Chapters 5-7

Question 0

Moment of a force =

Answer 0

Force x perpendicular distance of the pivot from the line of action of the force

Question 1

Define centre of gravity?

Answer 1

The point through which the entire weight of the object maybe be considered to act.

Question 2

What is the principle of moments?

Answer 2

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

Question 3

What is a couple?

Answer 3

A pair of equal and opposite forces that are not acting in the same straight line.

Question 4

Define torque?

Answer 4

The product of one of the forces of a couple and the perp distance between them (Nm).

Question 5

Torque of a couple =

Answer 5

One of the forces x perpendicular distance between the two forces.

Question 6

When is the equilibrium of a body achieved?

Answer 6

When the resultant force and resultant torque on the body are both zero.

Question 7

Density =

Answer 7

Mass/volume

Question 8

Define pressure?

Answer 8

The normal force per unit cross sectional area.

Question 9

Pressure =

Answer 9

Normal force / cross sectional area

N/m^2

Question 10

Define thinking distance?

Answer 10

The distance travelled by the car in the time the driver reacts to a particular situation.

Question 11

Define braking distance?

Answer 11

The distance travelled by the car as the brakes are engaged and the car comes to a stop.

Question 12

Define stopping distance?

Answer 12

Thinking distance + braking distance

Question 13

Power = ?

Answer 13

Rate at which energy is transferred or rate at which work is done.

P=work(j)/time(s)

Question 14

Work done =

Answer 14

Displacement in direction of force x force

Question 15

What is a joule?

Answer 15

Energy transferred when a force of 1N moves a distance of 1m in the direction of the force

Question 16

GPE =

Answer 16

Mgh

Question 17

KE =

Answer 17

0.5x m x v^2

Question 18

What does the principle of conservation of energy state?

Answer 18

That for a closed system, energy can be transformed to other forms but the total amount of energy remains constant.

Question 19

Efficiency =

Answer 19

Useful output energy / total input energy x100

Question 20

What is a watt?

Answer 20

A transfer of energy of one joule per second.

Question 21

Explain how crumple zones help to make cars safer?

Answer 21

Front and rear crumple zones mean that if the car crashes, they squash up, and therefore absorb a lot of the energy from the impact. This means that less of the energy reaches the passengers and so they are safer.

Question 22

Explain how seat belts help to keep passengers safe?

Answer 22

When a car crashes or stops suddenly, the car will stop but the people inside will keep on moving forwards, possibly into the windscreen or steering wheel. Seat belts hold passengers onto their seats so this doesn’t happen. They are also slightly stretchy so the passenger isn’t brought to a halt too suddenly. Finally, they extend across the whole torso so that the pressure of the seatbelt on the body is spaced out rather than being confined to one small space on the body.

Question 23

Explain how air bags help to keep people safe?

Answer 23

If a car suddenly decelerates, the air bag will inflate so that as the person moves forward they are cushioned as they move forwards. It stops them hitting the windscreen/steering wheel. There are holes in the bag so that it deflates quickly after being inflated to prevent the person from bouncing back off the airbag.

Question 24

How do all car safety features help to slow people down slower?

Answer 24

Change in momentum = force x time taken for change

Airbags increase the amount of time taken for a person to come to a halt, and therefore must decrease the force since change in momentum is constant.

Question 25

How does GPS work?

Answer 25

30+ satellites orbiting in such a way that on any point of Earth at least 3 are above horizon.
3 satellites are sending out signals from space to receivers, and between them they look at the time lapse and speed of radio waves to work out the distance from each satellite to the receiver. The receiver now knows the distance between itself and 3 satellites and so can work out where on the earth it is. This is called trilateration.