forces exam question Flashcards
On a bicycle, how does the force the pedal cause a moment about the rear axle?
The force causes a moment about the pedal axle which causes a force on the chain which causes a moment about the rear axle.
How does protective pads which are filled with foam help to reduce injury when the players collide?
When the collide the foam increases the time taken for the players to stop during the collision so the rate of change of momentum decreases reducing the force on the ice hockey player.
What is inertia?
Inertia is an object’s tendency to continue in the same state of motion or rest.
Explain the changing motion of the skydiver in terms of the forces acting on the skydiver?
Initially, the air resistance is less than the gravity/weight so the sky diver accelerates.
Acceleration causes air resistance to increase which gradually decreases the acceleration until the air resistance is equal to gravity meaning the resultant force decreases to zero so the skydiver falls at terminal velocity.
Jumping from 39 000 metres allowed the skydiver to reach a much higher speed than a skydiver jumping from 4000 metres.
Explain why.
The higher the altitude the less dense the air
so the air resistance on the skydiver (falling from 39000 m) was less (at the same speed)
so the skydiver was able to accelerate for longer before reaching (a higher) terminal velocity
Explain why atmospheric pressure decreases with increasing altitude.
air molecules colliding with a surface create pressure
at increasing altitude distance between molecules increases
or
at increasing altitude fewer molecules (above a surface)
so number of collisions with a surface decreases
or
or so always less weight of air than below (the surface)
(d) The higher the altitude the less dense the air
so the air resistance on the skydiver (falling from 39000 m) was less (at the same speed)
so the skydiver was able to accelerate for longer before reaching (a higher) terminal velocity
allow constant velocity/speed for terminal velocity
or
so the skydiver was able to accelerate for longer before air resistance = weight / gravity
The ramp is lowered by pulling on the rope or by pulling on the handle.
The hinge acts as a pivot.
Explain why it is easier to lower the ramp by pulling on the rope rather than pulling on the
handle.
the (perpendicular) distance from the pivot / hinge to (the line of action of) the force is
greater
allow distance from the rope to the pivot / hinge is greater (than distance between handle and pivot / hinge)
so a smaller force is required
The further a swimmer dives below the surface of the sea, the greater the pressure on the
swimmer. Explain why.
increasing depth increases the height / mass / volume (of the water column) above
the swimmer
allow more water above (the swimmer) more water is insufficient
increasing the weight / force (of water) acting on the swimmer
Describe one technique that you could have used to improve the accuracy of the measurements of extension of the spring taken by the student.
any practical technique that would improve the accuracy of length measurement eg
use a set square
to line up the bottom of the spring with the ruler scale
or
attach a horizontal pointer to the bottom of the spring (1)
so that the pointer goes across the ruler scale (1)
There are places on the journey where the train accelerates without changing speed.
Explain how this can happen.
train changes direction so velocity changes
acceleration is the rate of change of velocity
What is meant by the braking distance of a vehicle?
the distance travelled under the braking force
Explain why the stopping distance of the car increases if the driver is very tired.
the reaction time will increase, increasing the thinking distance (and so increasing stopping distance)
(increases stopping distance is INSUFFICIENT)
If the sled is pulled at an angle to the surface the value calculated for the constant of friction would not be appropriate.
only a (the horizontal) component of the force would be pulling the sled forward
the vertical component of the force (effectively) lifts the sled reducing the force of the surface on the sled
Scientists at the accident research laboratory want to develop a seat belt that
produces a constant force throughout a collision.
Use the idea of momentum to explain why this type of seat belt would be better for a
car driver.
to reduce the momentum of the driver
a smaller (constant) force would be needed
do not accept reduces the impact / impulse on the driver
What is meant by the term resultant force?
a single force that has the same effect as all the forces combined
accept all the forces added / the sum of the forces / overall force
As the aircraft moves along the runway to take off, its acceleration decreases even
though the force from the engines is constant.
Explain why.
as speed increases air resistance increases
accept drag / friction for air resistance
reducing the resultant force
The passengers ride in capsules. Each capsule moves in a circular path and accelerates.
