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Constant speed and acceleration for Edexcel GCSE Physics

Constant speed and acceleration

This page covers the following topics:

1. Newton's first law
2. Objects at rest
3. Objects moving with constant velocity
4. Accelerating objects

Newton's first law states that unless an object is acted upon by an external resultant force, it remains at rest or continue moving at a constant velocity. When there is no resultant force acting on an object, the object is said to be in equilibrium. Usually resultant force is found by combining forces on the same axis as vectors.

Newton's first law

By Newton's first law, an object at rest will have no forces acting on it. An object lying at rest on the ground will only experience its downwards weight and the upwards normal reaction exerted by the ground on it. If an external resultant force is exerted on the object, then the object will no longer be at rest and will accelerate in the direction of the force.

Objects at rest

By Newton's First Law, an object travelling at constant velocity must have balanced forces acting on it. If an external resultant force acts on the object, then the object will accelerate in the direction of the force.

Objects moving with constant velocity

By Newton's first law, an object that is accelerating or decelerating must have unbalanced forces acting on it. Newton's second law can then be used to calculate the acceleration using the resultant force acting on the object.

Accelerating objects

1

An object is in vertical equilibrium. There are two forces acting on the object horizontally: a 53 N force in the East direction and a 70 N force in the West direction. Describe the object's motion using Newton's first law.

According to Newton's first law, since the object has a resultant horizontal force acting on it, it will accelerate in the direction of the force.

acceleration westwards

An object is in vertical equilibrium. There are two forces acting on the object horizontally: a 53 N force in the East direction and a 70 N force in the West direction. Describe the object's motion using Newton's first law.

2

A ball is rolling across the ground at a constant velocity when it is kicked. Explain what happens to the ball in terms of Newton's first law.

By Newton's first law, the forces acting on the ball when it is rolling at constant velocity are balanced. When the ball is kicked, an external resultant force acts on the ball, and thus by Newton's first law, the ball will accelerate in the direction of the force exerted on it by the kick.

The ball accelerates in the direction of the kick.

A ball is rolling across the ground at a constant velocity when it is kicked. Explain what happens to the ball in terms of Newton's first law.

3

State Newton's first law.

Newton's first law states than an object will remain at rest or continue to move at a constant velocity unless it is acted upon by an external resultant force.

An object will remain at rest or continue to move at a constant velocity unless it is acted upon by an external resultant force.

State Newton's first law.

4

A ladder of weight 200 N resting on rough ground is leaning against a smooth wall. The normal reaction exerted by the wall on the ladder is 200 N. Find the normal reaction and the frictional force exerted by the ground on the ladder.

Newton's first law states that an object will remain stationary unless it is acted upon by an external resultant force. Since the ladder is stationary, the forces acting on it must be balanced.

Resolving vertically: weight = N, therefore the normal reaction exerted by the ground on the ladder is 200 N.

Resolving horizontally: normal reaction at wall = F, therefore the frictional force exerted by the ground on the ladder is 200 N.

200 N, 200 N

A ladder of weight 200 N resting on rough ground is leaning against a smooth wall. The normal reaction exerted by the wall on the ladder is 200 N. Find the normal reaction and the frictional force exerted by the ground on the ladder.

5

An object is experiencing forces A, B, C and D as shown in the diagram. Explain the relationships which need to hold between these forces for the object to be in equilibrium.

By Newton's first law, the forces on an object must be balanced for it to be in equilibrium.
Therefore, the following equalities must hold: A = C and B = D.

balanced forces โ†’ A = C, B = D

An object is experiencing forces A, B, C and D as shown in the diagram. Explain the relationships which need to hold between these forces for the object to be in equilibrium.

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