SP2z - Forces Flashcards
From a diagram with forces on it, how can you tell if the object is stationary or moving at a constant speed?
In both diagrams the speed is not changing as the resultant force is 0N, however:
- if it is stationary, there will be no drag
- if it is moving at a constant speed there will be drag
In terms of forces explain why an object would be decelerating
The resultant force is in opposite direction to motion
In terms of forces explain why an object would be accelerating
The resultant force is in same direction as motion
How do you calculate the rate of acceleration when given the figures of an objects mass and the resultant force?
Acceleration = resultant force / mass
Newton meters have a spring that extends when weights hang off them, stretching down a scale drawn on the side. Explain how this shows the weight of an object
The spring stretches down a scale due to the applied force on it that is in turn caused by the hanging weight. When the spring stops stretching, the force pulling it (which is the same as the weight) is equal to the tension in the spring. The scale on the side is drawn on once it is known how easy the spring stretches
Bathroom scales contain a spring that compresses when someone stands on them, moving a needle. How does this indicate the weight of a person?
The spring needs a contact force to push on it, which when it stops squashing, is equal to the weight the person above
Outline the experiment you did to find out how the acceleration of a trolley is linked to the mass of the trolley if a constant force is used to accelerate it
- place trolley on a slightly inclined ramp so friction is accounted for
- attach a known weight to the trolley by string, run the string over a pulley so the weight is hanging off the side of the bench
- add discs to the trolley to change its mass
- initial speed is 0
- use a light gate to measure the final speed of the trolley at the end of the track
- time with a stopwatch the trolley along the track
- do acceleration = (final speed - initial speed) / time
- graph mass against acceleration to find the relationship
To demonstrate circular motion, a teacher whirls a bung around on a piece of string. In this example, what provides the centripetal force to keep the bung moving in a circle?
The tension in the string
What force is needed for things going in a circle?
The centripetal force
What is inertial mass, and what is the equation for it?
Inertial mass is the object‘s resistance to acceleration when subjected to a force. The bigger the inertial mass, the less the object will accelerate for a fixed force applied
m = f/a
(Inertial) mass = resultant force / acceleration
How do you calculate momentum?
Momentum = mass x velocity P = mv
What are the SI units for momentum?
Kgm/s
How can you work out the force an object feels from another object during impact?
Force = change in momentum / time
How could you measure the reaction time of a human?
- hold a ruler above their hand so they are ready to catch
- drop the ruler at a random time
- note the distance on the ruler that it fell
- convert that to a time using known data
- repeat for accuracy
What is stopping distance?
The distance gone from the position a driver is at when they realise they need to stop to the point the car reaches when it is stopped
What is thinking distance?
The distance gone from the position a driver is at when they realise they need to stop to the point at which they begin to brake the car
What is braking distance?
The distance gone from the position a car is at when it begins to brake to the point it is at when it is stopped
What equation links together: stopping distance, thinking distance, and braking distance?
Stopping distance = thinking distance + braking distance
How do you calculate thinking distance?
Thinking distance = average speed x reaction time
How does the mass of the vehicle affect the braking distance?
More force is needed to decelerate larger inertial mass
How does the speed of the vehicle affect the braking AND thinking distance?
- more KE to get rid of while braking
- further gone while reacting (thinking) - thinking distance = speed x reaction time
How does the driver‘s reaction time affect the thinking distance?
Thinking distance = speed x reaction time
Give a general rule for how speed affects thinking distance
It is a DIRECTLY PROPORTIONAL relationship: the higher the speed, the higher the thinking distance - as you double the speed, the thinking distance doubles etc.
Give a general rule for how speed affects braking distance
As the speed or velocity increases, the braking distance increases with the SQUARE of the speed or velocity
(Eg. Double the velocity means 4x the braking distances, triple the velocity means 9x the braking distance etc.)
