Section 4: Mechanics

Question 1

What is Newton’s 1st Law of motion?

Answer 1

“The velocity of an object will not change unless a resultant force acts upon it.”

Question 2

State the principle of moments.

Answer 2

For a body to be in equilibrium, the sum of the clockwise moments about a point equals the sum of the anticlockwise moments. The resultant force will be 0.

Question 3

What is Lift? When does it occur? And what direction does it act?

Answer 3

Lift is an upwards force on an object moving through a fluid. It happens when the shape of an object causes the fluid flowing over it to change direction. The force acts perpendicular to the direction in which the fluid is flowing.

Question 4

What causes objects to topple?

Answer 4

If the line of action of its weight falls outside its base area. This is because a resultant moment will occur causing it to fall away from its original position.

Question 5

What is meant by the centre of mass of an object?

Answer 5

The single point that you can consider its whole weight to act through. An object will balance around this point. For a uniform solid, this will be at the centre of the object.

Question 6

What is meant by freefall?

Answer 6

When there’s gravity acting on an object and nothing else. Defined as the motion of an object undergoing an acceleration of ‘g’.

Question 7

What are the conditions needed for a body to be in equillibrium?

Answer 7

All forces acting on the object must be balanced - the resultant force must be 0. An object in equilibrium will be at rest or at a constant velocity.

Question 8

What is friction? What three important things relate to friction?

Answer 8

Friction is a force that opposes motion.

1) Friction forces always act in the opposite direction to the motion of the object.

2) They can never speed things up or start something moving.

3) They convert kinetic energy into heat.

Question 9

What is terminal speed?

Answer 9

Terminal speed is the maximum velocity a free falling object will reach, it happens when frictional forces equal the driving force. The resultant force will be 0, therefore the resultant acceleration will also be 0.

Question 10

What is the principle of conservation of energy?

Answer 10

Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change.

Question 11

Give 5 examples of vector quantities

Answer 11

1) Displacement 2) Velocity 3) Force 4) Acceleration 5) Momentum

Question 12

What is the principle of linear momentum?

Answer 12

Assuming no external forces act, linear momentum is always conserved. This means the total linear momentum of two objects before they collide equals the total linear momentum after the collision.

Question 13

What is a projectile and why does it follow a curved path?

Answer 13

Any object given an initial velocity and then left to move freely under gravity is a projectile. The horizontal and vertical components of the motion are completely independent. They follow a curved path because the horizontal velocity remains constant, while the vertical velocity is affected by the acceleration due to gravity - ‘g’.

Question 14

What is Impulse? How do you find it from a force - time graph?

Answer 14

The change in momentum. Area under the graph.

Question 15

What is Newton’s Third law?

Answer 15

If object A exerts a force on object B, then object B exerts an equal but opposite force on object A.

Question 16

How would you work out the Moment of a couple?

Answer 16

Question 17

What is meant by Displacement, Velocity and Acceleration?

Answer 17

Displacement - How far an object’s travelled from its starting point in a given direction

Velocity - The rate of change of an objects displacement

Acceleration - The rate of change of an object’s velocity

Question 18

How does air resistance affect projectile motion?

Answer 18

The horizontal component of drag reduces the horizontal speed of the projectile, and reduces the horizontal distance the projectile can travel. The vertical component of drag reduces the maximum height the projectile will reach, and steepens the angle of descent.

Question 19

How would you find the centre of mass of an object by experiment?

Answer 19

1) Hang the object freely from a point

2) Draw a vertical line downwards from the point of suspension

3) Hang the object from a different point and draw another vertical line

4) The centre of mass is where the two lines cross

Question 20

How do you calculate average velocity from a displacement-time graph?

Answer 20

The average velocity is just the total change in displacement of the object divided by the total time taken.

Question 21

What is drag? What main three things affect fluid friction?

Answer 21

Drag is friction due to a fluid.

1) The force depends on the thickness of the fluid.

2) The force increases as the speed of the object increases.

3) The force depends on the shape of the object moving through it - larger area pushing against the fluid, the greater the resistance force.

Question 22

What two ways can you increase the top speed of a vehicle travelling through a fluid.

Answer 22

1) Increasing the driving force, e.g by increasing the engine size.

2) Reducing the frictional force, e.g making the body more streamlined.

Question 23

Give 6 examples of scalar quantities

Answer 23

1) Length

2) Speed

3) Mass

4) Temperature

5) Time

6) Energy

Question 24

Define a couple.

Answer 24

A couple is a pair of forces of equal size which act parallel to each other, but in opposite directions. A couple doesn’t produce any resultant linear force, but does produce a turning effect.

Question 25

What is an elastic collision?

Answer 25

A collision where momentum is conserved and kinetic energy is conserved. No energy is dissipated as heat, sound, e.t.c.

Question 26

Why did Galileo used an inclined plane to investigate free fall rather than just normal free fall?

Answer 26

Free falling objects fell too quickly for him to be able to take any accurate measurement - he only had a water clock. Air resistance also affects the rate at which objects fall. Rolling the ball down a plane slowed down the ball’s fall as well as reducing the effect of air resistance.

Question 27

Describe an experiment where you can investigate the efficiency of an electrical motor as it lifts a mass through a measured vertical distance.

Answer 27

Use an ammeter to measure the current through the motor, You can then find the electrical energy of the motor using the equation E = IVt, where I is the current you’ve measured, V is the voltage of the power supply and t is the time taken to lift the mass.

Question 28

How do you calculate instantaneous velocity from a displacement-time graph?

Answer 28

Draw the tangent at the point, and find it’s gradient.

Question 29

What is Newton’s second law?

Answer 29

The acceleration of an object is proportional to the resultant force acting upon it.

Question 30

In equilibrium, what is special when resolving the forces?

Answer 30

They form a closed loop.

Question 31

What is the difference between a scalar quantity and a vector quantity?

Answer 31

A scalar quantity has no direction , just a magnitude whereas a vector quantity has both a direction and a magnitude.

Question 32

What is an inelastic collision?

Answer 32

A collision where some of the kinetic energy is converted into other forms during the collision. Linear momentum is always conserved though.

Question 33

How do you find the work done for a variable force where its value is constantly changing?

Answer 33

Plot a graph of force against distance and find the area under the graph.

Question 34

How do you decrease the force of an impact? Give three ways this is done in industry.

Answer 34

Since F = Δmv/Δt , increasing the time of the impact decreases the force felt.

1) Crumple zones

2) Seat belts

3) Airbags