Newton’s Second Law: Force and Acceleration

8SCIENCE

Newton’s Laws

• Newton’s first law of motion describes motion of an object with NO net force – That an object will stay at rest or stay in constant

velocity (unless acted on by an outside force)

• Newton’s second law of motion:– An object affected by a net force will accelerate in

the direction of the force – Ex. If an object like a ball is kicked forward, it will

move faster in a forward direction

Newton’s second law of motion

• Acceleration = net force mass

a = __F__ m

F = force measured in Newtons (1 N = kg · m/s²)

• If force acts on a small mass and a large mass, the small mass will accelerate more

• Ex: if you try to push an empty box across the floor, it will accelerate faster if the box is packed with books

Calculate the dogs’ acceleration

Remember: a = f/m where a = acceleration, f = force and m = mass

Balanced/Unbalanced Forces

• If you are sitting, you are at rest: all the forces are balanced (you are not moving)– Gravity is pulling you down, chair is pushing you up

• The upward force of the chair surface is called the normal force – It is straight up and balances your weight

• If you put a heavy weight on the chair, it may no longer have enough normal force and the chair may break

• Also, if the surface becomes tilted (ex. Like a ramp/incline) the normal force can no longer balance this weight and you move downhill

Unbalanced forces**Remember: When forces acting on an object are not balanced, it accelerates in the direction of the net force• What is an object doing if it positively accelerates?

Negatively accelerates?– It may speed up, slow down or turn

• Ex. Turning: if you jump off a diving board into a pool you will not continue straight across the pool at that height - gravity will cause you to turn and accelerate downwards