Forces Triple
Force of a car crash example question
A car travelling at 20 m/s collides with a stationary lorry and is brought to rest in just 0.02 m/s. a women in the car has a mass of 50kg. she experiences the same decleration when she comes into contact with a hard surface of the car ( such as the dashboard or the wind screen). What force does that person experience? Force = change in momentum / time Force = (50 x20) - (50 x 0) / 0.02 Force = 50 000N
What is a turning effect?
A force can have a turning effect - it can make an object turn around a fixed pivot. When the anticlockwise turning effect of forces are balanced by turning forces in the clockwise direction, the object will not turn - it is in balance. Unbalanced forces acting on objects can make them accelerate or decelerate. Ion a see sae, the forces are having a turning effect. The making of the see-saw turn around a fixed point called a pivot. We can use the turning effect all the time. In our bodies for example the forces of our muscles makes part of our bodies turn around joints like our elbows or knees. When you turn a door handle, open a door or remove a lid off a tun of paint with a knife you are using the turning effect of forces.
Why are car crashes dangerous
A force on a person in a car crash can be very large. You can work out the force in a crash using deceleration and also momentum equations.
What does it mean by a object is in balance
A object will be in balance ( that is it will not try to tur about a pivot point) of: - sum of anticlockwise moments = sum of clockwise moments
Momentum and collision example question
A railway truck with a mass of 5000kg rolling at 3 m/s collides with a stationary truck of mass 10 000 kg. the trucks join together. At what speed do they move after the collision? We shall assume that friction forces are small enough to ignore so we can apply the conservation law of momentum: Total momentum before the collision = total momentum after the collision So, momentum of A before collision + momentum of B before collision = momentum of A and B moving together after collision. (m1 x u) + (m2 x 0 m/s) = (m1 +m2) x v Where m1 is the mass of the truck A, u is its velocity before the collision, m2 is the mass of truck B ( at rest before the collision so its velocity is o m/s) and v is the velocity of the two trucks after the collision. Substituting in these values: (m1 x u) + (m2 x 0 m/s) = (m1 +m2) x v ( 5000 kg x 3 m/s) + (10 000 kg x 0 m/s) = 5000 + 10 000) x v v = 15000 kg m/s / 15000kg = 1 m/s After the collision the trucks move with a velocity of 1 m/s in the same direction that the original truck was travelling.
What happens when Objects not pivoted at the centre of gravity:
A simple see-saw is a uniform beam (plank) pivoted in the middle. The centre of its gravity of a uniform beam is in the middle, so the see-saw is pivoted though its centre of gravity. When an object is not pivoted though its centre of gravity the weight of the object will produce a turning effect.
What is the difference between balanced forces and action and reaction forces?
Balanced forces act in opposite directions on the same object Action and reaction forces also act the opposite direction but are acting on different objects.
What do cars have to make it more safe
Cars are now designed with various safety features that increase the time over which the car's momentum changes in an accident. crumple zones seatbelts airbags
Force=
Change in momentum / time
How do crumple zones prevent massive accidents
Crumple zones are just one safety features now used In modern cars to protect the passengers in an accident. If they hit something that does not crumple they will be brought to rest in a very short time, which, as we can see, means a large deceleration and therefore a large force acting on them.
How is a crane balanced?
For a large crane, the larger the masses of load, the closer to the pivot it should be, where the crane tower supports the crosspiece of the crane. Without careful balance the turning forces on support tower could cause it to bend or collapse.
Example for in balance question
If a person pushes the door with a force of 200N at a distance of 20cm from the hinge (pivot of this example). Person B opens the door by pulling the door handle which is 80 cm from the hinge. What is the minimum pulling force that person B must use to open the door? The door will be in balance ( on the point of opening) if: sum of anticlockwise moments = sum of clockwise moments f x 80 cm = 200 N x 20 cm F = 200 N x 20 cm / 80 cm = 50 N This means that the turning effects are balanced and we want person B to apply a big enough strength to make the anticlockwise moments bigger than the clockwise moment. The answer is that person B must apply a force greater than 50N Here is a simplified sketch:
turning effect diagrams
If you think about the simple door opening competition, you will realize that for a force to have the biggest turning effect it should be applied perpendicular ( at 90°) to the door. The force has no turning effect at all because the line along which the force is acting passes though the pivot. This shows how the distance to the pivot must be measured to get the correct values for the moment. The distance is the perpendicular distance from the line of action of the force to the pivot.
