Forces Flashcards
How do you measure weight?
Calibrated spring-balance (a newtonmeter).
How do you identify centre of mass? Define it.
If you hang the body somehow and let it in equilibrium, a line parallel to gravity will connect the suspension point and the centre of mass.
The point from where mass appears to be concentrated and where weight may be considered to act.
Can you draw free body diagrams?
Sketch and check.
Can you draw vector diagrams, including resolving vectors?
Sketch and check.
Provided the spring is not inelastically deformed…
Work done on the spring and EPE are equal.
Can you interpret force, extension graphs?
Sketch and check.
RP6. Investigate the relationship between force and extension for a spring.
- Clamp stand on edge of bench with a 2kg counterweight on its base, attach two clamps, attach a spring with a horizontal splint on the bottom to the top clamp, a ruler to the other, and adjust the ruler so 0 is next to the top of the spring.
- Measure and record the length of the unloaded spring.
- Add 0.1kg masses up to 1kg, recording the length for each weight.
- Calculate extension for each weight.
- Plot a line graph with extension on the vertical axis and force on the horizontal axis, draw a curve of best fit. Identify the range of force where they are directly proportional. K = gradient, on this graph thee less steep line will be the greater k.
Systematic error: zero error
Random error: the ruler not being vertical, misreadings
Hazards: equipment + masses falling, sharp end of the spring recoiling.
Risks: heavy objects on feet so bruises or fractures, damage to eyes or skin.
Precautions: counterweight, gently lower masses onto the spring, eye protection.
Explain why, in a liquid, pressure at a point increases with the height of the column of liquid above that point and with the density of the liquid?
The weight of the particles above increases pressure on the particles beneath and with density there are more particles and therefore more collisions and therefore a greater pressure.
Why do objects float?
The pressure of the water particles colliding with the bottom of the object is upthrust. If the upthrust can be greater than the weight of an object, it will float.
Why does atmospheric pressure vary with height above a surface?
There are more particles closer to the surface, and those particles colliding with a surface creates pressure.
Compare typical speed values.
walking = 1.5
running = 3
cycling = 6
car = 25
train = 55
plane = 250
sound in air = 330
Near the Earth’s surface any object falling freely under gravity has an acceleration of about …
9.m/s^2
Work with distance time and speed time graphs.
Sketch and check.
What are Newton’s three laws?
- Inertia
- F=ma
- =+opposite
RP7 : investigate the effect of varying the force on the acceleration of an object of constant mass, and the effect of varying the mass of an object on the acceleration produced by a constant force.
- Set up an air track with a pulley at one end, two light gates and an interrupt card of 10cm and attach it to a glider that is attached to a hanging mass by string over the pulley. Add 100g of mass, this mass causes the force applied to the pulley.
- Release the glider and record the weight and the acceleration from the data-logger software.
- Repeat and calculate a mean.
- Repeat with different masses, removing one mass each time, record 5 values in total. Plot a line graph with acceleration on the y and force on the x. This will show that acceleration and force are directly proportional.
- Do the same, but keep force constant and add increasing numbers of slotted masses to the glider.
- Plot a graph of acceleration on the y and 1/mass on the x and this will show that acceleration and mass are inversely proportional.
Systematic error: incorrect data-logger, zero error.
Random error: incorrect weight attached, calculation errors.
Hazards: electrical appliance, masses/glider falling.
Risks: electrical fault, fractures/bruises.
Precautions: check mains and plug are intact and there is no exposed wiring, use small masses and step back after releasing glider.
Explain the significance of inertial mass.
Since Newton’s Second Law is F = ma, we can rearrange this to give us m = F / a. This means that inertial mass can be defined as follows: Inertial mass is the ratio of force over acceleration. We can use the inertial mass to determine how ‘difficult’ it is to change the velocity of an object.
Describe a method to identify reactions times, and what are typical human reaction times?
0.2s - 0.9s.
LOOK IT UP.
Explain the dangers caused by large decelerations.
Force is applied to brakes, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases.
The greater the speed of a vehicle the greater the braking force needed to stop the vehicle in a certain distance.
The greater the braking force the greater the deceleration of the vehicle. Large decelerations may lead to brakes overheating and/or loss of control.
How does reaction time determine thinking distance when braking?
s=vt, so longer time = longer distance.
