Topic 2 - Motion and Forces

Question 1

SP2a - If a rocket experiences 800kN of thrust upwards and its weight is 300kN, what is the resultant force?

Answer 1

800 is up 300 is down.

800 - 300 = 500

500kN upwards

Question 2

SP2b - What is Newton’s first law?

Answer 2

• A moving object will remain at a constant velocity until an external force acts on it.
• A stationary object will remain stationary until n external force acts on it.

Question 3

SP2b - What is centripetal force?

Answer 3

• An object moving in a circle at a constant speed is contantly changing velocity and so is constantly accelerating towards the centre of the circle.
• As the car is accelerating, there must be a force acting on it.
• This is centriputal force and always acts towards the centre of the circular motion.

Question 4

SP2b - What three things can provide centriputal force?

Answer 4

• Tension
• Friction
• Gravity

Question 5

SP2b - How can the amount of centripetal force needed for an object to remain in circular motion be altered?

Answer 5

The amount of force needed increases if you:

• Increase the mass of the object
• Increase the speed of the object
• Decrease the radius of the circle

Question 6

SP2c - What is the difference between weight and mass?

Answer 6

Mass is the quantity of matter in an object and weight is the pull force the object experience depending on the gravitational field strength.

Weight - N

Mass - Kg

Question 7

SP2c - What formula links weight and mass?

Answer 7

w = m x g

(Weight = mass x gravitational field strength)

Question 8

SP2c - Describe the forces acting on a skydiver as they jump out of a plane?

Answer 8

• As soon as they jump out, the only force acting on them is their weight and so they accelerate downwards
• Air resistance increases with speed and so as they accelerate, it increases until eventually it is equal to their weight
• At this point the resultant force acting on the skydiver is 0 and so they stop changing velocity as they have reached terminal velocity
• Once the parachute is launched, the air resistance greatly increases causing a decceleration
• As the velocity decreases so does the air resistance until it is again equal to the weight
• It now reaches a new lower terminal velocity

Question 9

SP2d - What is Newton’s second law of motion?

Answer 9

The acceleration of an object depends on:

• Its mass
• The Force acting on it

Question 10

SP2d - What equation uses Newton’s second law?

Answer 10

F = m x a

(Force = mass x acceleration)

Question 11

SP2d - What is inertial mass?

Answer 11

• A measure of how difficult it is to accelerate an object.
• The ratio of the force needed to accelerate an object over the acceleration produced.
• If an object has a lerger inertial mass, it will require more force to produce a given acceleration than an object with a larger inertial mass.

Question 12

SP2d CP - How can you investigate the acceleration of a trolley using light gates?

Answer 12

• Attach masses to the end of the trolley hanging off the end of the table over the ramp and the pulley .
• Set up two light gatesa measured distance apart.
• Connect the light gates to a data logger.
• Release that masses. using your data log information, divide the length of the card by the time the light gate was active for each of the light gates.
• Divide the difference between these two velocities by the time between each of them to get the acceleration.
• To investigate how mass affects this, repeat with different masses.

Question 13

SP2e - What is Newtons third law?

Answer 13

When objects interact the forces exerted are equal and opposite.

Question 14

SP2e - When a person is standing, describe the action-reaction forces.

Answer 14

The downward force of weight from the person on the ground and the upward reaction force of the ground on the person.

Question 15

SP2e - What is the difference between balanced forces and action-reaction forces?

Answer 15

Balanced forces all act on the same object, action-reaction forces act on different objects.

Question 16

SP2f - What is the formula for momentum?

Answer 16

p = m x v

(momentum [kg m/s] = mass x velocity)

Question 17

SP2f - What equation connects force and momentum, and how can this be derived?

Answer 17

F = Δp / t

(Force = change in momentum ÷ time)

As F = m x a

and a = Δv / t

we can see that

F = (m x Δv) ÷ t

Since m x v = p

then m x Δv = Δp

So, F = Δp ÷ t

Question 18

SP2f - What is the law of the conservation of momentum, and how id this affected by the fact that momentum is a vector quantity?

Answer 18

• Total momentum before = total momentum after
• Since momentum is vector, an object moving in the opposite direction has a negative momentum and this must be taken into account when calculating momentum.

Question 19

SP2g - What is stopping distance?

Answer 19

• The total distance it takes for a car to stop once the driver reacts to an external stimuli.
• This consists of: Thinking distance & braking distance

Question 20

SP2g - What is the average reaction time to visual stimuli and what can affect this?

Answer 20

• 0.25 seconds.
• This can be longer if the driver has taken drugs/ alcohol or hasn’t had enough sleep

Question 21

SP2g - What can increase the braking distance of a car?

Answer 21

• Increasing its momentum:
  
  • If it is moving faster
  • Higher mass
• Reducing friction:
  
  • Brakes are worn
  • Rain/ice/snow on the roads
  • Loose gravel

Question 22

SP2h - How can you combine the formulae for work done and KE to have a formula for braking distance?

Answer 22

E = F x d

KE = 1/2 x m x v²

(1/2 x m x v²) ÷ F = d

d = (m x v²) ÷ 2F

Mass x velocity² divded by 2 force = braking distance

[Also can be (p x v) ÷ 2F = d Momentum x velocity ÷ 2 Force = Braking distance]

Question 23

SP2i - What are the four main safety features of a car?

Answer 23

• Air bags
• Crumple zones and side impact bars (Easily give way causing themsleves to crush)
• Seat belts

Question 24

SP2i - Explain how the safety features of a car link to the equation linking force and momentum.

Answer 24

• F = Δp / t
• All the safety features of a car absorbe kinetic energy, increasing the time taken for the momentum to reduce.
• As the time is increased, the force is reduced.