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Aim: What is Newton’s 2 nd law of motion? Do Now: What is the net (resultant) force? m = 5 kg 10 N8 N Answer will be given shortly

What is a force? A push or a pull If we apply a force, what happens? DEMO Add more weight (force) – what happens? The object accelerates Acceleration is directly proportional to force a α F

Add more mass to the cart – what happens? or F = ma The object accelerates at a lesser rate Acceleration is indirectly proportional to mass a α F

Newton’s 2 nd Law of Motion – Law of Acceleration Unbalanced forces produce acceleration Units F = ma Named after Sir Isaac Newton 1642-1727 “Father of classical physics”

1 a. What force accelerates a 3 kg mass at 4 m/s 2 ? m = 3 kg a = 4 m/s 2 F = ? b. If the same force produces an acceleration of 6 m/s 2, what is the mass? F = 12 N a = 6 m/s 2 m = ? F = ma F = (3 kg)(4 m/s 2 ) F = 12 N F = ma 12 N = m(6 m/s 2 ) m = 2 kg

m = 5 kg 10 N8 N Free – Body Diagram A picture that shows only the forces Illustrated as vectors drawn to a simplified object (a box) Vectors originate from the center of the object What is the net (resultant) force? 2 N right

Redraw the free – body diagram with just the resultant force: 2 N What is the acceleration of the object? m = 5 kg F Net = 2 N a = ? F Net = ma 2 N = (5kg)a a = 0.4 m/s 2

Gravity Force of attraction between the Earth and other masses Weight – measure of the gravitational force acting on an object F = ma F g = mg F g = weight *Weight is a force!!! g = acceleration due to gravity = 9.81 m/s 2 (we can approximate it as 9.8 m/s 2 )

How much does a 50 kg man weigh on the surface of the Earth? m = 50 kg g = 9.8 m/s 2 F g = ? F g = mg F g = (50 kg)(9.8 m/s 2 ) F g = 490 N

The ratio of an object’s weight to its mass is equal to the object’s (1) Momentum (2) Inertia (3) Gravitational force (4) Gravitational acceleration

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