Forces Flashcards
Describe what is Meant by a Scalar Quantity
-Scalar quantities only have a magnitude. They do not have a direction.
Examples include:
-Distance, Speed, Temperature, Mass, Energy
Describe what is Meant by a Vector Quantity
-Vector quantities have both a magnitude and a direction.
Examples include:
-Force, Velocity, Momentum, Acceleration, Displacement
-Vector quantities are usually shown by an arrow- the length of the arrow shows the magnitude, and the direction of the arrow shows the direction of the quantity.
Describe how to Draw a Free Body Diagram
-Draw the arrow with a ruler to scale.
-Label the arrow with the quantity.
-Show the object with a dot on the end of the arrow.
-Positive quantities are shown to the right and negative quantities are shown to the left.
Describe the Effects of Forces in Physics
-A force is a push or a pull experienced when one object interacts with another object.
-Forces can change the speed or direction that an object moves. Forces can also change an object’s shape.
-Because forces have magnitude (size) and direction, this means that forces are vector quantities. The unit of force is the newton (N).
-A contact force can take place when objects are touching. If the objects experience a force when they are not touching, then this is an example of a non-contact force.
Describe the Features of the Main Non-Contact Forces
Gravitational Force is always an attractive force. It acts between all objects e.g. between a planet and a satellite.
Electrostatic force can attract or repel. It acts between objects that have a charge e.g. between two positive particles.
Magnetic force can attract or repel. It acts between magnets and certain objects e.g. between two magnetic north poles.
Describe the Features of the Main Contact Forces
-An object at rest on a surface experiences reaction force.
-An object that is being stretched experiences a tension force.
-Two objects sliding past each other experience friction forces.
-An object moving through the air experiences air resistance.
Describe what is Meant by Gravitational Force
-Gravity attracts all masses. All objects with mass produce a gravitational field.
-On the surface of a planet, gravity causes all things to fall to the ground. It also gives everything a weight.
Describe the Difference Between Weight and Mass
-The mass of an object tells us the amount of matter in the object. The mass does not depend on where the object is. Mass is measured in Kg with a mass balance.
-Weight is the force acting on an object due to gravity. Because weight is a force, the unit of weight is the newton. It can be measured using a newtonmeter.
-The weight of an object depends on the strength of the gravitational field at the location of the object. The single point in an object where the weight is considered to act is called the centre of mass.
Give the Calculation for Weight
Weight (N) = Mass (kg) x Gravitational Field Strength (N/Kg)
W = mg
-Weight and mass are directly proportional.
-The gravitational field strength on Earth is around 9.8 N/Kg
and on the moon is around 1.6 N/Kg.
Describe what is Meant by the Resultant Force
-When several forces are acting on an object, we could replace all of the forces with a single force which has the same effect.
-Scientists call this single force the resultant force.
-If the forces are acting in parallel to each other then we simply subtract the smaller force from the larger force.
-If all the forces acting on an object give a resultant force of zero, the object is in equilibrium.
-When the forces are acting on an angle, we draw a vector diagram.
Describe how to Draw a Vector Diagram
-Draw one arrow horizontally with a ruler to scale.
-Use a protractor to draw the other arrow at the given angle.
-Label the arrows with the given quantities.
-Complete the parallelogram with dotted lines.
-Measure the distance between the top right and bottom left angles. This distance is the resultant force.
Describe what Happens when a Resultant Force Moves
-When a force moves an object through a distance, energy is transferred and work is done on the object.
-To make something move, (or keep it moving), a force must be applied. The thing applying the force needs a source of energy.
-The force does ‘work’ to move the object and energy is transferred from one store to another.
Describe the Energy Transfers Taking Place When a Boy Pulls a Sledge
-Tension in the rope transfers energy from the boy’s chemical energy store to the kinetic energy store of the sledge.
-The force of friction is acting between the ground and the boy pulling the sledge.
-Friction transfers energy from the kinetic energy store of the sledge to the thermal energy stores of the sledge
runners and the ground.
Explain why the Temperature of the Brakes in a Car Increases when the Car Brakes
-The force of friction acts between the brakes and the wheels.
