3.1-3.3 & 3.5 Forces

Question 1

Define displacement.

Answer 1

Displacement is the distance travelled in a particular direction.

Question 2

Define instantaneous speed.

Answer 2

The rate of change of displacement at one particular instant in time.

Question 3

Define average speed.

Answer 3

Average speed = total distance travelled / total time taken

Question 4

Define velocity

Answer 4

Velocity is the rate of change of displacement.

Question 5

Define acceleration.

Answer 5

Acceleration is the rate of change of velocity.

Question 6

What does the gradient on a displacement-time graph represent?

Answer 6

The object’s velocity.

Question 7

What does the area under a velocity-time graph represent?

Answer 7

The object’s displacement.

Question 8

What does the gradient on a velocity-time graph represent?

Answer 8

The object’s acceleration.

Question 9

What does a horizontal line on an acceleration-time graph represent?

Answer 9

Constant acceleration.

Question 10

Which two quantities in the equations of constant acceleration have the same unit?

Answer 10

u and v (both are measured in ms^-1)

Question 11

Why can’t we apply the equations of constant acceleration to the motion of a skydiver during a skydive?

Answer 11

Because a skydiver’s acceleration isn’t constant.

Question 12

In a rebound question, why must u and v have opposite signs?

Answer 12

Because they are in opposite directions.

Question 13

What is the acceleration g of free fall?

Answer 13

The force acting per unit mass

Question 14

What is the value of g on and near the surface of the earth?

Answer 14

9.81 m s^-1

Question 15

What is thinking distance proportional to?

Answer 15

Initial speed.

Question 16

What is breaking distance proportional to?

Answer 16

The square of the speed.

Question 17

Define 1N.

Answer 17

The force required to accelerate a 1kg object at 1ms^-1
1N = 1kg X 1ms^-2

Question 18

Define the net (resultant) force.

Answer 18

The sum of all the forces acting on an object.

Question 19

What is a projectile?

Answer 19

An object in freefall / an object that is only acted on by weight / an object whose acceleration is g

Question 20

If you have a two-dimensional motion problem with constant velocity in one direction and constant acceleration in a perpendicular direction, how do you solve it?

Answer 20

• Split the page into vertical and horizontal - break the initial motion down into vertical and horizontal components of velocity
• Use positive and negative to indicate direction (e.g. positive = up, negative = down)
• Use SUVAT to solve vertically - generally for the time.
• Use s = v t to solve horizontally

Question 21

If you are given a SUVAT question and a force starts off at an angle, how should you tackle it?

Answer 21

Resolve it into horizontal and vertical components.

Question 22

If a projectile is launched and lands at the same horizontal level, which two sets of values are the same?

Answer 22

Time up = Time down
Vertical take off speed = Vertical landing speed

Question 23

What is a free body diagram?

Answer 23

A diagram that represents the forces or the net (resultant) force acting on an object.

Question 24

What direction does normal contact force act in?

Answer 24

Perpendicular to the surface.

Question 25

True or false: An object will take the same time to fall a given distance, regardless of the horizontal component of its velocity.

Answer 25

True.

Question 26

Define tension.

Answer 26

The force experienced by a cable, rope, or string when pulled or hung. It acts away from the mass of the object.

Question 27

Define normal contact force.

Answer 27

The reaction force between an object and a surface.

Question 28

Define upthrust.

Answer 28

The upwards force that a fluid applies on an object. The size of the upthrust force can be found by applying Archimede's principle.

Question 29

Define friction.

Answer 29

The resistive force that one surface encounters when moving over another.

Question 30

Define drag.

Answer 30

The frictional force experienced by an object travelling through a fluid.

Question 31

What direction do drag forces such as friction act in?

Answer 31

The opposite direction to the movement causing it.

Question 32

List four factors that would affect the size of the drag force exerted on an object.

Answer 32

- The object's cross sectional area - The object's speed - The density of the fluid that the object is travelling through (if applicable) - The object's shape and texture

Question 33

Describe the motion of an object falling in a uniform gravitational field in the presence of drag.

Answer 33

- Initially, the object accelerates with an acceleration value of g because weight is the only force acting. Initially, there is no velocity. - Drag increases as the object falls due to increased speed. - Eventually the force of drag will be equal to the weight, the object will no longer accelerate as the net force is 0.

