Mod 3: Forces In Motion Flashcards
What is needed for an object to accelerate
A resultant force
Define resultant force
The resultant force is the vector sum of all the forces
What does a free body diagram show
All the forces acting on the object, not the forces the object exhorts onto the world
What does galileo’s theory state
All objects fall at the same rate, acceleration is the same and it does not depend on the objects mass
What is the mass of an object defined as
The volume of matter the object retains
Define inertia
The tendency of an object to move
What happens to the change in velocity of an object when it mass is increased
The object will have a greater resistance to the change in velocity
Define weight
The force experienced by a mass sue to gravity
Define the centre of mass
The single point where the mass appears to act
How can you find the centre of mass of a regular object
Use symmetry as the centre of mass will be where the symmetry lines cross
How would you find the centre of mass of an irregular object
Hang an object freely from a point
Draw a vertical line downwards from the point of suspension (use plumb line)
Hange object from a ifferent point and repeat.
Where the lines cross is where the objects centre of mass is
What does the drag force depend on
The viscosity( thickness) of the fluid
What does drag force depend on
The speed of the object
The shape and size of the object
How does a parachuter prevent themselves from crashing
When the person has reached terminal velocity they open their prachute. This increases their air resistance so that it is larger than their weight. Therefore they will decelerate until their speed has dropped so that the air resistance is equal to their weight this new terminal velocity will prevent them from crashing
How can you measure the terminal velocity of a ball bearing
Place elastic bands around a long tube at fixed distances of 10cm. Fill the tube with a viscous liquid such as water.
Drop a ball bearing into the tube and use a stopwatch to record the time at which it reaches each band and record your results in a table.
Repeat this to reduce random errors.
Then calculate the time taken for the ball bearing to travel between the bands
And then calculate an average for each reading. Use the average times and distance between each band to find the terminal velocity of the ball.
How can you improve the ball bearing experiment
Use a more viscous liquid so the ball will drop slower
Change the size of the ball to see how it affects terminal velocity
Change the shape of the ball
Define density
The measure of the compactness of a substance, how much space a certain mass takes up
Define pressure
The amount of force applied per unit area measured in pascals.
Define upthrust
An upwards force that fluids exert on objects that are completely or partially submerged in the fluid.
Why is there an upthrust force
Due to the differences in depth of the top and bottom of the object, therefore there is a difference in pressure which causes an overall upwards force known as the upthrust.
State archimedes principle
When a body is completely or partially submerged in a fluid, it experiences an upthrust equal to its weight of the fluid has displaced
What is a moment
The turning effect of a force
The force x perpendicular distance from the pivot to the line of action of force
When will an object turn
When its moments are not balanced
State the principle of moments
The sum of the clockwise moments are equal to the sum of the anticlockwise moments about the same point
Define a couple
A pair of forces equal in size which act parallel to; to each other but in opposite directions
What does the size of the torque depend on
The size of the forces and the distance between them
Define 1 newton
The force which gives a mass of 1 kg an acceleration of 1ms^-2
What is meant by tension
A force carried through a wire or string
What is normal contact force
The force at right angles to the surface
How many kg are in a tonne
1000
Define drag
A resistive force which opposes the motion of a body moving through a fluid
Describe and explain the motion of a ball falling through oil
Initially the weight f the ball would be equal to its acceleration therefore acceleration=9.81, this is because weight is the only force acting on the ball initially.
The drag will then begin to increase as the speed increases, this will cause acceleration to decrease as the resultant force decreases.