-Because of this, the kinetic energy store of the car has been transferred to the thermal energy store of the brakes.
-This causes the temperature of the brakes to increase.
Give the calculation for Work Done
Work Done (J) = Force (N) x Distance (m)
W = fs
-Work done can be calculated in Joules or Newton-metres
Describe how Objects Change Shape when a Force Acts Upon It
-When a force acts on an object, the object may change shape by bending, stretching or compressing.
-However, there must be more than one force acting to change the shape of a stationary object.
-If one force is applied to a stationary object, then the forces would be unbalanced and we would have a resultant force. This would cause the stationary object to move.
What is an Elastic and an Inelastic Material
-An object has been elastically deformed if it can go back to its original shape and length after the force has been removed. Objects that can be elastically deformed are called elastic objects.
-An objects has been inelastically deformation if does not fully reverse when the force is removed - there is a permanent change in shape. Objects that behave like this are called inelastic objects.
Describe the Energy Transfers when Someone Stretches or Compresses an Elastic Object
-When a person stretches or compresses an elastic object, chemical energy stores in the person’s muscles transfer to the elastic potential energy store in the elastic object.
-This is an example of work done. The total work done is equal to the energy transferred as long as the object is not inelastically deformed.
State Hooke’s Law
The extension of a spring (or any elastic material) is directly proportional to the force applied.
Give the Calculation for the Force Needed to Compress or Stretch an Object
Force (N) =Spring Constant (N/m) x Extension (m)
F = ke
What is Meant by the Spring Constant
-Spring constant is a measure of the stiffness of a spring up to its limit of proportionality.
-The limit of proportionality refers to the point beyond which Hooke’s law is no longer true when stretching a material
-The higher the spring constant, the stiffer the spring. The spring constant is different for different elastic objects.
-For a given spring and other elastic objects, the extension is directly proportional to the force applied.
What is Meant by the Elastic Limit of an Object
-The elastic limit of a material is the furthest point it can be stretched or deformed while being able to return to its previous shape. It is also called the limit of proportionality.
-When an elastic object is stretched beyond its elastic limit, the object does not return to its original length or shape when the force is removed.
-Once a material has gone past its elastic limit, its deformation is said to be inelastic.
Describe a Method for the Investigating Springs Required Practical
-Secure a clamp stand to the bench using a large mass on the base. Use bosses to attach two clamps to the clamp stand. Attach the spring to the top clamp, and a ruler to the bottom clamp.
-Adjust the ruler so that it is vertical, and with its zero level with the top of the spring. Measure and record the unloaded length of the spring.
-Hang a 100 g slotted mass carrier from the spring. Measure and record the new length of the spring.
-Add a 100 g slotted mass to the carrier. Measure and record the new length of the spring. Repeat the method until you have added a total of 1,000 g.
-Record the results in a table and use them to plot a force-extension graph.
How does the Large Mass on the Base of the Clamp Stand Make the Investigating Springs Required Practical Safer
-In this practical, we are hanging weights from the spring.
-At some point, the weight that we add could be enough to make the apparatus topple.
-If this happens then the weight could land on our feet.
-To stop this, we place a heavy weight on the clamp stand.
Why is it Important to Keep the Ruler Straight in the Investigating Springs Required Practical
-In this experiment, the metre rule must be kept vertical and the pointer must be kept horizontal.
-This helps to make the results accurate. Accurate results are close to the true value.
Describe how to Work Out the Extension of the Spring in the Investigating Springs Required Practical
To work out the extension of the spring, we take the value with the weight added and subtract the value when no weight is added.
Describe how to More Certainly Find the Limit of Proportionality in the Investigating Springs Required Practical
-Because we are adding 1 N weights to the spring, we cannot be certain of the exact point where we exceed the limit of proportionality.
-The way to address this is to add 0.1 N weights. This will allow us to more accurately pinpoint the weight where we exceed the limit of proportionality.
Describe what is Meant by a Moment
A moment is the turning effect of a force.
