Forces, Energy and Power

Question 1

Describe what is meant by ‘friction’.

Answer 1

A force which opposes the motion of an object.

Question 2

Name one type of friction.

Answer 2

Air resistance/drag

Question 3

If two forces act in the same direction, should you add or subtract them to find the resultant force?

Answer 3

Add

Question 4

If two forces act in opposite directions, should you add or subtract them to find the resultant force?

Answer 4

Subtract

Question 5

Describe what is meant by an object having ‘balanced forces’ acting on it.

Answer 5

The object has the same force acting on it in two directions.

Question 6

Draw a free-body diagram of a block with balanced forces acting on it.

Answer 6

Any object with the same value of force on opposing sides.

Question 7

Describe what is meant by an object having ‘unbalanced forces’ acting on it.

Answer 7

The object has a larger force in one direction than the other.

Question 8

Draw a free-body diagram of a block with unbalanced forces acting on it.

Answer 8

Any object with different values of force on opposing sides.

Question 9

Define ‘Newton’s 1st Law’.

Answer 9

An object will remain at rest or move at a constant speed in a straight line unless acted on by an unbalanced force.

Question 10

What can be said about the forces acting on an object if it moves at a constant speed?

Answer 10

The forces are balanced.

Question 11

In terms of Newton’s 1st Law, describe what would happen to a passenger not wearing a seatbelt in the event of a crash.

Answer 11

They would continue to move forward, at the original speed before the crash, into the dashboard/steering wheel etc.

Question 12

Define ‘Newton’s 2nd Law’.

Answer 12

When the forces acting on an object are unbalanced, the object will accelerate in the direction of the unbalanced force.

Question 13

Write down the equation relating unbalanced force, mass and acceleration.

Answer 13

Fu=ma

Question 14

State the symbol and units for unbalanced force.

Answer 14

F, N

Question 15

State the symbol and units for mass.

Answer 15

m, kg

Question 16

Draw a free-body diagram to show the four forces acting on an aeroplane.

Answer 16

Weight, drag, thrust, lift

Question 17

Define ‘Newton’s 3rd Law’.

Answer 17

If an object A exerts a force on object B, then B exerts an equal but opposite force on A. OR For every action there is an equal and opposite reaction.

Question 18

The action and reaction forces for Newton’s 3rd Law are also known as ___________ ________.

Answer 18

Newton pairs

Question 19

Give two examples of newton pairs (i.e. action and reaction forces).

Answer 19

Force of driver on seatbelt and force of seatbelt on driver/balloon pushing air out and air pushing back on balloon/rocket engines pushing hot gases downwards and hot gases pushing back on rocket/cannon firing cannonball outwards and cannonball exerting a force back on cannon/force of foot kicking ball and force of ball back on foot

Question 20

When a car crashes, the driver moves against the seatbelt, exerting a force on it (action force). State the reaction force for this newton pair.

Answer 20

The seatbelt locks in place and exerts a force back on the driver.

Question 21

When a rocket launches, the rocket engines push hot gases downwards (action force). State the reaction force for this newton pair.

Answer 21

Hot gases push back on the rocket (upwards).

Question 22

Explain, in terms of forces, why a rocket will eventually move upwards during launch.

Answer 22

When the thrust from the engines becomes greater than the weight, an unbalanced force upwards is produced, causing the rocket to accelerate upwards.

Question 23

As a rocket travels upwards, what happens to the air resistance acting on it as its speed increases?

Answer 23

It increases

Question 24

As a rocket uses up fuel, what happens to its weight?

Answer 24

It decreases

Question 25

As a rocket gets further away from the Earth's surface, what happens to its weight?

Answer 25

It decreases

Question 26

Draw a free-body diagram to show the forces acting on a rocket during launch.

Answer 26

Drag, thrust, lift, weight

Question 27

Describe what is meant by a 'connected system'.

Answer 27

When two or more objects are physically joined in some way.

Question 28

Describe what is meant by 'tension'.

Answer 28

The pulling force of a string, rope, cable or chain.

Question 29

If a mass is suspended (hanging) from a point via some string, in which direction will the tension in the string act?

Answer 29

Towards the point it is being suspended from.

Question 30

Describe what is meant by 'free-fall'.

Answer 30

An object that is accelerating towards the ground due to the influence of gravity alone.

Question 31

Explain how a skydiver in free-fall will reach terminal velocity.

Answer 31

When a skydiver jumps out of a plane, they are in free-fall. Air resistance/drag increases as they accelerate downwards, and eventually the weight of the skydiver downwards and the drag force upwards become balanced. The skydiver now travels at a constant speed i.e. terminal velocity.

Question 32

Define the term 'terminal velocity'.

Answer 32

For an object in free-fall, a constant speed is reached when the upward force on the object (air resistance) is balanced by the downward force on the object (weight).

Question 33

Sketch a velocity-time graph for a free-falling skydiver. No numerical values are required on the axes.

Answer 33

Axes with a sloping acceleration till it reaches a constant speed then a rapid deceleration till steady speed.

Question 34

Describe what is meant by 'mass'.

Answer 34

The quantity of particles that make up an object. It is a scalar quantity.

Question 35

Describe what is meant by 'weight'.

Answer 35

The force due to gravity acting on an object. It is a vector quantity.

Question 36

Explain the difference between mass and weight, including their units.

Answer 36

Mass is the quantity of particles that make up an object, measured in kilograms, whereas weight is the force due to gravity acting on an object, measured in newtons.

Question 37

Write down the equation relating weight, mass and gravitational field strength.

Answer 37

w=mg

Question 38

State the symbol and units for weight.

Answer 38

W, N

Question 39

State the symbol and units for gravitational field strength.

Answer 39

g, N/kg

Question 40

Name the two forces acting on a person who is standing on a set of scales in a lift.

Answer 40

Weight, reaction force

Question 41

For a person standing on a set of scales in a lift, what is the reaction force equivalent to?

Answer 41

The reading on the scales.

Question 42

Sketch a free-body diagram for a person standing on a set of scales in a stationary lift.