The eight and drag will eventually become equal therefore the resultant force=0 as the ball has reached terminal velocity
What are the factors effecting the drag of an object moving through a fluid
Shape, texture, speed, cross sectional area of object
The viscosity of the medium
Define terminal velocity
The constant velocity which is reached when the net force acting on the body is 0
Explain how the velocity time graph of a ball moving through a vacuum would differ if the ball was moving in air
The velocity time graph in a vacuum would remain constant at 9.81 ms^-2
Define torque
One of the forces in a couple of x perp. Distance between the forces
Describe what conditions must be met for an object to float
The object average density must be less than the fluids density
When will an object topple
When its centre of mass is not acting through the base of the object
Describe an example of how pivots are used in the human body
Joints are pivots allowing us to carry weights
Why would a long handled tool spanner be better than a short handed one
The large handed one requires less force to lift the weight as it has a larger distance from the pivot
Equation for pressure
Force perpendicular to the cross sectional area / cross sectional area
Describe an experiment to calculate the density of air
Weigh a contain of air
Remove the ir using a vacuum pump
Re weight the container
Calc th meads of the air by substracting the mas before and after the air as removed
Calc density using equation
Write an equation used to find pressure in liquids
P=pgh
P- pressure
Density of liquid= p
Depth-h
Gravity-g
A metal sphere is falling at a steady speed in a tube containing oil. Describe the forces acting on the sphere and explain if it is accelerating
Vertically down weight is acting on the spher and vertically up: up thrust. The net force = 0 therefore the s[here is to accelerating
Describe an experiment to determine the centre of mass of a lamina on earth
From a point on the lamina suspend a plumb line and mark a vertical line
Repeat this from another point
Where these two lines intersect, that is the centre of mass
Describe a source of experimental error when determining the centre of mass of an irregular shaped lamina. How can it be minimised
When marking the vertical line , the plumb line may move
We can minimise by drawing vertical line from several points and multiple times
Why do tidal streams produce more power than wind turbines when they are identical in size and have the same water or wind speed
Water has a greater density
Define work done by a force
W= force X distance moved in direction of force
A rider on a sledge is sliding down a slope which is 30 degrees to the ground. Explain why the normal contact force does not work on the sledge
It is at right angles to motion
A metal cylinder of diameter of about 5 cm placed on a horizontal table. How can you measure the pressure exerted by the cylinder on the table.
Use a newton meter to find the weight of the cylinder.
Measure the diameter using a ruler then use the formula of a circle to find area.
Then use pressure formula
Use a micrometer to reduce the uncertainty
When will an object float in a fluid
When it has a lower density than the fluid
How do you find the area of contact between a cylinder and an object
Circumference X thickness
An object is completely submerged in water. And it is floating. What can be said about the upthrust acting on the object
Upthrust is equal to the weight of the object
What physical quantity has the same base units as energy
Moment
A small block of wood is placed in deep water. The block is at rest with 80% of its volume under the surface of water. The wight of the block is 6N. What is the upthrust
Upthrust= weight, it is ate rest therefore upthrust = 6N
Why is using a psi (pounds per square inch) better than using a pressure gauge when measuring pressure
The psi has a greater resolution as it has smaller increments
A sealed hollow tube is partially submerged in water. A string is attached to the bottom of the tube and to the bottom of the beaker of water. The string exerts a downwards force. The container is slowly filled with more water until the container is completely full. What are the forces on the tube and how do they change as more water is poured in
The weight of the tube and upthrust are the forces acting on the tube. And as the water increases upthrust increases because the weight of water displaced increases
How can a student precisely measure the diameter of a sphere
Use a micrometer and repeat readings and find mean
A cylinder is completely submerged under water, and the cylinder is moving vertically down. What are the forces acting on the cylinder and explain how the resultant force on the cylinder varies from the moment the cylinder is completely submerged until it reaches the deepest point.
The forces acting on it are drag and upthrust and weight.
The resultant force decreases as the cylinder moves downwards as drag increases however the upthrust remains constant. At the lowest point upthrust> weight therefore there is an upwards resultant force
The frictional force acting on an object falling vertically through water is directly proportional to its speed squared. What is the relationship between p, the rate of work done against the frictional force and the speed V of the object
P= Fv
F is proportional to V^2 therefore p will be proportional to v^3
A paper cone is held above ground and dropped. It falls vertically and reaches terminal velocity before it hits the ground. How do you describe the resultant force on the falling cone before it reaches terminal velocity.
It is decreasing downwards
A golfer hit 2 balls with the same speed but ball A is hit 60 degrees to the horizontal and ball B is hit 30 degrees to the horizontal. Compare the magnitude and direction of the two balls and which trajectory will have a longer distance
Ball A and B both have the same speed but at diff angles to the horizontal. Ball B will have a larger horizontal velocity therefore it will travel further.
A student investigated how a tennis ball falls vertically from rest. They plotted a velocity time graph and it had constant acceleration. How would the graph be different if the tennis ball was affected by air resistance
The line will curve and the ball will have a lower maximum velocity
A student investigates the motion of a hollow ball falling through air. The ball reaches terminal velocity before reaching the ground. The ball is now filled with a small amount of sand. How will the forces acting on the sand filled ball compare, if it was also dropped from rest and also reaches terminal velocity before hitting the ground.