Question 34

For an object falling in a uniform gravitational field in the presence of drag, what is the initial acceleration when it is stationary?

Answer 34

g

Question 35

For terminal velocity to occur, which two forces must be equal?

Answer 35

Drag and weight.

Question 36

Name two practical techniques that can be used to determine terminal velocity in fluids.

Answer 36

- Ball bearing in a viscous liquid - Cupcake case falling through air

Question 37

What equation can be used to find the moment of a force?

Answer 37

Moment = force x perpendicular distance

Question 38

What is a couple?

Answer 38

A pair of forces that together cause a net (resultant) moment, but not a net (resultant) force.

Question 39

State the conditions for a pair of forces acting on the same object to be a couple.

Answer 39

The two forces have to be the same size but acting in opposite directions.

Question 40

What is a torque?

Answer 40

A force that causes something to turn.

Question 41

State the equation that can be used to find the torque due to a couple.

Answer 41

Torque = Force x distance

Question 42

State the principle of moments.

Answer 42

For an object to be in equilibrium, the sum of the clockwise moments must equal the sum of the anti-clockwise moments exerted on it.

Question 43

What is the centre of gravity of an object?

Answer 43

The point through which the weight appears to act.

Question 44

What is the centre of mass of an object?

Answer 44

The average position of the object's mass.

Question 45

State the two conditions required for an object to be in equilibrium.

Answer 45

The net resultant force on the object must be zero and the net resultant moment on the object must be zero.

Question 46

Describe the motion of an object that has a net resultant force of zero exerted on it.

Answer 46

Its acceleration is zero, however it may be stationary or moving at a constant velocity.

Question 47

Explain what is meant by resolving a force in the horizontal and vertical directions.

Answer 47

Finding the horizontal and vertical components of the force.

Question 48

What does coplanar mean?

Answer 48

In the same plane.

Question 49

If three coplanar forces add up to zero, what shape will their vector addition diagram make?

Answer 49

A triangle.

Question 50

State Archimedes principle.

Answer 50

The upward force exerted on an object immersed in a fluid is equal to the weight of the fluid displaced by the object

Question 51

State the equation which can be used to find pressure of fluids which have a constant density.

Answer 51

p = h ρ g where p is the pressure, h is the depth of the liquid, ρ is the density of the liquid, and g is the acceleration due to gravity.

Question 52

What phrase means the same as work done by a force?

Answer 52

Energy transferred by the force.

Question 53

Define work done.

Answer 53

Work done = force x distance moved in the direction of the force.

Question 54

What is the formula for work done, if the object moves in the same direction as the direction of the force?

Answer 54

W = Fx

Question 55

Define power.

Answer 55

The rate of energy transfer or the rate of work done.

Question 56

Define 1 Joule.

Answer 56

1 Joule is the amount of work when a force of one newton is exerted over a distance of one meter.

Question 57

State the principle of conservation of energy.

Answer 57

Energy cannot be created or destroyed, only transferred between different stores and objects.

Question 58

How can you increase an object's gravitational potential energy store?

Answer 58

Lift the object higher in the gravitational field.

Question 59

Why are energy transfers in the real world never 100% efficient?

Answer 59

Work done against resistive forces, such as friction and air resistance, means that some energy is always transferred to the thermal energy stores of an object and its surroundings.

Question 60

State Newton's first law.

Answer 60

An object will maintain constant velocity, unless acted on by a resultant force.

Question 61

State Newton's second law (with the formula and also with words).

Answer 61

F = ma

Question 62

State Newton's third law.

Answer 62

If body A exerts a force on body B,then body B exerts the same size force on body A in the opposite direction.

Question 63

What is impulse?

Answer 63

The change in momentum:

Question 64

How can impulse be found from a force-time graph?

Answer 64

It is equal to the area underneath the graph.

Question 65

What is the difference between elastic and inelastic collisions?

Answer 65

In an elastic collision, the kinetic energy before is equal to the kinetic energy afterwards - no energy is lost. However, in an inelastic collision, the kinetic energy at the end is not equal to the kinetic energy at the start - some energy is lost to the surroundings.