Give the Calculation for Moments
Moment (Nm) = Force (N) x Distance from Pivot to Line of Action of Force (m)
M = Fd
-The distance from the pivot to the line of action of the force may also be called the perpendicular distance.
-This is when the distance from the pivot is at right angles to the line of action of the force.
Explain why Turning a Nut is Easier with a Longer Handled Spanner
-If we increase the length of the spanner handle, we increase the distance between the pivot and the line of action of the force.
-This increases the moment, making it easier to turn the nut.
Describe how a Wheelbarrow Could be Designed to make Lifting the Load Easier
-The wheelbarrow could be redesigned with a longer handle. This would increase the distance from the pivot to the line of action of force.
-The moment equals the force multiplied by the distance from the pivot to the line of action of the force.
-With a longer handle, we could achieve the same moment by applying a smaller force.
Explain why a Screwdriver with a Thicker handle will be Easier to Use
-A screwdriver with a thick handle will require less force to turn the screw than a screwdriver with a thin handle.
-This means that the distance from the pivot to the line of action of the force is greater with a thicker handle than a thinner one.
-Because of this, we will need to apply a smaller force to the screwdriver with a thicker handle than to the screwdriver with a thinner handle in order to achieve the same moment.
Describe the Moments in a balanced Object
If an object is balanced, the total clockwise moment about a pivot is equal to the total anticlockwise moment about that pivot.
Explain what Must happen for a Crane to Carry a Mass Greater than its Own
-If the crane is lifting a greater mass then the anticlockwise moment due to weight will increase.
-To balance the crane, we will need to increase the clockwise moment due to the counterweight.
-We cannot increase the mass of the counterweight so to increase the clockwise moment, we need to increase the distance between the counterweight and the pivot.
Describe the Purpose of Gears
-Gears have teeth which interlock so that turning one causes another to turn, in the opposite direction.
-They are used to transmit the rotational effect of a force from one place to another.
-Different sized gears can be used to change the moment of the force. A force transmitted to a larger gear will cause a bigger moment, as the distance from the pivot is greater.
-The larger gear will turn slower than the smaller gear.
Describe what is Meant by Pressure
Pressure is the force per unit area
Give the Calculation for Pressure
Pressure (Pa) = Force (N) / Area (m^2)
P = F/A
Explain why When Doing a Handstand, the Pressure is Greater than When Standing Upright
-The weight is unchanged.
-The area of the hands is less than the area of the feet so the pressure on the hands during the handstand is greater than the pressure on the feet when upright.
Explain why Snow Shoes are Useful for Walking Across Snow
-The contact area of snow shoes is greater than the contact area of soles of feet.
-Pressure equals weight divided by area so wearing snow shows means there is less pressure on the snow making them easier to walk in.
Explain how Particles in Fluids Produce a Pressure on the Walls of the Container
-When particles in a fluid collide with the walls of the container, they exert a force.
-This force acts at right angles to the surface of the wall.
Explain why the Pressure of the Atmosphere is Less at Higher Altitudes than the Earth’s Surface
-At higher altitudes the air is less dense than at sea level. This means that there are fewer particles per cubic metre of air.
-Because of this, the force exerted on a surface by the gas particles will be lower at higher altitude than at sea level.
-The pressure of a gas is due to the force exerted by the gas particles on the walls of the container. This means that gas pressure is lower at higher altitudes than at sea level.
Explain why Mountaineers Take a Supply of Oxygen When they Climb Very Tall Mountains
-The atmospheric pressure is low on tall mountains (ie at high altitude). This means that the number of gas particles per cubic metre is lower than it would be at sea level.
-Because of this, climbers may not be able to take in enough oxygen from the atmosphere for their body’s needs. To help with this, climbers often take a supply of oxygen.
Explain why Crisp packets Expand in Aeroplanes
-Crisp packets are full of gas. These gas particles collide with the walls of the crisp packet, applying a force.
-When the airplane is on the ground, the air pressure outside the crisp packet is the same as the gas pressure inside the crisp packet so the forces balance.
-However, when an airplane is flying, the air pressure in the cabin is lower than on the ground.
