3.2 Forces in Action

Question 1

If you apply a net force on an object, what will happen?

Answer 1

The object will accelerate in the direction of the net force applied.

Question 2

What equation relates force, mass and acceleration?

Answer 2

Force (N) = Mass (kg) x Acceleration (ms-2)

Question 3

What does 1 Newton give?

Answer 3

1 Newton gives 1kg of mass an acceleration of 1ms-2 in the direction of the force.

Question 4

What is the difference between weight and mass?

Answer 4

Weight is a gravitational force acting on an object - varies with location - and mass is the amount of matter of an object - constant.

Question 5

How is the weight of an object calculated?

Answer 5

Weight (N) = Mass (kg) x Gravitational force

Question 6

What device can be used to determine the weight of an object?

Answer 6

A newtonmetre.

Question 7

What is friction?

Answer 7

The force that arises when two surfaces rub against each other.

Question 8

What is drag?

Answer 8

The resistive force on an object travelling through fluid (liquid/gas).

Question 9

What is tension?

Answer 9

The force within a stretched cable or rope.

Question 10

What is upthrust?

Answer 10

An upward buoyancy force acting on an object when it is in fluid (liquid/gas).

Question 11

What is a normal contact force?

Answer 11

A force arising when one object rests against another object.

Question 12

How are forces represented in a free body diagram?

Answer 12

Each force is represented by an arrow labelled with the force, they’re drawn to scale (longer arrow = greater force).

Question 13

How do you find the vertical component of a force?

Answer 13

mgsinθ

Question 14

How do you find the horizontal component of a force?

Answer 14

mgcosθ

Question 15

What is a fluid?

Answer 15

A fluid is a liquid or gas substance.

Question 16

What is a drag force?

Answer 16

The frictional force that opposes motion of an object.

Question 17

What is the magnitude of drag dependent on?

Answer 17

• Speed of an object
• Object’s cross-sectional area

Question 18

How is drag related to speed?

Answer 18

Drag is directly proportional to speed².

Question 19

Describe how forces of weight and drag change when an object undergoes a vertical fall.

Answer 19

• As the object starts to fall, there is no drag force and acceleration is g.
• As it falls, the speed increases and the drag force increases, resultant force decreases and the acceleration becomes less than g.
• The object will reach terminal velocity, so drag and weight are equal and opposite, speed is constant and the acceleration is 0.

Question 20

What is the magnitude of drag at the instant an object begins to fall?

Answer 20

Zero.

Question 21

What is terminal velocity?

Answer 21

Terminal velocity is the constant speed reached when the drag force and weight are equal and opposite on an object.

Question 22

What is an experiment conducted to investigate the motion of an object falling through a fluid?

Answer 22

• A falling object is attached to a pulley and is dropped through a cylinder of liquid, pulling a light polystyrene ball upwards on the other side of the pulley.
• The motion of this ball is identical to that of the object falling through the fluid.
• The exact values of data can be collected using data logger software. and a motion sensor.

Question 23

What is the experiment to determine terminal velocity of a metal ball falling through fluid?

Answer 23

• Have two ball bearings made with the same material, but with two different radii.
• Draw marks on the cylinder that are a known distance apart.
• Start a stop watch and slow motion camera, and drop one of the bearings into a cylinder of glycerine.
• Do this multiple times and repeat with the second bearing, dropping it from the same height.
• When the time taken between markings becomes equal, it has reached terminal velocity.

Question 24

What is centre of gravity?

Answer 24

An imaginary point at which the entire weight of an object appears to act.

Question 25

What is centre of mass?

Answer 25

A point through which any externally applied forces produce straight line motion but no rotation.

Question 26

What is the experiment used to determine the centre of gravity of a 2D object?

Answer 26

- Make holes along the edges of card (2D object) and insert pins firmly in them. - Allow the object to swing freely and it will come to rest with the centre of gravity vertically below the pin. - Use a plumb line to hang from the pin and draw a line across the string. - Repeat for the other holes, and the point of intersection of the lines is the centre of gravity.

Question 27

What is meant by equilibrium?

Answer 27

A body is in equilibrium when the net force and net movement acting on it are 0.

Question 28

If an object is in equilibrium, what does this mean in terms of the horizontal and vertical forces acting on the object?

Answer 28

They must be equal so resultant force is 0.

Question 29

What is meant by the moment of a force?

Answer 29

The moment of a force is the turning effect of a force about an axis or a point.

Question 30

How is the moment of a force determined?

Answer 30

Moment (Nm) = Force (N) x Perpendicular Distance of the Line of Action of Force from the Point of Rotation (m)

Question 31

What is the principle of moments?

Answer 31

For a body in rotational equilibrium, the sum of anticlockwise moments about any point is equal to the sum of the clockwise moments about that same point.

Question 32

How can you make an object rotate without any translational motion?

Answer 32

Apply a pair of equal and opposite, parallel forces to the object, that are applied along different lines.

Question 33

What is a couple?

Answer 33

A pair of equal and opposite forces acting on a body but not in the same straight line.

Question 34

What is torque?

Answer 34

Torque is the moment of a couple.

Question 35

How is torque calculated?

Answer 35

Torque (Nm) = Perpendicular Separation Between Forces (m) x One of the Forces (N)

Question 36

How is density defined?

Answer 36

Density is the mass per unit volume of a substance.

Question 37

How is density calculated?

Answer 37

Density (kgm-3) = Mass (kg)/Volume (m3)

Question 38

How do you find density of a regular object?

Answer 38

- Mass = weigh using scales/balance. - Volume = Measure with rule/micrometre/digital calliper. - Density = m/v

Question 39

How do you find the density of an irregular object?

Answer 39

- Mass = weigh using scales/balance. - Volume = Displacement (putting object in water and measuring the volume before/after it is put in). - Density = m/v

Question 40

How do you find the density of a liquid?

Answer 40

Find the volume using a measuring cylinder, and weigh using scales or a balance for mass. Density = m/v

Question 41

How is pressure defined?

Answer 41

Pressure is the normal force exerted per unit cross-sectional area.

Question 42

How is pressure calculated?

Answer 42

Pressure (Pa or Nm-2) = Force (N)/Area (m2)

Question 43

Why do gases and liquids exert pressure on surfaces?

Answer 43

Fluids exert pressure on surfaces because of constant bombardment by their molecules.

Question 44

What is the equation used to calculate the pressure exerted by a vertical column of liquid?

Answer 44

Pressure (Pa or Nm-2) = Height (m) x Density (kgm-3) x Acceleration by Free Fall (ms-2)

Question 45

What does pressure of a fluid depend on

Answer 45

Pressure is directly proportional to height, so water pressure increases with depth and denser liquids exert greater pressure. Pressure of a fluid at a particular depth is the same in all directions.

Question 46

Why do objects submerged in water experience an upwards force?

Answer 46

It happens due to pressure differences at its upper and lower surfaces that result in an upwards resultant force.

Question 47

What is an expression for calculating upthrust?

Answer 47

Upthrust (N) = Volume of Liquid Displaced (m3) x Density of Liquid (kgm-3) x Acceleration due to Gravity (ms-2)

Question 48

What is Archimedes' principle?

Answer 48

The upthrust exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid the body displaces.

Question 49

What condition is required for a body to sink?

Answer 49

The upthrust is less than the weight of the object.

Question 50

What condition is required for a body to float.

Answer 50

The upthrust is equal to the weight of an object, and the weight must be equal to the weight of fluid displaced.