The drag will be the same however the weight will be greater therefore the downwards resultant for will also be larger.
A student investigates the motion of a hollow ball falling through air. The ball reaches terminal velocity before reaching the ground. The ball is now filled with a small amount of sand. Explain why the terminal velocity of the sand filled ball will have a greater terminal velocity
At terminal velocity weight= drag
And the weight of the sand ball is greater therefore drag will also be greater, and greater speed = greater drag
An oscillator decays by the same fraction every half oscilation. Explain which time the oscillator will dissipate maximum energy
When t=0 as it is at max speed therefore it will have the greatest friction
A student investigates terminal velocity of a metal sphere that varies with the radius of r. It is suggested that v=Kr^2
How can the student plot a graph to find t
Plot. Graph of v against r^2 so that the gradient of the graph will be K
How can a student use laboratory equipment to measure distance and time to find the acceleration of freefall of a steel ball.
Measure s with a ruler
Measure t with a timer
Measure the distance from the bottom of the ball to the ground
Take repeat readings and find the mean of t
How does the acceleration and velocity change of a cyclist as he travel along a horizontal road. The mass of the rider and bicycle is given, the resistive forces= R and acceleration is given
The resultant force = ma- R
Acceleration decreases to 0 when F=R
Overall velocity rises from 0 to a terminal velocity when F=R
A block moves at constant speed up a ramp. Which force acting on the block does no work on or against the block.
The normal contact force
Why do astronauts feel weightless in space stations
No contact force between the astronaut and the floor of space station
A car is travelling at constant speed along a straight road. The driver sees a hazard and applies brakes causing the car to stop. The displacement time graph shows a decrease in gradient at time t=0.75 seconds. How can you tell the brakes were applied
The gradient decreases with time indicating velocity is also decreasing
A ball is at rest on a horizontal table. The ball has a weight W. What object experiences the equal and opposite force of W
Earth
A cyclist is moving up a slope and comes accross a gap in the ground. How does drag and the frontt wheel losing contact with the slope affect the motion of the cyclist and how it may affect the size of the gap which can be crossed
Drag causes velocity to decrease, therefore there will be a longer crossing time across the gap. This results in the cyclist being at a lower point on the other side of the gap. The rotation of the front wheel also lowers the height of the wheels. Therefore the max gap which can be crossed will be smaller with drag.
A driver is driving at constant speed of 20 metres per second. He then sees a hazard and applies the breaks. How would thinking time and breaking time change if the driver was travelling at 10 metres per second
Thinking time would be the same
However the breaking time will be halved
A swimmer of weight 640 N is being lifted vertically upwards from the sea by a cable of negiligable mass. The tension T in the cable from the time she leaves the water t=0 to t=1.5 is 670N, then from t=1.5 the tension T reduces to 640N. Describe the motion for the first 4 seconds of her ascent
T=0-1.5, there is a constant force of 30N, so therefore constant acceleration
From t=1.5-4 there is 0 resultant force therefore constant velocity
A student wishes to find the efficiency of a small low voltage DC motor. The motor is used to lift loads. Describe an experiment to determine the efficiency
Set up a circuit diagram of a cell connected to an ammeter and motor. Then add a voltmeter in parallel to the motor. Then set up the motor on top of 2 planks attached to a load and a meter ruler next to it.
Switc on the circuit and start a stopwatch. Stop the stopwatch and read the ammeter and voltmeter to find p.d and current when the load is lifted 20 m. Then use p=IV to find the total input energy.
Then use gpe equation and sub into p=W/t to find useful power output. Then use efficiency formula
What is momentum measured in
N
How can you improve the efficiency of a crane with a cable
Use oil to reduce friction and energy dissipated through heat
Use a stiffer cable to reduce enrgy lost through stretching
How can you find the breaking distance
U^2/2a
How do you convert from kmh^-1 to ms^-1
Kmh^-1 X 1000/3600
A cyclist goes in a zigzag motion when travelling up a hill however she still maintains a steady speed which would be the same speed if she was going up a hill in a straight line how will her powerful output change as she goes in a zigzag?
Useful power output will be less this is because the slope will be less deep while she is travelling up as the is lower. This mean mgsintheta this means four decreases and a smaller force for the same distance each second means a less power output as power equals forced divided by velocity.
