Physics M3C4 Forces in Action

Define the Newton

• Force required to accelerate a mass of 1kg by 1ms^-2

How can I measure the weight?

• Use a Newtonmeter

Define centre of mass

Define centre of gravity

Where are they?

• Point where any externally applied force produces straight-line motion but no rotation

• Point at which we consider the entire weight of a body to act

• Same place

How do find centre of mass/gravity?

• “Freely suspended object will come to rest with its centre of mass/gravity vertically below the point of suspension”

• Plumbline method:

  • Make small holes along edges

  • Insert pin connected to clamp into hole, with plumbline tied to the end of the pin

  • Allow object to come to rest

  • Draw a line along vertical string of the plumbline

  • Repeat for other holes

  • Center of mass/gravity is at the intersection of the lines

  • Can check by balancing on finger

Find acceleration and time taken

• Resolve W into F parallel to slope (mgsinθ)

• a = F/m

• Use s = u t + ½ a t²

What affects drag?

Relationship between drag and speed?

• Speed, cross-sectional area of object. Density of fluid.

• Drag ∝ Speed²

Terminal velocity

• At t=0, no drag and a = 9.81, Net force unbalanced

• As t increases, speed increases and drag increases

• Net force decreases so acceleration decreases

• Eventually, drag = weight, 0 acceleration - ‘Terminal velocity’, net force is 0

Experiment to determine terminal velocity

• Mark cylinder at regular length intevals (e.g. 10cm)

• Fill cylinder with viscous liquid

• Release ball bearing from rest at top of cylinder

• Start timer, record time at every marked inteval

• Record time and calculate the speed of the ball at each inteval

• Repeat a total of 3 times, find the average time at each inteval

• Terminal velocity when time difference between each inteval is the same

• Plot graph os distance (y-axis) agianst time (x-axis)

• Terminal velocity when gradient is constant

• Gradient = velocity

Define moment

• Product of force and perpendicular distance to line of actioin to pivot

What is equilibrium?

What is the principle of moments?

• When a body is in equilibrium, net force and net moment is 0.

• Principle of moments - for a body in rotational equilibrium, the sum of anticlockwise moments about any point is equal to the sum of anticlockwise moments about the same point

What is torque?

• Product of one of the equal forces applied and the perpendicular distance between the forces

• Moment of a couple

How to appraoch this triangle of forces in equilibirum?

• Use trig to resolve into vertical and horizontal components

• Vertical resultant force = 0

• Horizontal resultant force = 0

Define density

How to determine density?

• Mass per unit volume

• Find mass by weighing on digital balance. Find volume by displacing liquid in eureka can

Define pressure

• Normal force exerted per unit cross-sectional area

Properties of pressure?

• Pressure increases with depth (p ∝ h)

• Denser fluids exert greater pressure

• Fluid pressure at any particular depth is the same in all directions

• Fluid pressure does not depend on cross-sectional area

What is Archimedes’ principle?

What happens if upthrust:

• > weight of fluid displaced

• < weight of fluid displaced

• Upthrust of a body immersed in fluid = weight of fluid displaced

• Applies to fully or partially submerged objects

• If u > weight of fluid displaced, it ascends

• If u < weight of fluid displaced, it sinks

• Floating objects - upthrust = weight of fluid