P3 Part B Flashcards
What is a moment and what is the equation to calculate it?
A moment is the turning effect of a force
Moment(N/m) = Force(N) x perpendicular distance from the line of action of the force to the pivot(m)
How do you get the maximum moment or turning effect?
You need to push at right angles (perpendicular), any other angle means a smaller moment because the perpendicular distance between the line of action and the pivot is smaller.
Where is the centre of mass of an object?
The centre of mass hands directly below the point of suspension.
In a symmetrical shape, it is the point where lines of symmetry cross.
If the ………. moments are ……… to the clockwise moments, the object won’t turn.
If the anticlockwise moments are equal to the clockwise moments, the object won’t turn.
If someone weighs 300N and sits 2m from the pivot of a seesaw, and another person weighs 700N - where should they sit to balance the seesaw?
Anticlockwise moment = Clockwise moment
300 x 2 = 700 x y
y = 0.86 m
How do levers use balanced moments to decrease the amount of force needed to get the same moment?
Name 3 examples of force multipliers:
Levers increase the distance from the pivot at which the force is applied - so less force is needed to get the same moment. Levers are known as force multipliers.
- Long sticks/bars
- Wheelbarrows
- Scissors
Why do objects need a wide base and low centre of mass to be stable?
If the line of action of the weight of the object lies outside of the base of the object, it’ll cause a resultant moment, tipping the object over.
What is the time period of a pendulum? How does the length of the pendulum effect it?
It is the time taken for a pendulum to swing from one side to the other and back again.
The longer the pendulum, the greater the time period.
How is the time period of a pendulum calculated?
Time period (s) = 1 ÷ Frequency (Hz)
Why are liquids virtually incompressible?
A force applied to one point int he liquid will be transmitted to other poiints in the liquid. Pressure in a liquid is transmitted equally in all directions.
Give the equation for pressure
Pressure = Force ÷ Cross-sectional area
How is the pressure in liquids used in force multipliers?
In hydraulic systems, a pressure is exerted on the liquid using a small force over a small area and this force is transmitted to the second piston. The second piston has a larger area so there will be a larger force.
Where are hydraulic systems used?
In car braking systems, hydraulic car jacks, manufacturing and deployment of landing gear on some aircraft.
If an object is travelling in a circle, what will be happening to its velocity and acceleration?
The velocity is constantly changing (but not necessarily speed) so the object is accelerating - towards the centre of the circle.
What is the force which causes objects moving in a circular movement to accelerate towards the centre called?
Centripetal force
Give three different examples where centripedal force is in affect, and name what form it takes.
- In a car going round a bend, the centripetal force is friction between the cars tyres and the road.
- When a bucket is whirling round on a rope, the centripetal force comes from the tension in the rope.
- In a spinning fairground ride, the centripetal force comes from tension in the spokes of the ride.
What three factors does centripetal force depend on?
- The faster an object is moving, the bigger its centripetal force will have to be to keep it moving in a circle.
- The bigger the mass, the bigger the centripetal force.
- A larger force is needed to keep something moving in a small circle because it has more ‘turning’ to do.
What is a magnetic field?
A magnetic field is a region where magnetic materials (like iron and steel) and also wires carrying currents experience a force acting on them
Draw a magnetic field diagram.
Where do the arrows always point?
The arrows on the field lines always point from the North Pole of the magnet to the South Pole.

What does the right hand thumb rule tell you about which way a magnetic field goes?
When your thumb points in the direction of the current, and your fingers are curled round, your fingers will indicate the direction of the magnetic field.
If you extend your first two fingers out, the upper one will point in the direction of the magnetic field, whilst the second one down will indicate the direction of the electric current.
The magnetic field inside a solenoid (a coil of wire) is strong and uniform. What is the magnetic field like outside?
It’s just like the one round a bar magnet
How can you increase the strength of a magnetic field around a solenoid?
Add a magnetically ‘soft’ iron core through the middle of the coil, so it becomes an electromagnet.
Why are magnetically soft materials useful for something that needs to be able to switch its magnetism on and off?
They magnetise and demagnetise easily. When curent through the solenoid is turned off the magnetic field disappears - because the iron doesn’t stay magnetised.
Give an examle of when a magnet that can be turned on and off is useful
In cranes that lift iron and steel, electromagnets can be used. When the electromagnet is turned on, it will pick up magnetic materials and drop them when turned off.