What can we get from the momentum question
Momentum is a vector quantity and is measured in kilogram metres per second (kg m/s) - provided that mass in the above equation is measured in kg and velocity in m/s You can see from the equation that the more mass the object has the more momentum it will have when moving. The momentum of an object also increases its speed.
What is the difference between momentum and a moment
Momentum is the product of mass and the velocity of a body. The moment is a concept that gives a measure of the effect of a physical property around an axis.
What is the equation for momentum
Momentum, p (kg m/s) = mass, m (kg) x velocity, v (m/s) p = m x v
what is Newtons cradle?
Newton's cradle is an entertaining toy but it also demonstrates a physics conservation law. When one of the balls is drawn back a short way and released, it swings and collides with the remaining group of balls. After the impact the ball at the opposite end springs away and swings out as far as the first ball was drawn back to start with. If two balls are drawn back and released then two balls will move away at the opposite end as the collision occurs. The moving ball has momentum, and momentum is conserved in collisions.
How is turning forces linked to opening a door
One person trying to close the door 20cm from the hinge (the part of the door that fastens it to the wall) and one person trying to open the door at the handle. You will see that the person opening the door will find it much easier than the person closing it, and its not about strength. The person opening the door has an advantage, because the turning effect of your force doesn't just depend on the size of the force you but also the distance from the hinge, or pivot at which you apply it. You have an advantage of greater leverage.
Example experiment of newtons third law
Person X is clearly pushing person Y but it is not obvious that person Y is pushing back. When both X and Y move it is clear that X has been affected by the force pushing him to the left. The force felt by X is the reaction force Another eg: the weight of a car pushing down on the ground, although the weight is shown coming from the centre of gravity of the car, it acts through the wheels pressing down on the ground. The total reaction force from the ground acting upwards on the wheels is the same as the weight of the car. It is this reaction force that stops the car from sinking into the ground.
What do roads have to make it safer for car
Steep roads often have escape lanes filled with deep, soft, sand. The soft sand slows heavy lorries that are out of control slowly - by making time, t, for the lorry stop longer, the force, F slowing the lorry is smaller and the driver is less likely to suffer serious injury.
Explosions and momentum
The conservation of momentum principle can be applied to explosions. An explosion involves a release of energy causing things to fly apart. The momentum before and after the explosion is unchanged, though there will be a huge increase in movement energy. Eg: Rocket motors use the principle of conservation of momentum to propel a space aircraft though space. They produce a continuous, controlled explosion that forces large amount of fast-moving gases (produced by the fuel burning) out the back of the rocket. The spacecraft gains an equal amount of momentum in the opposite direction to that of the moving exhaust gases.
Momentum example
The first stage of the type of rocket used in Moon missions provides an unbalanced upward (away from the Earth) force of 30MN and burns for 2.5 minutes. a) Calculate the increase in the rocket's momentum that results b) If the rocket has a mass of 3000- tones what is the velocity of the rocket after the first stage has completed its burn? ( convert units into N, and s. 1 NM = 10(6)N 1 tonne = 1000kg a. The equation F = (mv-mu)/ time is another that can be rearranged - mv - mu = f x t - t = mv-mu / f - if you want mv or mu on its own add or subtract from both sides. F x t = (3x10(7) N) x ( 2.5 x 60) Increase in momentum = 4.5 x 10 (9) kg m/s b. the rocket starts from rest u=0. Therefore the initial momentum (mu) is 0. This means that the increase in momentum is mv = 4.5 x 10 (9) kg m/s. divide this by mass m = 3 x 10(6) kg to give: v = 4.5 x 10 (9) kgm/s / 3 x 10 (6) kg v = 1.5 x 10 (3) m/s
What is the centre of gravity? (ruler example)
The point through which the weight of a body acts When you balance a ruler on your finger, the ruler is balanced, the anticlockwise moment equals the clockwise moment, but there are no downward forces acting in this situation other than the weight of the ruler itself. We know that the weight of the ruler is due to the pull of the Earth's gravity on the mass of the ruler. The mass of the ruler is equally spread throughout its length. It is not, therefore, surprisingly to find that the ruler balances at its centre point. We say that the centre of gravity of the ruler is at this point. The weight force of the ruler acts though the centre of gravity - it is the point where the whole of the weight of the ruler appears to act. If we support the ruler at this point there is no turning moment in any direction about the point, and it balances. The centre of gravity is sometimes called the centre of mass.