Explain how the wheel can move at a steady speed and the capsules accelerate at
the same time.
- (acceleration occurs when) the direction (of each capsule) changes
- velocity has direction
- acceleration is (rate of) change of velocity
In which direction is the resultant force on each capsule?
to(wards) the centre (of the wheel)
The designers of the London Eye had to consider three factors which affect the
resultant force described in part (b).
Two factors that increase the resultant force are:
* an increase in the speed of rotation
* an increase in the total mass of the wheel, the capsules and the
passengers.
Name the other factor that affects the resultant force and state what effect it has on
the resultant force.
the greater the radius / diameter / circumference (of the wheel) the smaller the (resultant) force (required)
What is ultrasound? (this is not forces this is waves)
(sound waves) which have a frequency higher than the upper limit of hearing for humans
State one way cyclists can reduce the air resistance acting on them.
streamline position streamline clothes
accept crouched position
accept tight clothes
accept design of cycle
accept cycle slower
What does total stopping distance depend on?
the distance the car travels during the driver’s reaction time and braking FORCE (NOT distance)
Give seven factors that could cause the total stopping distance of a car to be greater.
- driver’s reactions are slow(er)
accept driver could have taken drugs
or alcohol or due to tiredness or
distractions - poor weather conditions
accept raining or snowing or fog /
mist (poor visibility) - greater mass or weight
- poor road conditions
oil / gravel / mud / leaves / wet / icy
going downhill - poorly maintained brakes
do not accept driver’s weak foot force - worn tyres
- speed of the car (increased speed)
What happens to the kinetic energy of the bus as it is braking?
(kinetic) energy transferred by heating
to the brakes.
Applying the brakes of the car causes the temperature of the brakes to
increase. Explain why. (2)
mark point 1: work done by) friction (between brakes and wheel)
do NOT accept friction between road and tyres / wheels
mark point 2: (causes) decrease in KE and increase in thermal energy
What is the equation of spring constant, force and extension?
force = spring constant × extension
Before taking any measurements, the student adjusted the ruler to make it vertical.
Explain why adjusting the ruler was important.
to reduce the error in measuring the extension of the spring
accept length for extension throughout
as the ruler at an angle would make the measured extensions shorter
The student measured the extension of the spring using a range of weights. Describe one technique that you could have used to improve the accuracy of the
measurements taken by the student.
use a set square (1) to line up the bottom of the spring with the ruler scale (1)
or
attach a horizontal pointer to the bottom of the spring (1) so that the pointer goes across the ruler scale (1).
No other masses were provided.
Explain how the student could test if the spring was behaving elastically.
- remove mass
accept remove force / weight - observe if the spring returns to its original length / shape (then it is
behaving elastically)
Without any calculation, estimate his acceleration 632 seconds after leaving
the aircraft.
Explain your value of acceleration in terms of forces.
0
upward force = weight (gravity)
so resultant force zero
A suspended object will come to rest with its centre of mass directly
…………………………………………. the point of suspension.
below
State what is meant by the ‘centre of mass of an object’.
point at which mass of object may be (thought to be) concentrated
Two children, A and B, are sitting on a see-saw. The see-saw is balanced. Use the idea of moments to explain what happens when child B moves closer to the pivot.
- the clockwise moment (of child B) decreases
- making it is less than the anticlockwise moment (of child A)
accept so moments are no longer balanced - so child A moves downwards
or
so child B moves upwards
The wheel nut will not move and so the handle of the wrench is extended. It is now easy to loosen the wheel nut using the same force as before.
Explain why.
- force is (applied) further from the nut / pivot / axis of rotation
(handle is longer is insufficient
do not accept less force needed) - moment (on wrench) is larger
Use the terms ‘line of action of the weight’ and ‘resultant moment’ to explain why a stable concrete mixer does not fall over when it is given a small push.
- line of action (of the weight) lies / falls inside the base
- the resultant moment returns mixer to its original position
Explain why the monitor will tip over if the screen is tilted too far back.
the position of the centre of mass has changed (1)the line of action of the weight is outside the base (1)producing a (resultant) moment (1)
