MECHANICS

Question 1

Explain the significance of the gradient of the line for a displacement time graph.

Answer 1

The gradient is Δs/Δt = v

Question 2

Explain the significance of the gradient and area under the line for a velocity time graph

Answer 2

1. The gradient is Δv/Δt = a

2. The area under the line is the displacement

Question 3

Explain the significance of the gradient and area under the line for an acceleration time graph

Answer 3

The area under the line is the velocity.

Question 4

Name the Scalar quantities

Answer 4

Mass
Time 
Distance 
Speed 
Work done 
GPE 
Kinetic energy

Question 5

Name the Vector quantities

Answer 5

Force 
Acceleration 
Velocity 
Displacement 
Momentum

Question 6

State Newton’s first law

Answer 6

An object will remain at a constant velocity or stationary if the resultant force is zero.

Question 7

State Newton’s second law

Answer 7

The resultant force acting on an object is equal to the mass of the object multiplied by the acceleration it experiences in the direction of the resultant force.

Question 8

State Newton’s third law

Answer 8

When object a exerts a force on object b, object b exerts an equal
and opposite force on object a

Question 9

State the criteria that Newton’s third law pairs must satisfy

Answer 9

1. Act on different objects
2. Be the same size
3. Be the same type of force (eg. Both gravitational/both electrostatic)
4. Act in different directions

Question 10

A rocket ejects gas backwards whilst in deep space. Explain, using Newton’s laws, what
happens to the motion of the rocket

Answer 10

1. The rocket applies a force on the gas backwards.
2. Due to Newton’s third law, the gas applies an equal and opposite force on the rocket forwards.
3. The rocket experiences a resultant force, so due to Newton’s second law it accelerates forwards.

Question 11

Define centre of gravity

Answer 11

The centre of gravity of an object is a point where the entire weight
of an object appears to act

Question 12

Define moment

Answer 12

Moment = force x perpendicular distance between the line of action of the force and the pivot

Question 13

State the principle of moments

Answer 13

The sum of the anticlockwise moments is equal to the sum

of the clockwise moments for a system in rotational equilibrium.

Question 14

Explain how the velocity changes throughout a skydive:

Answer 14

1. Initially, when the skydiver has just left the plane, the resultant force is weight.
2. Resultant force = W= mg = ma so a = g due to Newton’s second law. So the person accelerates. at g = 9.81m/s 2,(the acceleration due to gravity)
3. As their speed increases, air resistance increases
4. This causes the resultant force to decrease
5. Until air resistance = weight and the resultant force is zero
6. At this point the acceleration is zero due to Newton’s first law– they are falling at
   terminal velocity

Question 15

Conservation of energy

Answer 15

sum of energy remains constant before and after the collision

Question 16

Conservation of momentum

Answer 16

The vector sum of the momenta before a collision is equal to the vector sum of the momenta after a collision provided no external forces act

Question 17

Impulse

Answer 17

resultant force x time or change in momentum

Question 18

Linear momentum

Answer 18

product of mass and velocity

Question 19

Explain how conservation of momentum applies to a situation where an object ejects gas
and starts moving eg. a rocket leaves the surface of the earth:

Answer 19

1. Before the gas is ejected, the total momentum of the system is zero.
2. When the gas is ejected, the gas experiences a resultant force downwards.
3. As force = rate of change of momentum, the gas’s momentum increases in the downwards direction.
4. Therefore the rocket must also experience a change in momentum equal and opposite to the gas’s momentum (due to Newton’s third law, the force is equal and opposite)
5. So it has a momentum upwards that is equal in size and opposite in direction to the
   gas’s momentum – the total momentum is still zero.