SP2z - Forces

Question 1

From a diagram with forces on it, how can you tell if the object is stationary or moving at a constant speed?

Answer 1

In both diagrams the speed is not changing as the resultant force is 0N, however:

• if it is stationary, there will be no drag
• if it is moving at a constant speed there will be drag

Question 2

In terms of forces explain why an object would be decelerating

Answer 2

The resultant force is in opposite direction to motion

Question 3

In terms of forces explain why an object would be accelerating

Answer 3

The resultant force is in same direction as motion

Question 4

How do you calculate the rate of acceleration when given the figures of an objects mass and the resultant force?

Answer 4

Acceleration = resultant force / mass

Question 5

Newton meters have a spring that extends when weights hang off them, stretching down a scale drawn on the side. Explain how this shows the weight of an object

Answer 5

The spring stretches down a scale due to the applied force on it that is in turn caused by the hanging weight. When the spring stops stretching, the force pulling it (which is the same as the weight) is equal to the tension in the spring. The scale on the side is drawn on once it is known how easy the spring stretches

Question 6

Bathroom scales contain a spring that compresses when someone stands on them, moving a needle. How does this indicate the weight of a person?

Answer 6

The spring needs a contact force to push on it, which when it stops squashing, is equal to the weight the person above

Question 7

Outline the experiment you did to find out how the acceleration of a trolley is linked to the mass of the trolley if a constant force is used to accelerate it

Answer 7

• place trolley on a slightly inclined ramp so friction is accounted for
• attach a known weight to the trolley by string, run the string over a pulley so the weight is hanging off the side of the bench
• add discs to the trolley to change its mass
• initial speed is 0
• use a light gate to measure the final speed of the trolley at the end of the track
• time with a stopwatch the trolley along the track
• do acceleration = (final speed - initial speed) / time
• graph mass against acceleration to find the relationship

Question 8

To demonstrate circular motion, a teacher whirls a bung around on a piece of string. In this example, what provides the centripetal force to keep the bung moving in a circle?

Answer 8

The tension in the string

Question 9

What force is needed for things going in a circle?

Answer 9

The centripetal force

Question 10

What is inertial mass, and what is the equation for it?

Answer 10

Inertial mass is the object‘s resistance to acceleration when subjected to a force. The bigger the inertial mass, the less the object will accelerate for a fixed force applied
m = f/a
(Inertial) mass = resultant force / acceleration

Question 11

How do you calculate momentum?

Answer 11

Momentum = mass x velocity
P = mv

Question 12

What are the SI units for momentum?

Answer 12

Kgm/s

Question 13

How can you work out the force an object feels from another object during impact?

Answer 13

Force = change in momentum / time

Question 14

How could you measure the reaction time of a human?

Answer 14

• hold a ruler above their hand so they are ready to catch
• drop the ruler at a random time
• note the distance on the ruler that it fell
• convert that to a time using known data
• repeat for accuracy

Question 15

What is stopping distance?

Answer 15

The distance gone from the position a driver is at when they realise they need to stop to the point the car reaches when it is stopped

Question 16

What is thinking distance?

Answer 16

The distance gone from the position a driver is at when they realise they need to stop to the point at which they begin to brake the car

Question 17

What is braking distance?

Answer 17

The distance gone from the position a car is at when it begins to brake to the point it is at when it is stopped

Question 18

What equation links together: stopping distance, thinking distance, and braking distance?

Answer 18

Stopping distance = thinking distance + braking distance

Question 19

How do you calculate thinking distance?

Answer 19

Thinking distance = average speed x reaction time

Question 20

How does the mass of the vehicle affect the braking distance?

Answer 20

More force is needed to decelerate larger inertial mass

Question 21

How does the speed of the vehicle affect the braking AND thinking distance?

Answer 21

• more KE to get rid of while braking

- further gone while reacting (thinking) - thinking distance = speed x reaction time

Question 22

How does the driver‘s reaction time affect the thinking distance?

Answer 22

Thinking distance = speed x reaction time

Question 23

Give a general rule for how speed affects thinking distance

Answer 23

It is a DIRECTLY PROPORTIONAL relationship: the higher the speed, the higher the thinking distance - as you double the speed, the thinking distance doubles etc.

Question 24

Give a general rule for how speed affects braking distance

Answer 24

As the speed or velocity increases, the braking distance increases with the SQUARE of the speed or velocity
(Eg. Double the velocity means 4x the braking distances, triple the velocity means 9x the braking distance etc.)

Question 25

How do you calculate braking force?

Answer 25

Find Kinetic energy (KE = 1/2 x m x v^2)
This is the work done
Then use work done equation to find the force
F = work done / distance moved in direction of force

Question 26

How do you find braking distance?

Answer 26

Find Kinetic energy (KE = 1/2 x m x v^2)
This is the work done
Then use work done equation to find the distance
Distance moved in direction of force = work done (which is kinetic energy) / force applied

Question 27

What is the overall equation to find braking distance?

Answer 27

Braking Distance = (1/2 x mass x velocity^2) / force applied

Question 28

In terms of braking distances, what is the kinetic energy?

Answer 28

Work done in braking (work is done to brake and reduce the kinetic energy of a car)