-This means that the pressure due to gas inside the crisp packet is greater than the pressure of the air in the cabin.
Now the forces are unbalanced and the crisp packet expands.
Give the Calculation for Pressure in Liquids
Pressure (Pa) = Height of Column (m) x Density of the Liquid (Kg/m^3) x Gravitational Field Strength (N/Kg)
p = hρg
Explain why the Walls of a Dam Needs to be Stronger at the Base
-The pressure at the base will be greater than the pressure near the top because the base has a greater force of weight acting upon it so the bottom of the wall needs to be thicker to hold the pressure.
Describe how Pressure in a Liquid Changes with Depth
-The pressure of a liquid depends on the depth. A greater depth has a greater pressure. Because the bottom of an object is at a greater depth than the top, it experiences a greater pressure.
-This means that the bottom of an object experiences a greater force than the top. Because of this, we have a resultant force acting upwards. Scientists call this upward force upthrust.
-The upthrust is equal to the weight of fluid that has been displaced by the object.
Describe how Upthrust Causes an Object to Float or Sink
-An object will float if the upthrust is equal to the weight of the object. This will happen if the object can displace its own weight of liquid.
-An object that is denser than the fluid it is placed in is unable to displace enough fluid to equal the weight. This means that its weight is always larger than the upthrust so it sinks.
Describe the Difference Between Distance and Displacement
-Distance is just how far an object has moved. It has magnitude (size) but no direction and is therefore a scalar quantity.
-Displacement measure the distance and direction in a straight line from an object’s starting point to its finishing point. It has both magnitude and direction which means that displacement is a vector quantity.
Describe the Difference Between Speed and Velocity
-Speed is how fast an object is travelling with o regard to the direction. It has magnitude (size) but no direction and is therefore a scalar quantity.
-Velocity is speed in a given direction. It has both magnitude and direction which means that velocity is a vector quantity.
Give the Calculation for Speed
Distance (m) = Speed (m/s) x Time (s)
s = vt
Give the Typical Speeds of Different Everyday Objects
A person walking- 1.5 m/s
A person running- 3 m/s
A person cycling- 6 m/s
A car- 13 m/s
A train- 50 m/s
A plane- 250 m/s
Sound- 330m/s
Describe what can Affect the Speed at Which a Person can Run
-A younger person will generally be able to achieve a faster speed than an older person.
-A fitter person will be able to run faster than an unfit person.
-A person running on flat ground will usually be able to run faster than a person running uphill.
-A person who has only run a short distance should be able to run faster than a person who has
already run a great distance.
State what Happens to the Speed of Sound When the Air is Warmer
The speed of sound increases when the air is warmer.
Explain why the Velocity of a Car Changes but the Speed remains Constant When Travelling Around a Roundabout
-When objects move in a circle, the object’s velocity is constantly changing even though its speed is constant.
-That is because velocity is a vector quantity so it has both magnitude (size) and direction.
-For objects moving in a circle, their direction is constantly changing.
-This means that their velocity is also constantly changing, even when their speed is constant.
State what the Lines on a Distance - Time Graph Shows
-The line on a distance time graph shows the movement of the object. The steeper the graph, the faster the object is travelling at.
-A straight line across shows that the object is stationary.
-A straight uphill or downhill line shows the object is travelling at steady speed.
-A line uphill is moving away from the starting point and a line downhill is moving towards the starting point.
-A steepening curve means the object is speeding up and a levelling off curve means its slowing down.
-If the object is accelerating, its speed at a certain point can be found by finding the gradient of the tangent to the curve.
Describe what is Meant by Acceleration
Acceleration is the change in velocity in a certain amount of time.
Give the Calculations for Acceleration
Acceleration (m/s^2) = Change in Velocity (m/s) / Time Taken (s)
a = Δv / t
Final Velocity (m/s) - Initial Velocity (m/s) = 2 x Acceleration (m/s^2) x Distance (m)
v2 - u2 = 2as
State what the Lines on a Velocity - Time Graph Shows
-The gradient on a velocity time graph shows the acceleration of the object. The steeper the line, the greater the acceleration or deceleration.
