Forces, momentum and safe stopping
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| momentum/məʊˈmentəm/ | động lượng |
| resultant force/rɪˈzʌltənt fɔːs/ | lực tổng hợp |
What would explain this observation?
- A passenger continues moving when a vehicle brakes. The seat belt provides the force needed to change the passenger momentum 动量.
Build the model
- Resultant force 合力 causes acceleration, not motion itself.
- Momentum is mass multiplied by velocity.
- For an isolated system, total momentum is conserved even when kinetic energy is not.
- momentum: Mass multiplied by velocity.
- resultant force: The vector sum of forces on an object.
For fixed momentum change, which reduces average stopping force?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
For a fixed momentum change, average force = momentum change / time, so a longer stopping time divides the same change by a larger number and gives a smaller force. Crumple zones and airbags apply exactly this.
Match each technical term to its precise meaning.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Impulse equals momentum change.
- Increasing stopping time for the same momentum change reduces average force.
- Identify external forces before applying momentum conservation.
In the lab: Draw a free-body diagram containing only forces on the selected object. For spring measurements, add loads in steps within the elastic range and measure extension from the unloaded position.
Which two habits make the investigation or model in this case more defensible?
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: a 60 kg passenger slows from 10 to 0 metres per second in 0.50 s. Δp = m(v-u) = 60(0-10) = -600 kg metres per second.
- Average F = Δp/Δt = -600/0.50 = -1,200 N.
- The sign shows the force opposes the initial motion.
Your turn: A 2 kg trolley travels at 3 metres per second. Find its momentum.
A 2 kg trolley travels at 3 metres per second. Find its momentum.
Độ khó 2/5 · 140 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
The result is 6 kg·m/s. Known: mass 2 kg; velocity 3 metres per second. Use momentum p = mv. p = 2 x 3 = 6 kg metres per second. Momentum is a vector; the value is in the direction of travel.
Check the conclusion and its limits
- Balanced forces do not require the object to be at rest.
- Mass and weight have different units and meanings.
Not true: An object moving at constant velocity must have a nonzero resultant force. By Newton's first law, constant velocity - including steady motion - means zero RESULTANT force. Forces can be present, but they balance; a nonzero resultant would change the velocity.
- Resultant force causes acceleration, not motion itself.
- Impulse equals momentum change.
- Watch for: balanced forces do not require the object to be at rest.
Write from a momentum of 20 kg metres per second
- A 4 kg trolley travels at 5 metres per second, then stops in 0.40 s.
- Find the momentum and the average force magnitude. Constant velocity means zero resultant force.
An object moving at constant velocity must have a nonzero resultant force.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
By Newton's first law, constant velocity - including steady motion - means zero RESULTANT force. Forces can be present, but they balance; a nonzero resultant would change the velocity.
Mass multiplied by velocity: write the technical term.
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
momentum means Mass multiplied by velocity.
A 4 kg trolley travels at 5 metres per second and then stops in 0.40 s. What is its initial momentum, what is the average force magnitude, and what resultant force does constant velocity need?
Momentum is mass times velocity. Average force is the momentum change divided by the time.
What is the trolley's initial momentum?
Độ khó 1/5 · 100 xu + một phần thưởng nhỏ cho lần hoàn thành đầu tiên trong ngày
4 times 5 is 20 kg metres per second. The 50 N figure is the force, not the momentum. Other options — 50 N (the average force magnitude); 5 kg metres per second (the speed).