Mechanics 2 Flashcards
Newton's Laws Freefall Momentum Energy Efficiency
What does freefall mean?
Freefall is when the only force acting on an object is gravity.
In which direction does g act?
Vertically down
At what acceleration do objects in freefall accelerate towards the ground at?
g (9.81m s-2)
What’s freefall motion called when the object is given an initial velocity?
Projectile motion
What should you do if you need to use the SUVAT equations on an object that has an initial velocity at an angle to the horizontal?
Resolve the velocity into its vertical and horizontal components. Using the vertical velocity for the calculation with the SUVAT equations, to calculate how long it’s in the air. Then use the horizontal component of velocity along with the time calculated from the vertical component to calculate the distatance the projectile travelled.
What is the only force present in free fall motion?
Weight
Describe an experiment you could do to obtain data that you could use to determine the value of g.
Using light gates or an electomagnetic timer. Time the time taken for a ball bearing the fall through a certain distance. Take readings for each height and obtain an average.
Explain how you can plot the data obtained from the measuring g practial on a graph to obtain the value for g.
Plotting height against time squared and drawing a line of best fit. Then find the gradient of that line. 1/2 the gradient will give you the acceleration due to gravity.
Ignoring air resistance, which ball will accelerate fastest. One with a mass of 5kg or one with a mass of 50kg?
They will both accelerate at the same rate, g.
What is the effect of air resistance on the trajectory of a projectile?
It will reduced the distance travelled by the projectile. If the projectile starts with vertical velocity, it will reduce the height the projectlie reaches.
What is meant by friction?
A force which opposes the motion of an object.
What is meant by drag?
A force which opposes the motion of an object in a fluid.
In what direction does a frictional force act?
In the opposite direction to the motion of the object.
In what direction does lift act, relative to a flowing fluid?
At right angles to the flowing fluid.
What can you say about the frictional forces and the driving forces acting on an object when it reaches terminal velocity?
They are equal and opposite (balanced)
Sketch a velocity-time graph for an object falling through the air and reaching terminal velocity.
Initially there is some uniform acceleration, the rate of acceleration slows as the the air resistance increases, eventually leading to a constant velocity.
How can a skydiver reduce her terminal velocity?
Open her parachute to increase the drag force acting on her.
How does friction change the energy store of an object?
Converts kinetic energy into thermal energy.
Explain how air resistance affects the maximum speed of a car.
As the speed of the car increases, the air resistance acting on the car also increases, until these forces are balanced and the car is travelling at it’s top speed. The force of air resistance limits the maximum speed of the car. A larger air resistance will mean a slower top speed, a smaller force of air resistance means a higher top speed.
Other than increasing the power of the engine, how could the maximum speed of the car be increased?
Make the body of the car more aerodynamic to reduce air resistance.
What affect does speed have on air resistance?
Air resistance increases as speed increases.
How can frictional forces acting on an object be reduced?
By making the object more streamlined.
Which of these graphs shows the velocity-time graph of an object falling through air and reaching terminal velocity?
Teacher to draw
What does the acceleration-time graph look like for a parachutest falling and reaching terminal velocity?
Acceleration starts at 9.81 ms-2 and reduces in a non-uniform way until reaching 0 ms-2 where the parachutest reaches terminal velocity.
What is the initial rate of acceleration on earth? Why does it decrease as the object falls?
9.81 ms-2. Air resistance increases reducing the resultant force.
Why does a kicked football move in a parabolic curve/projectile motion
Because the horizontal velocity remains constant, while the vertical velocity is affected by acceleration downwards due to gravity.
When looking at projectile motion questions, how should you treat the horizonal and vertical components of motion?
Horizontal and vertical components of motion should be treated independently.
Define terminal velocity
The maximum velcoity an object will fall at when the force of weight is balanced by the force of air resistance
State Newton’s first law of motion
If there is no resultant force acting on an object it will either stay at rest or move with a constant speed in the same direction.
State Newtons second law of motion
If there is a resultant force acting on an object it will accelerate in the direction of the resultant force. The acceleration of an object is proportional to the resultant force acting on it.
State Newtons third law
If object A exerts a force on object B, object B will exert and equal force on A but in the opposite direction.
Explain with reference to Newton’s third law how we are able to walk across the floor.
In order to walk across the floor, you must push back on the floor with your foot; then, according to Newton’s Third Law, the floor pushes forward on your foot, which propels you forward. This, of course, requires friction to work.
Explain why the equal and opposite force of a books weight on the table is not the reaction force of the table acting on the book.
The forces are acting on the same object, the forces are of different types.
Name the equal and opposite force of a book’s reaction force on a table according to Newton’s third law.
The reaction force of the table acting on the book.
How much resultant force is needed to accelerate a 24.1 kg mass by 3.5 ms-2?
84 N (2.s.f)
A resultant force of 18N is applied to a toy car with a mass of 0.61 kg. What will the magnitude of the car’s acceleration be?
30 ms-2 (2.s.f)
Two ice skaters of mass 55.0 kg and 60.0 kg push against each othere. The heavier ice skater accelerates away at 2.3 ms-2. What will the magnitude of the lighter ice skaters acceleration be?
2.5 ms-2 (2.s.f)
Explain how a swimmer moves forwards in the water.
The swimmer pushes back against the water with their arms and legs, the water pushes the swimmer forwards with an equal sized force.
State Newton’s three laws of motion and briefly explain what they mean.
