Mechanics

Question 1

What is a scalar quantity? Give examples.

Answer 1

• Scalar quantities only have magnitude.
• E.g. distance, speed, energy, mass, temperature.

Question 2

What is a vector quantity? Give examples

Answer 2

• Vector quantities have both magnitude and direction.
• E.g. displacement, velocity, acceleration, force, momentum.

Question 3

How do you find the resultant of two vectors?

Answer 3

• Use Pythagoras for the magnitude and trigonometry for the angle.
• Draw a tip to tail scale diagram, measure magnitude with a ruler and angle with a protractor.

Question 4

How can a diagram be used to determine whether three vectors are in equilibrium?

Answer 4

• Draw a tip to tail vector diagram to scale.
• A closed triangle of vectors means there is an equilibrium.

Question 5

What are the two conditions for an object to be in equilibrium?

Answer 5

• Resultant force = zero.
• Resultant moment = zero.

Question 6

Define a moment about a point.

Answer 6

• Moment = force x perpendicular distance from the point to the line of action of the force.

Question 7

Give the principle of moments.

Answer 7

• For a system to be in equilibrium
• the sum of clockwise moments about a point
• must be equal to the sum of the anticlockwise moments.

Question 8

What does the word ‘uniform’ mean for a rod/beam?

Answer 8

• Weight acts through the centre of mass,
• which is in the centre of the rod/beam.

Question 9

Define a couple.

Answer 9

• A pair of forces of equal size which act parallel to each other, but in opposite directions. (think turning a steering wheel)

Question 10

Describe the effect of a couple.

Answer 10

• Produces a rotational effect/moment.
• Does not produce a translation.

Question 11

Give the equation for the moment of a couple.

Answer 11

• Moment of a couple = size of one of the forces x perpendicular distance between the line of action of the forces.

Question 12

Define centre of mass.

Answer 12

• The single point that you can consider the whole weight to act through.

Question 13

Where is the centre of a mass of a uniform, regular solid?

Answer 13

• At its centre.

Question 14

How could you find the centre of mass of an irregular object?

Answer 14

• Suspend freely from one point.
• Hang a plump bob down and draw on the vertical line.
• Repeat for a different point and find where lines intersect.

Question 15

What do the gradients represent: a. displacement-time graph? b. velocity-time graph?

Answer 15

• a. velocity. - b. acceleration

Question 16

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

Answer 16

• a. displacement.
• b. change in velocity.

Question 17

Describe the forces acting on a projectile. Ignore air resistance.

Answer 17

• Only force is weight acting vertically down.

Question 18

Describe the horizontal motion of a projectile

Answer 18

• Horizontal component of velocity is constant – no acceleration.
• Horizontal displacement = horizontal velocity x time.

Question 19

Describe the vertical motion of a projectile.

Answer 19

• Accelerates down due to gravity.

Question 20

Give Newton’s 1st law.

Answer 20

• An object will remain at rest, or continue to move with uniform velocity, unless acted on by an external, resultant force.

Question 21

Give Newton’s 2nd law in words.

Answer 21

• The acceleration of an object is directly proportional to the resultant force and inversely proportional to the mass.

Question 22

Use the data sheet to give the equation for Newton’s 2nd law

Answer 22

• F = ma.

Question 23

Give three facts about the two forces in a Newton’s 3rd law pair.

Answer 23

• Equal in size.
• Opposite in direction.
• Act on different objects.

Question 24

Define momentum in words

Answer 24

• Momentum is the product of the mass and velocity of an object

Question 25

Give the equation for momentum and give two possible units.

Answer 25

- p = mv - Units = kgms-1 or Ns.

Question 26

Give the law of conservation of momentum.

Answer 26

- When no external forces act (or isolated/closed system), - the total momentum before an event is the same as after the event.

Question 27

Give the similarities and differences between elastic and inelastic collisions.

Answer 27

- Momentum and total energy are conserved in both. - Elastic collision – kinetic energy is conserved. - Inelastic collision – kinetic energy not conserved.

Question 28

Use the data sheet to give the equation that links force and momentum. Explain in words.

Answer 28

- Resultant force = rate of change of momentum - f = Δ (mv)/Δ t

Question 29

Define impulse and uses the data sheet to give the equation

Answer 29

- Impulse = change of momentum. - FΔt = Δ (mv)

Question 30

The area under a force-displacement graph represents…

Answer 30

- Impulse = change in momentum

Question 31

Explain why safety features like air bags and seat belts reduce the chance of serious injury.

Answer 31

- Person slows down more gradually / over a larger period of time. - Slower rate of change of momentum. - So lower resultant force on the person. - As f = Δ (mv)/Δ t

Question 32

Use the data sheet to give the equation for work done.

Answer 32

W = F x s x Cosθ

Question 33

Area under a force-displacement graph represents…

Answer 33

- Work done

Question 34

Define power and give it's units

Answer 34

- Rate of doing work/energy transfer. - Units = W (or Js-1).

Question 35

What is a kilowatt-hour (kW h)?

Answer 35

- A unit of energy equal to 1 kW of power sustained for 1 hour.

Question 36

Give the law of conservation of energy

Answer 36

- Energy cannot be created or destroyed. - It can only transferred from one form to another.

Question 37

Use the data sheet to give the equation for efficiency.

Answer 37

efficiency = useful output power/input power