain moving. Again due to inertia. It should move forever. However, on earth, friction opposes this movement.
Due to inertia, the force to get an object to move is greater than the force required to keep an object moving.
However, due to inertia, once moving, the object will continue to resist changes to its velocity. It want's to keep the velocity constant. Keep 0km/h constant, or keep 100km/h constant.
To change this motion, force needs to be applied. To slow a car down from 100km/h to 50km/h frictional force needs to be applied. This can be applied gradually, such as taking your foot off the accelerator and gradually slowing down, using the rolling friction of the car. Or it can be applied quickly, such as releasing a parachute from the boot, tied to the towbar. Or, by applying the brakes. The first increases air resistance (drag), the second increases rolling friction between the tires and the road. (But don't lock up your brakes, or you moving into sliding friction, the tires get hot, the rubber melts, and your friction reduces!)
The car will happily then stay at 50km/h. It has momentum (momentum = velocity x mass). The car's inertia, which is its need to stay the same, means that you don't need to any petrol to keep the mass of the object moving, rather the fuel is used to overcome friction.
Think of the International space station. It moves at 27000km/h without using any boosters or rockets, without applying any additional force - this is because it uses its inertia - it will stay doing exactly what it is doing unless a force acts on it. Because it is in space it doesn't need to overcome the force of friction. But, it did need thrusters to get to the position where it can move at 27000km/h and that was done as by the huge rockets that lifted it to its position. As those rockets enter space, they don't need to keep applying their thrusters. As there is no air to slow them down, they can just keep on going. What about gravity - well that is pulling the ISS back down, whilst its momentum is at a right angle to gravity - so it wants to fly into outerspace - the two interactions result in the ISS orbiting the planet.
The same with the Starman Tesla car - it was traveling over 20,000km/h when it was the rockets finally powered down. Over two years later it is still traveling at over 20,000km/h as there is nothing to slow it down.
So to summarize, to change motion, you need to input a force. In space, any force will cause a change in motion. On earth, the force of friction is always opposing motion, so you need to input an equal and opposite force to over friction and maintain your velocity. If you want to speed up, then you'll need to put in more force to overcome the inertia of the object.
Kyle and Ronak use a force-meter and a skateboard to show this in the video below.
Thanks to Kyle and Ronak, MHJC Class of '18
You'll notice the readings on the Force meter that Kyle and Ronak are using is in Kg and N
Whats the difference?
Kg or Kilogram is the unit for the mass of an object
N or Newton the unit for the Force applied
In this experiment Kyle and Ronak are investigating force so they discuss it in Newtons
Side note: 1 Newton is equivalent to 1kg being accelerated at a rate of 1 meter per second, per second
If all the forces are balanced, you get no change in motion, so an object in motion will stay at that velocity, it won't get slower or faster
If one of the forces is greater than the others, thus unbalanced forces, then you will have increased, or decreased motion in that direction
Newtons Laws
Mr Cowley Lectures on Acceleration
Isaac Newton thought about Forces. He condensed the phenomena of motion into 3 Laws. Naturally, they are named after the author of the Laws.
Newton's First Law
The Law of Inertia
An object at rest or in motion, will remain at rest or in motion, unless acted upon by a force
This is why you need to use more fuel, and the revs in the car are higher whenever you accelerate
Newton's Second Law
Force = Mass * Acceleration
The rate of acceleration of the object is proportional to the force acting on it, and inversely proportional to the mass of the object
This is why more revs in the car, the faster it will accelerate. However, if you are towing a trailer, then for the same revs, and thus force, you will accelerate slower, because your mass is greater
Newton's Third Law
Equal and Opposite
For every action there is an equal and opposite reaction.
If you pour oil under your tires, you can push down on the accelerator, your wheels will spin, and you wont move. This is because the road needs to push against your tires for you to move.
When you accelerate away at the lights, your tires push against the road, the road pushes equally against the tires.
When you push against a door, the door pushes against your hand. With the forces being unbalanced, the door swings open. The bigger the door, like a nuclear bunker door, the more force it will push against you and the more force you will have to exert for the force to be unbalanced.
When gravity pushes you onto the ground, the ground pushes you back up (Normal force, or supportive force). Otherwise, you'd sink, like in a swamp, or in a quicksand.
When you shoot a rifle, the explosion of the gunpowder in the bullet, in the chamber of the gun, pushes the bullet forward, and it pushes the gun backwards into your shoulder
Newtons Second Law
Force = Mass x Acceleration
Newtons three Laws apply to any motion and all motion
Newtons second law can be used to quantify that motion. It turns it into numbers
Force will be measured in Newtons
Mass will be measured in Kilograms
Acceleration will be measured in m/s/s
F = m x a
Rearranging F=ma
Like all equations, we can rearrange the formula to find each of it's parts.
For instance, If we rearrange F = ma to isolate 'a' then we get: a = F/m
So, if I launch two bottle Rockets
Bottle rocket 1 is large, has a greater mass and is launch with less thrust force.
Bottle rocket 2 is small, has less mass and is launched with more thrust.
Which one will accelerate the fastest?
Acceleration = F/m
With my car data, you can hear when the engine working the hardest, it is exerting the greatest force. Its during these times, that the cars acceleration is the greatest
Acceleration = F/m
With my car data, you can hear when the engine working the hardest, it is exerting the greatest force. Its during these times, that the cars acceleration is the greatest
Why doesn't the train 'appear' to slow down when it makes contact with the trees?
Mass = F/a
Each of the trees that hit the train exert a force that causes the train to negatively accelerate (deceleration).