What is the link between force and momentum?
The relationship between f=ma, saying when an unbalanced force acts on an object it causes a change in momentum of the object in the direction of an unbalanced force. Newton discovered that the rate of change of momentum of an object is proportional to the force applied to that object. This means that if you double the force acting on an object, its momentum will change twice as quickly. This diagram shows the effect up thrust force (speeding up) on velocity of a space shuttle, and, therefore, on its momentum: Initial momentum of an object = mu Final momentum of an object = mv Therefore, increase of momentum = mv - mu So, rate of increase of momentum = mv - mu / t Newton identified a proportional relationship between the rate of increase of momentum and the force applied, but with the system of units we use the relationship: Force, f = change in momentum (mv-mu) / time taken F = mv-mu / t If you look at this equation you will notice it can be arranged to give a more familiar equation F =ma F = mv-mu / t Since : (v-u)/t = a Then : F = ma
What is the link between momentum and collisions?
The total momentum of objects that collide remains the same: Total momentum before the collision = total momentum after the collision You will need to use this to work out what happens to objects that collide when they're moving along the same straight line.
what is the equation for turning effect of a force?
The turning effect of a force about a hinge or pivot is called a moment. The moment of a force is defined like this: Moment of a force (nm) = force, f (N) x perpendicular distance from pivot, d (m) Moment = F x d The moment of a force is measured in newton metres (Nm) because force is measured in Newtons and distance is measured in metres. We need to be precise about what distance we measure when calculating the moment of a force.
How to seat belts prevent massive accidents
They only work best If the passengers are wearing seat belts so that the reduced deceleration applies to their bodies too. Without seat belts, the passengers will continue moving forward until they come into contact with some part of the car or with a passenger in front.
Forces on a beam
This diagram shows a boy standing on a beam across a stream. The bean is not moving so the upward and downward force must be balanced. As the boy is standing in the middle of the beam, the upward forces on the ends of the bream are same as each other. If he moved to one end of the beam, then the upward force at the ends of the beam change. Another example is he is one quarter of along the beam. The upward force on the support nearest to him is three quarters of his weight, and the upward force on the end furthest away from him is only one-quarter of is weight.
Example of support forces:
This sketch diagram shows a uniform wooden metre ruler with a mass of 0.12kg. a pivot at the 40cm mark supports the ruler. When a brass weight is placed at the 20cm mark the ruler just balances in the horizontal position. Take moments about the pivot to calculate the mass of the brass weight. For balance: Anti-clockwise moment = clock-wise moment (m x g) x 20 = (0.12 x g) x 10 The weight of the brass weight is 0.06kg. Here g is gravitational strength, but it cancels.
What is conservation of momentum
Total momentum before the collision = total momentum after the collision
What is momentum
When we say that object 'gain momentum' we are trying to get cross the idea of something becoming more difficult to stop. For example as a car is rolling down the hill we say that the car is 'gaining momentum' as it speeds up. Momentum is a quantity possessed by masses in motion. Momentum is a measure of how difficult it is to stop something that is moving the product of an object's mass and velocity
What is newtons third law?
When you push something it pushes you back just has hard, but in the opposite direction. As well as: for every action there is an equal and opposite reaction When you sit down, your weight pushes down the on the seat. The seat pushes back on you with an equal weight, but upward.
object in balance experiment
for example, children sitting on a see-saw: in this diagram a: the anti-clockwise moment = 200N x 1.5m = 300 Nm the clockwise moment = 300N x 1.5 = 450 Nm So the see-saw is not balanced and leans down to the right as it rotates clockwise about the pivot. The calculations for diagram a are approximate because the forces are not acting perpendicularly to the see-saw. For diagram b: the anti-clockwise moment = 200 x 1.5m = 300 Nm the clockwise moment = 300N x 1.0 = 300 Nm so the see-saw is balanced.
