P3.2 Using physics to make things work Flashcards
If an object is freely suspended, an object will come to rest with its….
centre of mass directly below the point of suspension
Where the the centre of mass in a symmetrical object?
along the axis of symmetry
In the equation
T = 1 (divided by) f
what does each letter stand for and what are the units?
T - periodic time - time period (s)
f - frequency (Hz)
Describe a simple pendulum
suspending a weight from a piece of string.
when you let it go, it will swing back and forth
What is the time period of a pendulum?
The time taken for the pendulum to swing from one side to the other and back again
What does the time period of a pendulum depend on?
the length of the pendulum
the longer the length of the pendulum, the greater the time period
the shorter the length of the pendulum, the shorter the time period
What are some applications of a pendulum?
old-style clocks
playground swings
fairground rides
What is the turning effect of a force called?
the moment
In the equation
M = F X d
what does each letter stand for and what are the units?
M- moment of the force (Nm)
F- force (N)
d- perpendicular distance from the line of action of the force to the pivot (m)
Describe the moments if an object is not turning
the total clockwise moment must be exactly balanced by the total anticlockwise moment about any pivot
If the anticlockwise moments are equal to the clockwise moments…..
the object wont turn
How do levers make it easier to do work?
they move the distance the force is applied further from the pivot
Give some example of simple levers as force multipliers
long sticks or bars
wheelbarrows
scissors
How are force multipliers helpful?
they increase the distance from the pivot at which force is applied
this means less force is needed to get the same moment
levers reduces the amount of force that’s needed to get the same moment by increasing the distance
When will an object topple over?
if the line of action of the weight of an object lies outside the base of the object (if centre of mass moves beyond edge of base) there will be a resultant moment causing the object to topple
this is because the total anticlockwise moments do not equal the total clockwise moments, causing a resultant moment
The most stable objects have a …..
wide base and low centre of mass
Why does a bunsen burner have a wide heavy base?
to give it a low centre of mass - heavier to knock over
Liquids are virtually ….
incompressible
What does it mean that liquids are virtually incompressible?
you can’t squash them, their volume and density stay the same
What will happen when you apply a force to one point in a liquid, why?
the pressure in the liquid is transmitted equally in all directions
(the force exerted at one point on a liquid will be transmitted to other points in the liquid)
This is because liquids are virtually incompressible and can flow
In the equation
P = F (divided by) A
what does each letter stand for and what are the units?
P- pressure in pascals (Pa)
F- force (N)
A- cross sectional area (m2)
What are hydraulic systems used as?
force multipliers - they use a small force to produce a bigger force
Describe a simple hydraulic system
2 pistons, one with a smaller cross sectional area than the other
Pressure is transmitted equally through a liquid so the pressure at both pistons is the same
At the first piston, a pressure is exerted on the liquid using a small force over a small area
This pressure is transmitted to the second piston
The second piston has a larger surface area and so (as force = pressure x area) there will be a larger force
Where are hydraulic systems used?
car braking systems
What happens when an object moves in a circle?
it continually accelerates towards the centre of the circle
This acceleration changes the direction of motion of the body, not its speed
What is the force called that keeps something moving in a circle?
a centripetal force
What direction is the centripetal force on an object which is moving in a circle?
the centripetal force is always towards the centre of the circle
What force provides the centripetal force when a car goes around a bend?
friction
What force provides the centripetal force when a bucket is whirled around a rope?
Tension in the rope
What force provides the centripetal force in a spinning fairground ride?
tension
The centripetal force needed to make an object perform circular motion increases as:
the mass of an object increases
the speed of an object increases
the radius of the circle decreases
What kind of objects may be unstable?
those with a narrow base and a high centre of mass
A centripetal force……………………. but is always provided by something else such as gravitational force, friction or tension
does not exist in its own right
What is the centre of mass of an object?
The point at which the mass of the object may be thought to be concentrated
The ……. an object is moving, the bigger the centripetal force needed to keep it moving in a circle
faster
The …. the mass of an object, the bigger the centripetal force needed to keep it moving in a circle
Larger
The …… the radius of a circle an objects moving in, the bigger the centripetal force needed to keep it moving in a circle
smaller
Describe how to find the centre of mass for an object that is not a regular shape
- suspend it from a point around its edge
- mark a vertical line directly under the point of suspension
- repeat 2 more times
- the centre of mass is where these points cross
Explain why double decker buses are less stable than single deck buses
the centre of mass is higher above the ground for a double decker bus.
It is more likely to lie outside the base of the bus
The time period of a pendulum does not depend on ….
the mass of the object or the amplitude of the swing
What are moments measured in?
Nm (Newton meters)
What is pressure measured in?
Pa (pascals)
What provides the centripetal force that keeps planets and satellites in orbit?
gravity
Describe the movement of a simple pendulum
swings back and forth
at the highest point of its swing, the pendulum briefly stops and stores energy as gravitational potential energy
at the lowest point of the swing, this energy is transferred to kinetic energy as the pendulum travels fastest
Explain how the centripetal force changes when a car slows down as it drives around a bend
the force decreases because centripetal forces are less when the speed is lower
Explain how the centripetal force changes when a bucket of water is swung around in circles when it is full of water and when it is empty
the force is greater when the moving mass is larger
Explain how the cylinder is rotating at a constant speed but at the same time the riders inside the cylinder are accelerating
The direction of the riders is constantly changing
therefore the velocity of the riders is changing
and because acceleration is the rate of change of velocity, the acceleration is changing
