Forces and motion levels 7-9

Question 1

What does the term ‘inertial mass’ mean

Answer 1

Inertia is the tendancy of a body to resist changes in motion. The larger the mass of an object the harder it is to start or stop it from moving, and it is thereofre called inertial mass.

A higher inertial mass means that you required a higher force to create the same amount of acceleration.

Question 2

Explain how to calculate the acceleration of an object acted on by more than one force.

Example: what is the acceleration of this body

Answer 2

To find the acceleration first you need to find the resultant force.

In this case it can be done by a scale diagram or pythagoras theoerem.

Resultant force = √(24² + 32²)

= 40

Next use Newons seond law to find the acceleration

F = ma

a = f/m

=40/4

= 10m/s²

Question 3

Explain the shape of the velocity time graph for a skydiver.

Answer 3

The skydiver starts with a high acceleration, this can be seen from the steep gradient of the graph, this ocyrs becuase the only force acting on the skydiver is the weight force

During second A the acceleration reduces as the velocity increases. This is becuase as the skydiver goes faster the size of the air resistance force increases. Eventually terminal velocity is reached, there is now a gradient of zero becuase air resistance and weight balance, leaving no resultant force.

At point B the paracute is opened, increasing the surface area and therefore air resistance. As air resistance is now much greater than weight there is a large deceleration (steep negative gradient)

During part C the deceleration gradually reduces as the air resistance force reduces with velocity, until a new lower terminal velocity is reached.

At point D the skydiver hits the ground, giving a sudden deceleration and reducing velocity to zero.

Question 4

State some factors that will increase both the thinking distance and the braking distance of a car.

Answer 4

The speed of the car

Rainy or snowy weather (affects braking distance by reducing friction and affects thinking distance by making hazard perception harder)

Question 5

Explain how to calculate the braking distance of a car if you know the mass, speed and force of the brakes

Example calculation: what is the braking distance of a car that weighs 2000 tonnes, has a frictional force in its brakes of 1500N and is travelling at 10m/s.

Answer 5

1. Calculate the kinetic energy of the car using the mass and the speed
2. use the equation:

work (energy transferred) = force x distance

to work out the distance required to transfer all of the kinetic energy of the car into other stores

Example:

Kinetic energy = 1/2 x mass x velocity²

= 0.5 x 2000 x 10²

= 100,000J

Distance = energy/force

= 100,000/1500

= 67m

Question 6

What is the relationship between:

1. Speed and thinking distance
2. Speed and braking distance

Answer 6

1. Thinking distance is directly proportional to speed. If speed doubles then thinking distance doubles
2. Braking distance is proportional to velocity squared. This means that if the velocity doubles then the braking distance goes up by 4 (2²)

Question 7

Explain how to use the law of conservation of momentum to find the velocities of objects after they collide.

Example: Trolley of mass 2kg and travelling at 3m/s collides a stationary trolley of mass 4kg and they stick together. What is the velocity of both of the trolleys after the collision

Answer 7

The momentum before any collision is always equal to the total momentum after the collision. This means that if you find the momentum at the start, you can use this to find out about what has happened afterwards.

Example:

1. Draw a diagram to help you to think about what is happening
2. find the total momentum at the start

momentum = 2 x 3 = 6 kgm/s

3. Use this to find the velocity at the end

final momentum = 6

6 = total mass x velocity

The total mass = 2+4 = 6 kg

Therefore velocity = momentum/total mass

= 6/6

= 1 m/s

Question 8

Explain how to use a graph of extension (y-axis) against force (x-axis) to find the spring constant of a spring

Answer 8

1. Mark the section of the graph where extension is proportional to force, this will be the part with the straight line.
2. Take the gradient of this line
3. perform the calculation 1/gradient to find the force

Question 9

What are the potential sources of error in the Hooke’s law practical

Answer 9

Ransom error:

Parallax - the eye, scale and the bottom of the spring may not all be on the same horizonral line

oscilaitons - the mass can oscilate up and down which makes it harder to read

mass errors - each of the masses may have a slightly different mass

Systematic error:

The scale on the ruler might be out.