The physics in plain English
Momentum equals mass times velocity. A 90kg flanker arriving at 6 metres per second carries 540 kg·m/s into contact. A 115kg prop arriving at 4.5 carries about 520. The lighter player wins that contest because speed counts for exactly as much as size. The measured tackles back this up. In the 60-tackle video sample, ball carriers averaged 100kg against 93kg for tacklers, and in front-on tackles the carrier had the mass advantage 77 percent of the time. But tacklers arrived faster in 67 percent of those front-on tackles, entered with more momentum in 60 percent, and finished on top in 57 percent. Speed into contact turned a weight disadvantage into a momentum advantage, and even then no single physical number reliably picked the winner.
100 kg
Ball carrier average mass
60 measured tackles, professional to under-19
93 kg
Tackler average mass
Same tackle sample
563 kg·m/s
Forward carrier momentum
Backs averaged 438 kg·m/s
Why smaller players can win the collision
Three things swing the contest toward the smaller tackler. A lower centre of gravity gives you leverage, because the player who gets underneath controls where the collision goes. Closing speed builds momentum, and speed is far more trainable than mass. And the way you line the tackle up matters as much as what you bring into it: across 4,479 coded tackles from two international competitions, tackles made front on cut the chance of the ball carrier offloading or breaking the tackle compared with chasing from behind or tugging the jersey. Get your body in front of the carrier and the physics start working for you.
The keys to a safe, dominant tackle
- Head up and on the right side. Keep your head up and your face forward, and place your head behind or to the side of the ball carrier, never in front of them. In elite match footage, tackles with these two habits were the ones less likely to end in a head injury check for the tackler.
- Get low before contact. Aim your shoulder between the carrier's hip and ribcage. World Rugby's community-game law variation caps the legal tackle height at the base of the sternum, and below the ball is also where your leverage lives.
- Shorten your steps on the approach. Chopping your feet as you close keeps you balanced and able to adjust.
- Plant your lead foot close with a straight back. Step in tight to the carrier and keep your spine straight with your weight forward, so you are driving through them rather than reaching at an angle.
- Drive your legs and wrap. The tackle is not finished at contact. Keep your legs pumping for three to five steps, lock your arms around the carrier and squeeze to control the descent.
Tackle types and when to use them
| Tackle type | When to use | Key focus |
|---|---|---|
| Front-on dominant | Ball carrier running straight at you | Low body, leg drive, head behind or to the side |
| Side-on / drift | Ball carrier running across the field | Timing, shoulder contact, roll to ground |
| Chop / ankle | Last-ditch, covering tackles | Hit below the knee, arm wrap immediately |
| Double / assist | Organised defensive line | First tackler goes low, second controls the ball |
| Smother | Close range, ball carrier receiving a pass | Wrap the ball and arms before the offload comes |
Gym exercises that build tackle power
- Trap bar deadlift, because it trains the low hips and flat back you tackle from
- Power clean, for the explosive extension of ankles, knees and hips that launches you into contact
- Sled push, the nearest thing in a gym to sustained leg drive through a collision
- Split squat and single-leg squat, because tackles launch off one leg, not two
- Neck and shoulder strengthening, so the parts that take the contact are prepared for it