Big Hits! PHYSICS IN SPORT LESSON 3

Big Hits!

• Apply Newtons second Law (F=ma)

• Know and use the quantity called momentum

(p=mv)

• Express and use Newtons Second Law in terms of

momentum F=Δp/ Δt

Learning Intentions

Newtons First Law

• A body will remain at constant speed

unless acted on by an unbalanced force

• Forces cause changes of velocity.

t

va

Newtons Second

Law

• The acceleration of a body is proportional to the force that causes it.

• Heavy objects are harder to accelerate than light ones

• This is often referred to as inertia

maF

aF

But….

Acceleration and therefore

force are vector quantities

a

F

Vector form of

Newton’s Second Law

• We need to ensure we are aware of the direction of our vectors (Which way is +ve?)

• A ball (0.5kg) rebounds from a wall at right angles. The impact takes 0.01s.

• What is the force acting on the ball? On the wall?

3 ms-1

2 ms-1

Vector form of Newton’s

Second Law – 2D

example

• A ball of mass 0.2kg collides

with a wall at an angle of

30o. It rebounds at the same

angle and the same speed.

The impact lasts 0.05s

a) Find the average

acceleration during impact.

b) The force on the ball (and on

the wall)

3 ms-1

30o

Momentum

• Newtons laws were originally expressed in terms of quantity called momentum

• Forces caused changes of momentum.

• Momentum is the product of an objects mass and velocity

m

v

mvp units – kgms-1 t

pF

t

vmF

maF

Impulse

• Why do you follow

through in tennis?

• Why do you draw your

hands back when you

catch a cricket ball

• The quantity “force x

time” is known as

impulse. units – Ns

1 Ns = 1 kgms-1

ptF

t

pF

Momentum form of Newton’s

Second Law

Momentum form of Newton’s

Second Law

• We need to ensure we are

aware of the direction of

our vectors (Which way is

+ve?)

• A ball (0.1kg) rebounds

from a wall at right angles.

The impact takes 0.01s.

• What is the force acting on

the ball? On the wall?

• What is the average

acceleration during impact?

2 ms-1

1.5 ms-1

A car of mass 1000kg, travels

East • A car of mass 1000kg, travels East at 6ms-1

and takes 2 seconds to round a bend, which

it does without loss of speed and then

heads South.

• What average force must turn the car?

• What is the radius of the bend?

Momentum form of Newton’s

Second Law – 2D example

• A ball of mass 0.25kg

collides with a wall at an

angle of 30o. It rebounds at

the same angle and the

same speed. The impact

lasts 0.05s. Find

a) The force on the ball (and on

the wall).

b) the average acceleration

during impact

4 ms-1

30o

Shunts • A NASCAR coming out of a spin is

coasting out of gear at 20 ms-1 when it

goes into the path of another car travelling

in the same direction at 40 ms-1. When

they collide the bumpers lock. How fast

do they move after the collision?

800 kg

20 ms-1

750 kg

40 ms-1

Slams • Two Hockey players collide

• If they hold onto each other

after the collision, how

do they move? 10 ms-1

80 kg

90 kg

12 ms-1

Stones • In curling the players slide 2kg “stones” along the ice

toward a target. They score the numbers of stones they

have nearest the centre of the target. Collisions to knock

out opponents stones are part of the game.

• In this shot explore the different

scenarios using calculations to

justify any conclusions.

8 ms-1

0 ms-1