Because the train is has a very, very large mass, the rate of deceleration due to the force the trees is unnoticeable
Force = ma
The greater the stopping distance, the lower the rate of deceleration
The lower the rate of deceleration the less force is being used to slow the vehicle - for example, if you drift to a stop because you run out of petrol
However, the shorter the stopping distance, the higher the rate of deceleration. Thus the greater the force. Because of this, cars have crumple zones. These marginally increase the time for deceleration (in microseconds) but this decreases the value for deceleration and thus decreases the force. This decrease, although slight, can be the difference between life and death.... As shown, in the video clip above
Questions for: Force = Mass x Acceleration
Have a go at the following questions. The answers are after the video clips.
Questions:
An empty truck and trailer with a mass of 18 tonnes pushes it to the limit and goes from 0 to 100km/h in 30 seconds.
What is its rate of acceleration ?
What was the forward force it generates?
Later that day the trailer is loaded up with 45 tonnes of goods. This brings the truck up to 60 Tonnes. Again the driver pushes it to the limit and goes from 0km/h to 100km/h. However, now that the truck is more massive, and assume that the same net thrust force is generated by the engine
How long will it take the driver to get to 100km/h?
Answers
An empty truck and trailer with a mass of 18 tonnes pushes it to the limit and goes from 0 to 100km/h in 30 seconds.
What is it's rate of acceleration?
a. 100km/h in m/s is 100km/h divided by 3.6. (remember, it is divided by as it is going from big units to small units). This gives 27.8m/s.
b. Then, it takes 30 seconds to go from 0 m/s to 27.8 m/s. So the rate of acceleration is a= (v2-v1)/t a = (27.8 - 0) / 20 = 0.927m/s/s
What was the forward force it generates?
a. 1 tonne is 1000kg, so 18 tonnes is 18,000kg
b. Force = mass x acceleration. So, F = ma. Then F = 18,000 x 0.927 F = 16,686N
Later that day the trailer is loaded up with 45 tonnes of goods. This brings the truck up to 60 Tonnes. Again the driver pushes it to the limit and goes from 0km/h to 100km/h. However, now that the truck is more massive, and assume that the same net thrust force is generated by the engine
How long will it take the driver to get to 100km/h?
a. 60 tones is 60 x 1000 = 60,000kg
b. F = ma so to isolate 'a' you divide the big F by m a = F/m
a = F/m a = 16,686N /60,000kg = 0.278m/s/s
c. Then to get the time taken you need to rearrange a=v/t so t = v/a ( is simple math'd it, I went 3=6/2 so 2=6/3)
t = v/a
t = 27.8ms / 0.278 = 100 seconds
What does F=ma mean for Velocity?
In space
The best example is in the rocket launch Youtube clip. Skip to the 17th or 18th minute. You'll see that as long as the thrusters are on, the Rocket is accelerating, as evidenced by the increasing velocity.
The moment the Thrusters are turned off, the Thrust Force drops to 0% and the acceleration immediately becomes 0 as evidenced by the now constant velocity of 16,957.7mph. The shuttle stays at this constant velocity and it will only change if a new force is applied to it.
What does F=ma mean for Velocity?
In a Car
On earth the consequence of F=ma is that to accelerate an object you need to apply a force
Conversely
An object will only accelerate if a force is applied!!
So, my car only accelerates if an unbalanced force is applied. You can hear this in the engine! - Watch the Force causes acceleration clip.
Once I'm at a velocity I want to be at, then I lift my foot from the accelerator. But, unlike the space shuttle, a forward force still needs to be applied to overcome the force of friction - but this is balanced by friction, causing constant velocity and zero acceleration (e.g. at 50km/h). During constant velocity and zero acceleration, the forces of thrust and friction are balanced so Net Force is 0 N
What does F=ma mean for Velocity?
When towed on a Skateboard
Kyle and Ronak show the difference between the Force required to accelerate an object.
When Accelerating an Object you apply an Thrust Force which is greater than the Frictional Force, this makes the Net Force Unbalanced. This Unbalanced Force accelerates the object.
When maintaining constant Velocity, the Thrust Force is balanced by the Frictional Force. This makes the Net force balanced, and thus zero. As Net Force is zero, their is no acceleration.
What does F=ma mean for Velocity?
When firing a Bullet
A bullet only accelerates when in the Muzzle of the gun.
For this reason, rifles have long muzzles, this gives more time for the acceleration.
This is because, a bullet accelerates by the explosive force caused by hot air expanding behind the bullet. Once the bullet leaves the muzzle, the pressure of this explosion goes around the bullet and into the air (this is why a bullet fits snuggly into the muzzle). You can see the hot gas leave the muzzle behind the bullet in the slow-mo video).
The second the bullet leaves the gun, it stops accelerating.
The bullet will now maintain constant velocity
(ignoring air resistance, which, by being an opposing force, will slow or decelerate the bullet and cause the velocity to actually decrease)
What does F=ma mean for Velocity?
When Throwing a Ball
The ball only accelerates when it is in your hand. As soon as you let go the balls velocity, in a vacuum, will be constant. It'd have constant velocity in a vacuum. However, you probably thew the ball in a normal situation, so It will actually start to decelerate due to air friction will start to slow the ball down.
As soon as the ball leaves your hand, Gravity will pull it towards the ground.
What does F=ma mean for Velocity?
When running (or biking or driving) off buildings and cliffs
The last moment that that the person, bike or car can accelerate in a forwards direction is in the last moment that they can apply that thrust force, which is whilst they are on the building, or cliff. As soon as they step off or drive off, then their is no more thrust force, and so no more forward acceleration. Acceleration immediately becomes zero. You then only have forward velocity due to inertia. Theoretically the forward velocity should be constant. Practically, the forward velocity will decrease over time, due to deceleration due to air resistance.
At the same time, one the person, bike or car is no longer supported with the Normal force of the cliff or building, then they will accelerate towards the earth, and their velocity towards the ground will increase in a linear manner.