Momentum

Question 1

What is the equation of momentum?

Answer 1

Momentum (p) (in kg m/s) = mass m (kg) x velocity, v (in m/s)

Question 2

What is the equation for force?

Answer 2

Force = change in momentum / time taken 
Force= (mv - mu) / t

Question 3

What is the relationship between momentum before and after a collision?

Answer 3

Moment before the collision = moment after the collision

Question 4

How do you increase momentum?

Answer 4

force x time = increase in momentum

Question 5

What is Newton’s 1st Law?

Answer 5

Things do not speed up, slow down or change direction unless you push (or pull) them

Question 6

What is Newton’s 2nd Law of motion?

Answer 6

• The bigger the force acting on an object the faster the object will speed up
• Objects with greater mass require bigger forces than those with smaller mass to make them speed up (accelerate) at the same rate

Question 7

What is force?

Answer 7

A vector quantity

Question 8

How do air bags help increase car safety?

Answer 8

• Air bags increase the time taken for the head’s momentum to reach zero, so reduces the forces on it.
• They also act as a sort of cushion and prevent cuts

Question 9

How do helmets help increase car safety?

Answer 9

-A motor cyclists safety helmets padded inside so as to extend time of any collision

Question 10

How do seat belts help increase car safety?

Answer 10

• Stop you tumbling around inside the car if there is a collision
• However they are designed to stretch a bit in a collision, this increases the time taken for the body’s momentum to reach zero so reduces the forces on it

Question 11

How do crumple zones help increase car safety?

Answer 11

• They are areas of a vehicle designed to crush in a controlled way in a collision
• They increase the time taken to change the momentum in a crush which reduces the force induced

Question 12

How do these car features help increase car safety?

Answer 12

• When you stop you momentum will reach 0
• Change in momentum = force x time
• If you increase the time taken to stop
• The force acting will be less
• Therefore you are less likely to be hurt

Question 13

What is Newton’s Third Law?

Answer 13

If Object A exerts a force on object B then object B exerts the exact opposite force on object A

Question 14

Explain Newton’s Third Law

Answer 14

1. That means is you push something say a shopping trolley, the trolley will push back against you just as hard
2. And as soon as you stop pushing so does the trolley
3. The two forces are acting on different objects e.g. ice skaters:
   - When skater A pushes on skater B (the ‘action’ force) she feels an equal force and opposite force from skater B’s hand (the reaction force). Both skaters feel the same sized force in opposite directions and so accelerate away from each other
   - However skater A would be accelerated more than skater B if she had a smaller mass because F =ma
4. It is the same sort of thing if you go swimming. You can push back against the water with your arms and legs and the water pushes you forwards with an equal-sized force in the opposite direction

Question 15

What is the difference between vector and scalar quantities?

Answer 15

1. Vectors have size and direction

2. Scalar quantities only have size

Question 16

What is a vector? What are some examples?

Answer 16

1. When there are multiple forces acting on an object, it is often useful to know the resultant force acting on the object to do this you need to know the size of all the different forces acting on the object and their direction
2. Force, Velocity, Displacement, Acceleration, Momentum
   - Vector quantities are usually represented by arrows

Question 17

What is a scalar quantity? What are some examples?

Answer 17

1. Some physical quantities only have size and no direction

2. Mass, Temperature, Time, Length

Question 18

How do you work out resultant force?

Answer 18

• Combine vectors

- Add them end to end

Question 19

Why does a force cause changes in momentum?

Answer 19

1. When a force acts in an object it causes a change in momentum (force acting (N) = change in momentum (kg m/s) / time taken for change to happen (s)
2. A larger force means a faster change of momentum, and so a greater acceleration
3. Likewise, if someone’s momentum changes very quickly e.g. in a car crash the forces on the body will be very large, and more likely to cause injury
4. This is why cars are designed to slow people down over a longer time when they have a crash because the longer it takes for a change in momentum the smaller the force and so the less severe the injuries are likely to be

Question 20

Describe resultant forces

Answer 20

1. In mot real situations there are at least two forces acting on an object along any direction
2. The overall effect of these forces will decide the motion of the object, whether it will accelerate, decelerate or stay at a steady speed
3. If the forces act along the same line, the “overall effect” is found by just adding or subtracting them
4. The overall force you get is called the resultant force and when you use the formula F=ma, F must always be the resultant force