-A straight line facing upwards shows a constant acceleration. A straight line facing downwards shows a constant deceleration.
-A straight line across shows a steady speed.
-A curved line shows a changing acceleration.
-The area under the graph shows the distance travelled.
Describe how Friction Causes Objects to Slow Down
-If an object has no force propelling it along, it will always slow down and stop because of friction.
-Friction always acts in the opposite direction to movement.
-To travel at a steady speed, the driving force needs to balance the frictional forces.
-Friction happens between two surfaces in contact or when an object passes through a fluid.
Describe how Drag is Different to Friction
-Drag is the resistance you get in a fluid (air resistance is a type of drag).
-To reduce the effects of drag, the object’s shape should be kept streamlined. This is when the object is designed to allow fluid to flow easily across it, reducing drag.
-Frictional forces from fluids always increase with speed.
Describe how a Skydiver Experiences Air Resistance
-A skydiver is an example of an object falling through a fluid. Initially, only the force of gravity is acting.
-This causes the skydiver to accelerate towards the ground at 9.8m/s^2. However, as they fall, the skydiver experiences an upward force due to friction with air particles. This is called air resistance.
-When air resistance balances gravity, the skydiver stops accelerating and moves at a constant velocity. This is called the terminal velocity.
Explain why a Skydiver Reaches Terminal Velocity When Their Parachute is Closed
-When the skydiver leaves the airplane, only the force of weight is acting. The size of this force will not change. Because there is a resultant force, the velocity of the skydiver increases as they accelerate towards the ground.
-As the skydiver falls, they collide with air particles. This causes the force of friction acting upwards. Because the air resistance is much smaller than weight, the skydiver continues to accelerate towards the ground.
-As the skydiver’s velocity increases, the air resistance increases. When air resistance balances the force of weight there is no resultant force. Now the skydiver moves down at a constant velocity.
Explain why a Skydiver Reaches Terminal Velocity When They Open Their Parachute
-The skydiver now opens their parachute. Air resistance now massively increases. Air resistance is now much greater than weight so there is a resultant force acting upwards. This causes the skydiver to decelerate (their velocity decreases).
-As the velocity decreases, the air resistance decreases. At some point, air resistance and weight are balanced. Now the skydiver moves downwards at a constant velocity. This is the new (lower) terminal velocity.
Describe Newton’s First Law of Motion
-If a resultant force of zero acts on a stationary object then the object will remain stationary.
-If a resultant force of zero acts on a moving object then the object will continue moving in the same direction with the same speed.
Explain why a Car Moving in a Straight Line at a Constant Speed will have a Resultant Force of Zero
-The car is moving in a straight line at a constant speed.
-Because it is not changing in direction and is not changing in speed, the forces acting on the car must be balanced.
-In other words the resultant force must be zero.
-The resistive forces acting on the car are friction with the road and friction with air particles.
Describe Newton’s Second Law of Motion
The acceleration of an object is proportional to the resultant force and inversely proportional to the mass of the object.
Give the Calculation for the Acceleration of an Object Due to a Resultant Force
Force (N) = Mass (Kg) x Acceleration (m/s^2)
F = ma
Describe what is Meant by Inertia
Inertia is the tendency of objects to either remain still or continue moving in the same speed and direction unless a resultant force is applied.
Describe what is Meant by the Inertial Mass
-If we know the force needed to accelerate the object, we can use this to calculate the object’s mass (using the equation Force = Mass x Acceleration).
-Scientists call this mass the inertial mass because it was calculated from force and acceleration rather than from density.
Describe how the Inertial Mass Affects the Force Needed to Produce a Given Acceleration
If an object has a larger inertial mass, it will require a greater force to produce a given acceleration compared to an object with a smaller inertial mass.
Describe Newton’s Third Law of Motion
Whenever two objects interact, the forces they exert on each other are equal in magnitude but opposite in direction.
Give Examples of Newton’s Third Law of Motion
-The wheels of a car exert a backwards force on the road. The road exerts a forward force on the wheels.
-The Earth exerts a downwards force of attraction due to gravity on a skydiver. The skydiver exerts an upwards force of attraction due to gravity on the Earth.
