Physics Unit 2: Motion
Momentum and Impulse
Momentum can be defined as ‘mass in motion’. Momentum is a vector quantity, the direction of the momentum is
the same as the direction of the velocity. All objects have a mass, so if an object is moving, it has momentum. The
more momentum that an object has, the harder that it is to stop. Momentum depends on two variables: Mass and
Velocity, and is given by the equation:
𝒑 = 𝒎𝒐𝒎𝒆𝒏𝒕𝒖𝒎, 𝒖𝒏𝒊𝒕𝒔: 𝒌𝒈 𝒎 𝒔−𝟏
𝒑 = 𝒎𝒗
𝒎 = 𝒎𝒂𝒔𝒔,
𝒖𝒏𝒊𝒕𝒔: 𝒌𝒈
𝒗 = 𝒗𝒆𝒍𝒐𝒄𝒊𝒕𝒚, 𝒖𝒏𝒊𝒕𝒔: 𝒎 𝒔−𝟏 𝒐𝒓 𝒎/𝒔
Check your understanding:
1. Determine the momentum of….
a) An 80 kg halfback moving east at a velocity of 9 𝑚 𝑠 −1
b) A 1000 kg car moving north at a velocity of 20 𝑘𝑚 ℎ𝑟 −1
c) A 60 gram tennis ball moving west at a velocity of 20 𝑚 𝑠 −1
2. A person jogging at a velocity of 12 𝑘𝑚 ℎ𝑟 −1 , has a momentum of 250 𝑘𝑔 𝑚 𝑠 −1 . Find the mass of the jogger.
3. A large rock of mass 100 𝑘𝑔 rolls down a hill with a momentum of 820 𝑘𝑔 𝑚 𝑠 −1 . Find the velocity of the rock.
4. An object has a momentum of 20,000 𝑘𝑔 𝑚 𝑠 −1 . What would be the objects new momentum if:
a) Its velocity was doubled
b) Its velocity was tripled
c) Its mass was doubled
d) Both its velocity was doubled and its mass was doubled.
Impulse is a vector quantity defined as the product of the force acting on a body and the time interval during which
the force is exerted. If the force changes during the time interval, F is the average net force over that time interval.
The impulse caused by a force during a specific time interval is equal to the body’s change of momentum during that
time interval: impulse, effectively, is a measure of change in momentum.
𝑰𝒎𝒑𝒖𝒍𝒔𝒆 = 𝑭 ∆𝒕
= 𝒎∆𝒗
𝐼𝑚𝑝𝑢𝑙𝑠𝑒, 𝑢𝑛𝑖𝑡𝑠 ∶ 𝑘𝑔 𝑚 𝑠 −1 𝑜𝑟 𝑁 𝑠
𝑡 = 𝑡𝑖𝑚𝑒,
𝐹 = 𝑓𝑜𝑟𝑐𝑒, 𝑢𝑛𝑖𝑡𝑠: 𝑁
= 𝒎𝒗 − 𝒎𝒖
= ∆𝒑
= 𝒑𝒇𝒊𝒏𝒂𝒍 − 𝒑𝒊𝒏𝒊𝒕𝒊𝒂𝒍
Other useful ways of expressing these relationships are:
𝑢𝑛𝑖𝑡𝑠: 𝑠𝑒𝑐𝑜𝑛𝑑𝑠 (𝑠)
𝑚 = 𝑚𝑎𝑠𝑠,
𝑢𝑛𝑖𝑡𝑠: 𝑘𝑔
𝑝 = 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚. 𝑢𝑛𝑖𝑡: 𝑘𝑔 𝑚 𝑠 −1
𝑭𝒏𝒆𝒕 =
∆𝒑
∆𝒕
∆ means
‘change in’
Check your understanding:
1. A soccer player kicks a 0.1 kg ball that is initially at rest so that it moves with a velocity of 20 m/s.
a) What is the impulse the player applies to the ball?
b) If the player’s foot was in contact with the ball for 0.01 𝑠, what was the force exerted by the player’s foot on the
ball?
2. A 60 𝑔 tennis ball hits a wall horizontally at a speed of 12 𝑚 𝑠 −1 and bounces back in the opposite direction at a
speed of 10 𝑚 𝑠 −1 . It is in contact with the ball for 0.02 𝑠.
a) What is the change in momentum of the tennis ball?
b) What is the impulse of the tennis ball?
c) What is the magnitude of the force exerted by the wall on the tennis ball?
Now try text book questions: Chapter 6 Questions 46, 47, 48, 49, 50, 51