Newton’s first law: The velocity of an object won’t change unless a resultant force acts on it. This means a body will stay still or move in a straight line at a constant speed unless there’s a resultant force acting on it. Newton’s second law: Resultant force = mass x acceleration. This means a larger force acting on an object causes a larger acceleration, and a larger mass means a smaller acceleration for a given force. Newton’s third law: If an object A exerts a force on object B, then object B exerts an equal and opposite force on object A. This means every action has an equal and opposite reaction.
Draw a diagram showing what forces are acting on a book when it’s sitting still on the floor.
Gravity pulls down on book, floor pushes up on book
What are the two conditions necessary for two equal and opposite forces to obey Newton’s third law?
The forces must act on different objects, they must be the same type of force.
Why does a bird lift into the air when it flaps its wings?
When a bird flaps its wings it pushes down on the air. The air then pushes back up o the bird’s wings with the same force, which causes it to lift.
How can Newton’s second law be written as a formula?
F = ma
Give the formula for momentum, including the units for each quantity
p= mv , p, momentum, measured in kgms-1 , m, mass, measured in kg, v, velocity, measured in ms-1
What unit is momentum measured in?
kgms-1
Is momentum a vector or a scalar quantity?
Vector
What two quantities does momentum depend on?
Mass and velocity
Which of the following gives a body’s change in momentum? A) The gradient of a force-time graph. B) The area under a force-time graph. C) The grdient of a force-distance graph. D) The area under a force-distance graph.
B) The area under a force-time graph.
Moving objects with mass are said to have __________?
Momentum
State the law of conservation of momentum
In a closed system the total momentum before an event is equal to the total momentum after an event.
What is conserved in an elastic collision?
Kinetic energy and momentum
What is conserved in an inelastic collision?
Only momentum
What is the formula for kinetic energy?
E = 1/2 mv2
How can Newton’s second law be written in terms of momentum?
F = ma –> F = m Δv/ Δt –> F = Δp/ Δt
How do crumple zones make cars safer?
They are designed to crumple easily, which increases the impact time, reducing the average force that is needed to stop you.
What is the formula for impulse?
F Δt = Δ(mv)
What is impulse?
Force x time
What are the units of impulse?
Ns
The impulse on a body is equal to ……
The change in momentum of that body
What does the area under a force-time graph give you?
Impulse
How do air bags work?
The person’s kinetic energy is absorbed by the air bag, the impact time is lengthened, reducing the force on the passenger.
How does a seat belt work to make a passenger safer?
The seat belt stretches, increasing the time it takes you to stop, reducing the force needed to stop you.
As force is equal to the rate of change of momentum, how can the force acting be increased?
Decreasing the time the change in momentum takes place over.
A squash ball moving horizontally hits a squash racquet. How would you calculate the impulse acting on the ball?
Impulse is equal to the change in momentum, so you would need to subtract the momentum before the collision from the momentum after the collision.
What are the units of work done?
Joules
What is the formula for work done when the force and direction moved are in the same plane?
W = Fs
How do you calculate work done when the direction of the movement is different from the direction of the force making it move?
Resolve the force in the direction of the motion. W =F s cos θ
What does work mean in physics?
Work is the amount of energy transferred from one form to another when a force causes a movement of some kind.
How do you calculate work done? Give the units for each quantity.
W = F s W is work done, measured in Joules (J) F is force, measured in Newtons (N) s is distance moved, measured in meters (m)
How do you calculate power?
P = ΔW/Δt
What unit is power measured in?
Watts (W)
What is the Watt defined as?
1 Watt is one joule per second
What is transferred when work is done?
Energy
Power is the rate of doing what?
Doing work
What does the ares under a force-displacement graph tell you?
Work done
Which equation links power and speed of a moving object?
P = W/t –> P = Fs/t –> P = Fv
What does the principle of conservation of energy state?
The principle of conservation of enegry states: Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change.
Efficiency is a measure of how well energy is __________.
Transferred.
What is the formula for kinetic energy?
E = 1/2 mv2
When an object falls, _______ energy is transferred to ___________ energy.
When an object falls, gravitational potential energy is transferred to kinetic energy.
Why are no machines 100% efficient?
Whenever energy is transferred, some energy is converted to less useful forms (such as heat and sound)
What is the formula for elastic potential energy?
E = 1/2k (ΔL)2
What is the formula for gravitational potential energy?
E = mgh
How is efficiency caculated in terms of energy?
Efficiency = useful energy output/ total input energy
Which of the following is the correct unit for efficiency? A) joules B) watts C) seconds D) Efficiency has no units
D) Efficiency has no units
How is efficiency caculated in terms of power?
Efficiency = useful power output/ total input power
Describe the energy transferes involved when a catapult string is drawn back and a pellet is fired out of the catapult.
Work is done by the hand to pull the catapult string back, transferring he kinetic energy of the hand to elastic potential energy is stored in the string. When the string is released the elastic potential energy is converted into kinetic energy, which is transferred to the pellet. As the pellet travels through the air the kinetic energy of the pellet is transferred to thermal energy of the surroundings.
True or false? The drag force felt by an object moving in a fluid depends on the objects shape.
True.
What is the difference between an elastic and inelastic collision?
In an elastic collision both momentum and kinetic energy are conserved. In an inelastic collision only momentum is conserved.
Explain, with regards to Newton’s second law, why landing on the crash mat is less likely to cause injury to a gymnast than the gymnast landing directly onto the floor.
From Newton’s second law, force is equal to the rate of change of momentum. The crash mat increases the time over which they gymnast is brought to rest from her initial velocity. This reduces the gymnast’s rate of change of momentum. Due to Newton’s second law, this means that the force acting on her is less than it would be if she landed directly only the floor, and this smaller force is less likely to cause injury.
In physics, what does it mean to say that ‘work is done’?
Energy is transferred when an object is moved by a force.