-A person uses a paddle to exert a backwards force into water. The water exerts a forwards push force onto the paddle.
Describe a Method for the Acceleration Required Practical
-Set up a light gate on a table and connect it to a data logger. Set up a trolley so it holds a piece of card with a gap in the middle that will interrupt the signal on the light gate twice.
-Connect the trolley to a string that goes over a pulley and is connected on the other side to a 100 g slotted mass carrier. Add ten 100g masses to the carrier.
-Mark a starting line on the table so the trolley always travels the same distance to the light gate. Place the trolley on the starting line, holding it so the string is taut. Then, release it.
-Record the acceleration measured by the light gate as the trolley passes through it. Repeat this twice more to get an average acceleration.
-Now, place the trolley back on the starting line and transfer a 100g slotted mass from the carrier to the trolley. Release the trolley again and repeat twice more for an average.
-Continue doing this up to 1000g and record the average acceleration for each force.
Describe how to Investigate the Effect of Mass in the Acceleration Required Practical Rather than the Effect of Force
-In the second part of the experiment we change the mass of the toy car but we keep the force
constant.
-To do this we keep the weight on the end of the string constant but we add progressively greater masses onto the trolley.
Explain why the Mass is Transferred from the Hook to the Trolley in the Acceleration Required Practical
-During the first part of this experiment, we are investigating the effect of changing the size of a force on the acceleration of an object of constant mass.
-We change the force by removing masses from the end of the string. However, if we simply removed mass then we would be reducing the mass of the object.
-So to prevent this, we have to take the mass that we remove from the string and place it on the toy car.
-This way we have reduced the force but we have not changed the total mass.
Describe the Effect of Decreasing the Force Applied While Keeping the Total Mass the Same on the Acceleration Required Practical
-Newton’s Second Law of Motion states that the acceleration of an object is proportional to the force applied but inversely proportional to the mass of the object.
-If we decrease the force applied (while keeping the total mass the same) then we will find that a smaller force will produce a smaller acceleration.
Describe the Effect of Increasing the Mass While Keeping the Force Applied the Same on the Acceleration Required Practical
-Newton’s Second Law of Motion states that the acceleration of an object is proportional to the force applied but inversely proportional to the mass of the object.
-If we increase the mass (while keeping the force applied the same) then we will find that a greater mass leads to a smaller acceleration.
Describe what is Meant by Thinking, Braking and Stopping Distance
-The thinking distance is the distance travelled by the vehicle during the driver’s reaction time (ie the time taken for the driver to spot an obstruction, make a decision and move their foot to the brake).
-The braking distance is the distance travelled by the vehicle from when the driver first applies the brakes to when the vehicle stops.
-The stopping distance is the sum of the thinking distance and the braking distance. In other words, the distance a vehicle moves from when the driver first spots an obstruction to when the vehicle comes to a complete stop.
Describe what is Meant by Ideal Conditions
-Ideal conditions mean that the driver is well rested, not distracted and not under the influence of any drugs such as alcohol.
-The weather should also be dry with good visibility (for example no mist or fog).
The car’s brakes and tyres should also be in good condition (ie not worn).
Describe the factors Affecting Thinking Distance
Speed- the faster the car is travelling, the further the car will travel during the time taken to react.
Reaction Time- the longer the reaction time, the longer the thinking distance
Other factors include:
-consuming drugs or alcohol, using a mobile phone, how tired the driver is
Describe the factors Affecting Braking Distance
Speed- for a given braking force, the faster the vehicle travels, the longer it takes to stop.
The Weather or Road Surface- if it is wet or icy, there is less grip (so less friction) between a vehicle’s tyres and the road, which can cause tyres to skid.
Tyre Conditions- if the tyres of a vehicle are bald then they cannot get rid of water in wet conditions. This leads to them skidding on top of the water.
Brake Conditions- if the brakes are worn or faulty, they won’t be able to apply as much force as well maintained brakes, which could be dangerous when you need to break hard.
