Collision Safety Review
Everything in one place, plus mixed practice
The Big Ideas
Stopping an object takes a fixed impulse: Δp = mΔv.
F = Δp/t: increase the time → decrease the force.
Area under a force-time graph = impulse.
Safety devices: crumple zones, seat belts, airbags, helmets, pads, mats.
Explore
Choose a speed and protection, then press Crash! Compare the force graph with the previous trial.
Mixed Practice
A 0.50-kg ball moving at 10. m/s is stopped in 0.020 s. Find the average force.
Δp = 5.0 kg•m/s; F = 5.0/0.020 = 250 N
In a fall, a 5.0-kg head moving at 6.0 m/s stops in 0.0020 s without a helmet, or 0.010 s with one. Find both forces.
Δp = 30. kg•m/s. Without: 15,000 N. With helmet: 3,000 N — five times smaller.
An airbag must keep the force on a 60.-kg driver moving at 20. m/s below 8,000 N. What is the minimum stopping time?
Δp = 1,200 kg•m/s; t = Δp/F = 1,200/8,000 = 0.15 s
Why do you bend your knees when you land from a jump?
Bending your knees increases the stopping time, so the same change in momentum happens with a smaller force on your legs.
A ball hits a wall at 4.0 m/s. In case A it stops; in case B it bounces back at 4.0 m/s. Which case has the larger impulse?
Case B. Its velocity changes by 8.0 m/s instead of 4.0 m/s, so Δp (and the impulse) is twice as large.
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. Impulse is equal to
2. Two identical cars stop from the same speed. Car A stops in 0.1 s, car B in 0.3 s. Compared with car A, the force on car B is
3. Which quantity is the same whether an egg lands on a pillow or the floor (from the same height)?
4. Which design feature mainly reduces landing speed rather than increasing stopping time?
5. A 1,000-kg car moving at 10. m/s stops in 0.50 s. The average force on the car is
Smart Football Helmets
Some football helmets contain sensors that measure the acceleration of a player’s head during hits. In one hit, a player’s 5.0-kg head changed velocity by 4.0 m/s in 0.010 s.
What was the average net force on the player’s head during this hit?
▶ Want a challenge? Open 6 more Regents-style questions (harder · mixed formats)
Challenge score: 0 / 6
C1. Substitute numerical values into a mathematical representation that can be used to determine the magnitude of the average acceleration of the player’s head during the hit. Then calculate the acceleration.
C2. A new helmet padding extends the same hit to 0.025 s. What is the new average force?
C3. East is to the right. The hit changes the head’s velocity by 4.0 m/s toward the west. Choose the direction of the impulse on the player’s head, and determine its magnitude.
●
C4. A trainer makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
C5. During the hit, how does the force the opposing player’s helmet exerts on this player compare with the force this player’s helmet exerts on the opponent?
C6. Engineers want to use the helmet sensors to compare a new padding with the old padding. Describe how they could use the sensor data to evaluate the new padding, and identify one limitation of the sensor data.