Mechanics

Question 1

What is velocity?

Answer 1

The rate of change of displacement per unit time and it is represented by the gradient of a displacement-time graph

Question 2

What is acceleration?

Answer 2

The rate of change of speed/velocity per unit time. It is represented by the gradient of a velocity-time graph

Question 3

How is displacement represented on a graph?

Answer 3

The area under a velocity-time graph

Negative area shows an object returning to its starting point

Question 4

What is a scalar?

Answer 4

A quantity with only magnitude

Question 5

What is a vector?

Answer 5

A quantity with both magnitude and direction

Question 6

Name the scalar quantities

Answer 6

• Mass
• Time
• Energy
• Temperature
• Length
• Speed

Question 7

Name the vector quantities

Answer 7

• Displacement
• Force
• Velocity
• Acceleration
• Momentum

Question 8

Why do projectiles follow a curved path?

Answer 8

Horizontal velocity remains constant but vertical velocity is affected by acceleration due to gravity. Horizontal and vertical components of the object’s motion are completely independent

Question 9

Steps for projectile motion calculations

Answer 9

• Resolve the initial velocity into its horizontal and vertical components
• Use v cos θ for horizontal components
• Use v sin θ for vertical components
• Use the vertical component to work out how long the projectile is in the air
• Use the horizontal component to work out how far it goes while its in the air

Question 10

How can projectile motion be investigated using a video camera?

Answer 10

• Set up a video camera with a known framerate in front of a metre ruler
• Record the position of a ball bearing (for example) in each frame after it is projected
• Use the distance travelled between frames to calculate the velocity

Question 11

How can projectile motion be investigated using strobe photography?

Answer 11

• Set a camera to a long exposure in front of a reference object like a metre ruler or a screen with a grid
• A strobe light flashes repeatedly, which will light up the projectile at regular intervals so it appears at multiple times in the same photograph in a different position each time
• Work out how far the object travels between flashes of the strobe, and use the time between flashes to work calculate the velocity of the projectile between the flashes
  > Horizontal distance between the objects remains constant, and the vertical distance between images is smallest at the top of the arc as the projectile slows down and changes direction

Question 12

Velocity time graphs - acceleration

Answer 12

Acceleration is the gradient of a velocity time graph, and uniform acceleration is a straight line. The steeper the gradient, the greater the acceleration.
y = mx + c can be arranged into v = u + at, so a is gradient (m) and initial speed is the y intercept (c)

Question 13

Acceleration time graphs

Answer 13

The height of the graph shows an object’s acceleration at a time and the area underneath it shows the object’s change in velocity. A negative gradient shows the object decelerating and if a = 0 then the object is moving with constant velocity

Question 14

Free body force diagrams

Answer 14

They should show a single body on its own and should include all forces that act on a body, but not the forces it exerts on other objects. Arrow labels should show the size and direction of the forces, which will be balanced if the object is in equilibrium

Question 15

Resolving forces

Answer 15

Separate the forces into 2 independent forces acting at right angles to each other which will not affect each other since they are perpendicular, then use trig to find the size of a component force in a particular direction.

Question 16

Resultant forces from vectors

Answer 16

Create a closed triangle with vectors, with the resultant force represented by the third side. Then use trigonometry and pythagoras to find relevant angles and sides

Question 17

Resolving forces on a slope

Answer 17

Have the object’s weight acting directly downwards from the centre of the object, use the angle of the slope to draw another line to create the vertical line of the triangle.

Question 18

Newton’s first law

Answer 18

A body will remain stationary or move at a constant velocity unless acted upon by a resultant force.
If forces become unbalanced then then an overall resultant force will make the object accelerate

Question 19

Newton’s second law

Answer 19

The rate of change of the momentum of an object is directly proportional to the resultant (net) force upon it.
F = ma - F in newtons, mass in kilograms, acceleration is always in same direction as resultant force

Question 20

Why do all objects fall at the same rate (if air resistance is ignored)?

Answer 20

Acceleration is independent of the mass
According to Newton’s second law, F = ma, then the only force acting on the object is weight (W = mg)
So F = ma = W = mg, then mass cancels out to leave a = g

Question 21

Newton’s third law

Answer 21

If object A exerts a force on object B, then B exerts a force equal in size and opposite in direction on A.
Newton’s third law pairs always have to be the same type of forces

Question 22

What is friction?

Answer 22

A force that opposes motion. Friction between solid surfaces, and friction in a fluid (e.g. air resistance)

Question 23

Friction in fluids

Answer 23

The force depends on the viscosity of the fluid, and it increases as speed increases. The shape of the object has an impact on the size of the frictional force, and projectiles are always slowed by air resistance, so calculated values of distance a projectile will travel will be too large

Question 24

Terminal velocity of a vehicle

Answer 24

• A car accelerates from rest using a constant driving force
• As velocity increases, frictional forces increase, which reduces the resultant force on the car, causing the acceleration to decrease
• Eventually, the frictional forces equal the driving force, meaning there is no overall resultant force, so no acceleration and the car travels at a constant velocity

Question 25

How can a vehicle’s maximum velocity be increased?

Answer 25

• Increase the driving force by increasing engine size/power

- Reduce frictional force by making the body more streamlined

Question 26

Terminal velocity of a parachutist

Answer 26

• After falling from a plane, the weight will initially be greater than air resistance, so the person will accelerate downwards, but as speed increases, air resistance increases
• Eventually, air resistance will equal the weight and the parachutist will reach terminal velocity
• Before landing, the parachutist will open the parachute, immediately increasing air resistance due to the increased surface area so it becomes much bigger than the weight
• This slows them down until the speed has decreased enough for air resistance to become equal to the weight, reaching terminal velocity again, but at a lower velocity so it is safe enough to land