Suggest why the Speed Limit is Often Reduced in Residential Areas
-Residential streets are more likely to present a driving hazard compared to a main road.
-The stopping distance of a car increases at higher speeds. At 20 miles per hour, a driver will stop in a shorter distance than at 30 miles per hour.
-Therefore it is safer to driver at 20 miles per hour (rather than 30 miles per hour) on a residential street.
Explain why it is Dangerous to Drive too Closely to the car Infront
-The speed limit on many roads in the UK is 30 miles per hour. At this speed, if a driver braked fully, it would take six car lengths to stop.
-Driving too closely to the car in front is dangerous because if they brake hard, it might not be possible for the driver behind to stop in time. This would cause a collision.
Describe how a Car Brakes
-When a car brakes, the brake presses against the wheel. This causes the force of friction to act.
-The kinetic energy store is transferred to the thermal energy store and the temperature of brakes increases as the car slows.
-If we increase the speed of the car then a greater braking force is needed to stop the car in a given distance.
-Braking while at high speed can cause the brakes to overheat or the driver to lose control of the vehicle.
Describe what is Meant by Momentum
Momentum is a measure of how difficult it is for something to stop.
Give the Calculation for Momentum
Momentum (Kg m/s) = Mass (Kg) x Velocity (m/s)
p = mv
Describe how Momentum Works in a Closed System
In a closed system, the total momentum before an event is equal to the total momentum after the event.
Describe how Conservation of Momentum Affects Objects in a Crash
-When two objects crash, the object hit will move faster as it gains momentum from the crash.
-The other object loses an equal amount of momentum because it is preserved.
-Both objects exert equal and opposite forces on each other but there are no forces acting on them.
Conservation of momentum applies when there are no external forces acting.
Explain why a Cannon Recoils
-The total momentum of the cannon and cannonball before firing is 0 kg m/s. Momentum is always conserved so the total momentum of the cannon and cannonball must be 0 kg m/s after firing.
-The cannonball has momentum in the forward direction. This means that the cannon must have momentum in the backward direction.
-The forward momentum of the cannonball is cancelled by the backward momentum of the cannon. So the total momentum after firing is 0 kg m/s.
-If the cannonball moves forwards then the cannon must move backwards in order for momentum to be conserved.
Give the Calculation for Force when Momentum Changes
Force (N) = ( Mass (Kg) x Change in Velocity (m/s) ) / Time (s)
F = (mΔv) / t
Explain how Seatbelts Reduce the Risk of Serious Injury
-If the driver is wearing a seatbelt then the driver will come to a stop more gradually. Their momentum will fall to zero but over a much longer time.
-This means that they will experience much lower forces
than if they were not wearing their seatbelt. These forces could cause an injury but are less likely to prove fatal.
Explain how Seatbelts Stretching Reduce the Force on Passengers
-Seatbelts are designed to stretch slightly when they are engaged. This stretching causes the driver to come to a stop more slowly than if the seatbelt was rigid.
-Because of this, the driver’s momentum will reduce more slowly, reducing the forces acting and reducing the risk of injury.
Explain how Airbags reduce the Risk of Serious Injury
-Airbags deploy during a collision. These inflate so that as the driver moves forward, the driver’s movement is cushioned by the airbag. This slows down the change in momentum and reduces the forces acting.
-They are designed to inflate and then deflate. This means that when a driver collides with the airbag, the bag loses some of its gas, again slowing momentum change.
-If the bag remained fully inflated, the driver would come to a stop more rapidly and experience greater forces.
Explain the Benefit of using Polystyrene in Helmets Rather than Using a Rigid Helmet
-The purpose of a cycle helmet is to slow down the change in momentum of the cyclist’s head during a collision (thereby reducing the forces acting).
-Helmets contain expanded polystyrene. During a collision, the cyclist’s head presses on the polystyrene. This crushes the polystyrene and slows down the rate of momentum change.
-A rigid helmet would not slow down momentum change as effectively so the forces acting would be greater and would be more likely to cause injury.
Explain why Crash Mats are used in Playgrounds
Crash mats increase the time to reduce the change in momentum thereby decreasing the force.
